Temporal shifts in the fatty acid profiles of rocky intertidal invertebrates

ORIGINAL ARTICLE

Temporal variability in the isotopic niches of rocky shoregrazers and suspension-feedersNicole B. Richoux & Rachel T. Ndhlovu

Department of Zoology and Entomology, Rhodes University, Grahamstown, South Africa

Keywords

Limpets; macroalgae; mussels; niche

partitioning; polychaetes; stable isotopes;

suspended particulate matter; urchins.

Correspondence

Nicole B. Richoux, Department of Zoology

and Entomology, Rhodes University, PO BOX

94, Grahamstown, 6140 South Africa.

E-mail: [email protected]

Accepted: 24 June 2014

doi: 10.1111/maec.12200

Abstract

Carbon and nitrogen stable isotope ratios were measured in rocky inter-tidal

suspension-feeders (brown mussels and cape reef worms) and grazers (goat’s

eye limpets and cape sea urchins) to determine the influences of lifestyle and

time on the diets of consumers. Niche partitioning between consumer species

within the same feeding guild was assessed using isotopic niche area (a proxy

for trophic niche). Specimens were collected monthly at a single site in South-

eastern South Africa from July 2010 to June 2011. Temporal variations in iso-

topic signatures were generally greater in the suspension-feeders compared

with the grazers, isotopic niche widths were smaller in the suspension-feeders

and intra-population variations in isotope signatures were larger in the grazers.

No inter-specific niche overlap (according to standard ellipse areas) was

observed within either feeding guild unless standardization calculations were

used. Temporal variations in the diets of all the consumers appeared uncou-

pled from temporal variability in the isotopic signatures of basal resources in

the region; as such, shifts in the consumer diets most likely pertained to feed-

ing behaviour and food preferences. Our data provide new insights into how

syntopic rocky shore consumers coexist by partitioning their temporally vari-

able food environment.

Introduction

Rocky shore organisms live under physically challenging

and highly dynamic conditions (through space and time),

and different species play varying roles as habitat provid-

ers, consumers and/or food sources. The complex inter-

play of organisms’ feeding relationships contributes sub-

stantially to the structure and dynamics of rocky shore

communities (Bustamante et al. 1995; Menge et al. 1999).

As such, coastal researchers have long been interested in

the feeding habits of animals in this energetic and pro-

ductive ecotone. The fluctuating nature of the habitat

contributes to a diversity of food resources available for

consumers, including macroscopic (i.e. macroalgae, mac-

rozooplankton) and microscopic (i.e. benthic and pelagic

microalgae, detritus, bacteria, microzooplankton, etc.)

material (Bustamante et al. 1995; Bode et al. 2006). Vari-

able inputs of terrestrial detritus via river run-off, or

resuspension of settled benthic material, can cause further

variability in the food available for coastal communities

(Airoldi et al. 1996; Takai et al. 2004; Richoux et al.

2014b).

Besides the obvious fundamental role of food availabil-

ity, lifestyles can differ amongst co-occurring aquatic con-

sumers and so their food is acquired through a variety of

mechanisms and behaviours (Ward & Shumway 2004;

Steinarsd�ottir et al. 2009; Aguilera et al. 2013). As such,

basic dietary compositions may contrast amongst con-

sumers, and different basal resources (e.g. primary pro-

ducers) may respond diversely to biotic and/or abiotic

drivers (e.g. Nordstr€om et al. 2009; Dethier et al. 2013).

This diversity in basal organismal responses can lead to

contrasting qualitative or quantitative fluctuations of pri-

mary producer populations, even over the same time per-

iod and in the same location. For example, macroalgal,

biofilm and phytoplankton communities respond

Marine Ecology (2014) 1–15 ª 2014 Blackwell Verlag GmbH 1

Marine Ecology. ISSN 0173-9565

differentially to changes in nutrient concentration, light

level, temperature and grazing pressure (Pedersen &

Borum 1996; Kinney & Roman 1998; Qian et al. 2003;

Thompson et al. 2004). Even within a feeding guild (e.g.

invertebrate suspension-feeders), different species can

capture suspended particles within a specific size range or

having a certain chemical composition (Kang et al. 2009)

owing to the range in sophistication and specificity of the

filtering structures that have evolved (Ward & Shumway

2004). In contrast to the pelagic feeding habits of suspen-

sion-feeders, invertebrate grazers tend to use scraping

devices to remove material associated with benthic sur-

faces. Such material can include macroalgae, bacteria,

microalgae, settled detritus and invertebrates (Thompson

et al. 2004; Camus et al. 2008). With generally different

feeding habits in the two groups, there can be an ecologi-

cal disconnect between the food sources consumed by

grazers versus suspension-feeders. It follows that we could

expect differences in the temporal variability in the diets

of rocky shore organisms occupying these contrasting

feeding guilds.

Different techniques are available to study the diets of

aquatic invertebrates including gut contents (i.e. direct

observations or DNA analysis) and tracers (i.e. fatty acid

or stable isotope signatures). We adopted stable isotope

concepts to approach our research questions because iso-

topic ratios can provide us with basic information about

consumer diets and the trophic niche occupied by a pop-

ulation (Bearhop et al. 2004; Layman et al. 2012).

Researchers have used isotopic techniques to explore the

diets of coastal invertebrates, and spatial variability has

been a topic of interest (Riera & Richard 1996; Yokoyama

& Ishihi 2007; Vanderklift & Wernberg 2010; Richoux

et al. 2014b). Questions involving seasonality in inverte-

brate community trophic interactions are also becoming

more prevalent in the literature (e.g. Rolff 2000; Decottig-

nies et al. 2007; Lefebvre et al. 2009; Nordstr€om et al.

2009; Woodland et al. 2012), although studies on inter-

tidal rocky shore communities are relatively rare (but see

Schaal et al. 2010; Richoux et al. 2014b). Temporal varia-

tions are commonly considered a source of unwanted

error in a data set, and primary consumers are often pref-

erentially used as baselines in food web studies because

their slower turnover times tend to dampen the effects of

temporal fluctuations in the isotopic signatures of pri-

mary food sources (Fukumori et al. 2008). A general

body size effect (decreased temporal variability in larger

animals; Cabana & Rasmussen 1996; Woodland et al.

2012) has further facilitated the usage of primary con-

sumers (particularly bivalves) as stable baselines for mix-

ing models. In our case, however, we focussed specifically

on temporal variability so that we may learn how com-

munity feeding interactions can change.

Regardless of the general approach taken, the combina-

tion of a diverse pool of resources subject to shifting

environmental conditions through time (all of which can

affect isotope signatures; Sato et al. 2006; Rolff 2000) cre-

ates a potentially variable dietary regime for any rocky

shore consumer. Isotopic data have been particularly suc-

cessful at revealing the range and complexity of diets in

numerous aquatic consumers (Bode et al. 2006; Decottig-

nies et al. 2007; Kang et al. 2009). Furthermore, stable

isotope metrics are becoming increasingly sophisticated

and can represent ecologically relevant proxies for trophic

niches of populations, and can even illuminate trophic

niche partitioning amongst co-occurring consumers

(Jackson et al. 2011; Layman et al. 2012). Broad-scale

patterns in aquatic consumers have been elucidated using

meta-analyses of published isotope literature, and influen-

tial biotic factors include body size and feeding habits of

organisms, whereas abiotic factors such as the trophic

state of the environment (i.e. oligotrophic, mesotrophic

or eutrophic) are equally important (Woodland et al.

2012). We chose to focus on the differences between

organisms from two trophic guilds that occupy the same

rocky inter-tidal site.

Our primary aim was to characterize the temporal vari-

ability in rocky shore consumer diets. We approached

this task by assessing the isotopic signatures of two sus-

pension-feeders and two grazers from one location on a

monthly basis over 1 year. Representative macroalgae and

suspended particulate matter (SPM) were assessed in a

similar manner to gain some understanding of changes in

local basal resources over time. We tested the hypothesis

that isotopic signatures of suspension-feeders are more

variable through time relative to those of grazers owing

to the greater potential variability in basal suspended

food resources relative to benthic algae. We were also

interested in comparing the isotopic niches of popula-

tions within each feeding guild (suspension-feeder and

grazer guilds), as any differences in isotopic niche would

suggest an influence of selective feeding.

Material and Methods

Study area

Specimen collections were carried out on a monthly basis

during low-tide dates between July 2010 and June 2011.

The collection location was in a rocky shore region

~2 km northeast of the mouth of the Kariega Estuary,

Kenton-on-Sea (Eastern Cape, South Africa; Fig. 1). The

region is warm-temperate and influenced by the warm

oligotrophic Agulhas Current flowing southeast along the

eastern coast of Southern Africa, so there is relatively

little seasonal variability in surface water temperatures

2 Marine Ecology (2014) 1–15 ª 2014 Blackwell Verlag GmbH

Isotopic niches of rocky shore consumers Richoux & Ndhlovu

(Lutjeharms 2005; surface temperatures measured during

our study ranged from 14.5 to 20 °C). The nearby Karie-

ga Estuary is a marine-dominated system with typically

low nutrient concentrations and freshwater outflow

(Grange et al. 2000), so terrestrial inputs do not influence

the diets of coastal invertebrates in the study area

(Richoux et al. 2014a,b).

Specimens

Four consumer species were targeted: the goat’s eye lim-

pet Cymbula oculus (Born 1778; formerly Patella oculus),

the cape urchin Parechinus angulosus (Leske 1778), the

brown mussel Perna perna (Linnaeus 1758) and the cape

reef worm Gunnarea gaimardi (Quatrefages 1848; for-

merly Gunnarea capensis). The limpets and sea urchins

were representatives of the grazing guild, and the mussels

and polychaetes of the suspension-feeding guild. Five ani-

mals per species per sample time were removed from the

rocks using a chisel, and the animals were stored in buck-

ets containing seawater. Polychaetes were located slightly

lower in the inter-tidal zone relative to the mussels, lim-

pets were collected from exposed rock throughout the

inter-tidal region, and sea urchins were found sheltered

in rock pools. We selected animals that were relatively

similar in size (mussel shell length averaged ~50 mm, sea

urchin test width averaged ~40 mm, limpet shell width

averaged ~55 mm and polychaete length averaged

~50 mm). Three replicates of 5-l surface seawater were

collected on each sampling occasion to derive isotopic

signatures for the SPM. Specimens of three macroalgal

species (Ulva sp., Bryopsis sp. and Gelidium pristoides)

were gathered by hand to ascertain the temporal variabil-

ity in isotopic signatures of some representative primary

producers from the area, although Bryopsis sp. was found

only on five occasions. It was not our intention to under-

take an exhaustive search for all the potential diet items

for the consumers we selected, as that task would have

entailed a very different approach.

Laboratory analysis

Animals were kept in aerated seawater overnight to allow

for gut clearance (particularly important for the polychae-

tes, as these were not dissected). Muscle tissues were

removed from the mussels, limpets and sea urchins and

frozen at �80 °C. Water samples were vacuum-filtered

onto pre-combusted (4 h, 450 °C) Whatman glass fibre

type F filters, acidifed with 1 M hydrochloric acid (HCL)

to remove carbonates and rinsed with distilled water.

Macroalgae were cleaned of visible epiphytes. All samples

were kept frozen (�80 °C), lyophilized for 24 h in a Vir-

Tis BenchTop 2K and the animal and macroalgal tissues

were then ground into homogeneous powders using a

mortar and pestle. Homogenized tissues were treated with

1 M HCL, rinsed twice in MilliQ-distilled water and dried

at 50 °C. Lipids were removed from the polychaete tis-

sues (to prevent unwanted influences of changing lipid

profiles from gonad development; Lorrain et al. 2002; this

procedure was carried out only for the polychaetes as we

were unable to successfully remove this potentially lipid-

rich non-muscle material through dissection) using 2 ml

of 2:1 chloroform-methanol and a few drops of MilliQ

water. The lipid-extracted samples were re-dried at 50 °Cprior to final homogenization.

Subsamples of ~1 mg (animals) to 2 mg (algae) of tis-

sues were weighed into tin capsules, and sections of the

filtered samples were placed inside tin capsules, and all

samples were analysed on a Europa Scientific ANCA-SL

Elemental Anayser coupled to a 20-20 isotope-ratio mass

Fig. 1. Study region and sampling site for all

specimen collections.


Richoux & Ndhlovu Isotopic niches of rocky shore consumers

spectrometer. The data were expressed in & in the stan-

dard d unit notation: dX = [(Rsample/Rstandard) � 1]1000,

where X = 13C or 15N and R = 13C/12C or 15N/14N. These

abundances were calculated in relation to in-house stan-

dards (casein and a mixture of beet sugar and ammo-

nium) calibrated against International Atomic Energy

Agency (IAEA)-CH-6 and IAEA-N-1 reference materials.

Data analyses

To address the main hypothesis (that isotopic signatures

of suspension-feeders are more variable through time rel-

ative to those of grazers), we visually examined the

changes in carbon and nitrogen isotope signatures in all

consumers and potential basal sources over time. For

each species, three-one-way analyses of variance (ANO-

VAs) were completed: one with d13C signature as the

dependent variable, the second with d15N signature and

the third with trophic position (see the next paragraph

regarding the calculation for trophic position). ‘Time’

was included as the predictor variable for each ANOVA

to determine whether the isotope signatures differed

amongst sampling dates. When residual analysis indicated

that the parametric model assumptions were not valid, a

non-parametric Kruskal–Wallis test was carried out

instead. ANOVAs were not performed using d13Ccorrected

(parameter described below) as the statistical output was

identical to those of the non-corrected d13C values. Raw

data for each consumer from four different dates were

examined in additional detail using d13C versus d15N bi-

plots to further compare temporal variability within the

suspension-feeder and grazer guilds.

In order to assess whether the isotopic niches of the two

suspension-feeding species were distinct from one another,

and the isotopic niches of the two grazers were distinct

from one another, we calculated the trophic position

(Vander Zanden & Rasmussen 1999) for each individual

consumer relative to a possible food source as a baseline

(SPM, Ulva sp. or Gelidium pristoides): trophic positioncon-

sumer = 1+[(d15Nconsumer � d15Nbaseline) � (TFd15N)�1], where

d15Nconsumer is the d15N value for an individual consumer,

d15Nbaseline represents the reference baseline value at posi-

tion 1 and TFd15N is the mean trophic fractionation factor

of 2& (McCutchan et al. 2003). These trophic position

calculations allowed for incorporation of potential shifts in

baseline food sources over time. As an additional indicator

of isotopic niche space occupied by each consumer, coeffi-

cients of variation (CVs) for d13C and d15N were calculated

on each date to provide standardized values of intra-spe-

cific variability (and mean CVs were calculated from these

monthly values to compute a measure for general intra-

specific variability over the study period). We postulated

that organisms feeding on a larger range of food items

would have larger mean CVs, indicating greater amongst-

individual variability in diet within any given time, relative

to more specialized feeders. Technically, CVs are intended

for data measured on a ratio scale, and as isotope signa-

tures are interval-scale data, we calculated the maximum

change within each time series for each population (Dd15N,Dtrophic position, Dd13C and Dd13Ccorrected, see next para-

graph for the d13Ccorrected calculation) as additional stan-

dardized measures of variability (Olsson et al. 2009;

Woodland et al. 2012).

The isotopic niche width of each consumer population

(data from all dates pooled) was calculated using both

convex hull (Layman et al. 2012) and standard ellipse

areas (SEAs; Jackson et al. 2011). These isotopic metrics

are proxy measures for the trophic niches of populations,

with the convex hull technique describing the niche space

occupied by all samples collected from a population, and

the SEA describing the average niche space occupied by a

population (Layman et al. 2012). As isotopic niche area

for a population can be influenced by variations in base-

line sources over space and time (Matthews & Mazumder

2004; Hoeinghaus & Zeug 2008), we also calculated the

convex hull and SEA metrics using consumer data

corrected for these shifts [trophic position instead of

d15N, and d13Ccorrected instead of d13C; where d13Ccor-

rected = (d13Cconsumer � d13Cmean of baseline)/carbon range of

baseline; Olsson et al. 2009]. The baseline used for the

suspension-feeders was SPM, and the baseline for the sea

urchins was all the macroalgae combined. For the limpets

we utilized Ulva sp. and Corallina sp. (the latter derived

from Allan et al. 2010) as the limpet d13C signatures ran-

ged between these two sources. These statistical proce-

dures using d13Ccorrected served as a sensitivity analysis to

determine how choice of baseline sources can influence

isotopic niche calculations. Isotopic metrics were calcu-

lated using Stable Isotope Analysis in R (SIAR; Parnell

et al. 2010), ANOVA and Kruskal–Wallis tests were per-

formed using SYSTAT 12 and graphics were created in

SIGMAPLOT 10.

Results

Temporal variations in rocky shore organisms

The d15N and d13C signatures of suspension-feeders

(Perna perna and Gunnarea gaimardi) and grazers (Pare-

chinus angulosus and Cymbula oculus) showed different

patterns of variability through time (Fig. 2). ANOVA or

Kruskal–Wallis tests indicated significant shifts in d15Nand d13C signatures and the trophic positions of the sus-

pension-feeders over time (Table 1). The d13C values in

both suspension-feeders were generally increasingly

depleted through summer (November to February;



Fig. 2B), whereas temporal patterns in d15N values were

very different in Pe. perna and Gu. gaimardi and fluctua-

tions in Gu. gaimardi were much greater overall

(Fig. 2A). Of the two grazers, the d13C signatures of

Pa. angulosus changed significantly [with generally higher

values occurring in summer/autumn (January through to

April); Fig. 1D], but there was no distinctive temporal

pattern in their d15N values (Table 1; Fig. 2C). There was

a significant effect of time on trophic position of Pa. ang-

ulosus (Table 1), a metric that takes into account shifts in

basal resources. The d15N and d13C signatures of the

limpets (C. oculus) were remarkably consistent, although

as in the sea urchins, the trophic position of the limpets

shifted significantly over time (Table 1; Fig. 2C,D).

The isotopic signatures of SPM and the three macroal-

gae were highly variable for the most part; only d13C val-

ues of SPM were not significantly influenced by time

(Table 2; Fig. 3). There was a general trend of depletion

in d15N values, particularly in SPM, Bryopsis sp. and Geli-

dium pristoides, during the summer months, but no com-

mon trend occurred in d13C values of the local basal

resources (Fig. 3).

Table 1. ANOVA and Kruskal–Wallis (KW) output for testing the influence of time on isotopic signatures and trophic positions (TP) of suspen-

sion-feeders Perna perna and Gunnarea gaimardi, and grazers Cymbula oculus and Parechinus angulosus.

Species df (factor, error)

d13C d15N TP

F/KW P F P F/KW P

Perna perna 10, 37 4.85 <0.001* 2.42 0.025* 7.86 <0.001*

Gunnarea gaimardi 9, 36 32.97 <0.001* 16.45 <0.001* 31.46 <0.001*

Cymbula oculus 10, 44 10.93 0.36 1.47 0.18 6.12 <0.001*

Parechinus angulosus 10, 30 18.79 0.043* 1.17 0.35 6.38 <0.001*

df, degrees of freedom; F, F ratio.

*Denotes a significant effect.

Underlined values are the Kruskal Wallis outputs.

A B

C D

Fig. 2. d15N (A, C) and d13C (B, D) signatures (&; mean � SD of three to five individuals per species and sample time) for suspension-feeders

(mussels Perna perna and polychaetes Gunnarea gaimardi) and grazers (sea urchins Parechinus angulosus and limpets Cymbula oculus).

Percentage values are grand mean coefficients of variation (CVs; averages of all CVs from each date); note the different scales in the d13C axes.



Isotopic niches of consumers

To explore the isotopic (trophic) niche occupied by each

population at different times of the year, we extracted

the raw data of suspension-feeders collected during

August, November, February and May (Fig. 4A). These

were four sampling occasions that showed different levels

of dissimilarity in isotopic signatures between Perna

perna and G. gaimardi (e.g. d15N signatures were most

similar in the two species during February, most distinct

during August and moderately distinct during November

and May; Figs 2, 4A). Even during February, when the

two suspension-feeders had the most similar isotopic sig-

natures and trophic positions, there was no isotopic

overlap between the populations (Fig. 4A). We subtracted

the relevant trophic enrichment factors from each con-

sumer and superimposed SPM values (from our study

and from Richoux et al. 2014b) and nearshore zooplank-

ton signatures (derived from Vorwerk & Froneman 2009)

to investigate the relationships between the consumers

and potential food sources. Interestingly, our SPM d15Nsignatures were exactly level with the mussels, but our

SPM d13C signatures were very distant from the mussels

and polychaetes, and SPM d15N and d13C signatures

from 2009 (Richoux et al. 2014b) were low and high,

respectively, relative to the mussels (Fig. 4A).

The isotopic niche occupied by each grazer population

during different times of the year was examined by focus-

sing on specimens collected during August, October,

Table 2. ANOVA output for testing the influence of time on isotopic

signatures of suspended particulate matter (SPM) and macrophytes

Ulva sp., Gelidium pristoides and Bryopsis sp.

Sample type df (factor, error)

d13C d15N

F P F P

SPM 10, 21 1.72 0.14 3.31 0.01*

Ulva sp. 9, 17 50.34 <0.001* 27.07 <0.001*

Gelidium

pristoides

9, 20 37.97 <0.001* 24.43 <0.001*

Bryopsis sp. 4, 10 56.19 <0.001* 48.78 <0.001*

df, degrees of freedom; F, F ratio.

*Denotes a significant effect.

A B

C D

Fig. 3. d15N (A, C) and d13C (B, D) signatures (&; mean � SD) for macroalgae (Ulva sp. and Gelidium pristoides) and macroalgae Bryopsis sp.

and suspended particulate matter (SPM). Percentage values are grand mean coefficients of variation (CVs; averages of all CVs from each date);

note the different scales in the d13C axes.



April and June (Fig. 4B). As in the suspension-feeders,

even when the two grazers were most similar during

October and April, there was no overlap of isotopic niche

between the populations. This lack of niche overlap was

apparent despite the considerable intra-specific variability

of isotopic signatures within each grazer population, par-

ticularly the sea urchins (Fig. 2C,D). The macroalgal spe-

cies that we collected, particularly Gelidium pristoides,

may all have contributed towards the diet of Parechinus

angulosus, but we probably failed to collect the main diet

item for Cymbula oculus (Fig. 4B).

Overall, CVs of isotopic signatures within each date

were largest in sea urchins (C range 5–24%, N range 0.8–12%), intermediate in limpets (C range 3–17%, N range

3–7%) and smallest in polychaetes (C range 0.2–2%, N

range 1–6%) and mussels (C range 0.2–2%, N range 0.5–5%; see Fig. 2 for grand means). Similarly, the maximum

changes within each time series (Dd15N, Dtrophic posi-

tion, Dd13C and Dd13Ccorrected; Table 3) were generally

largest in the sea urchins, smallest in the mussels and

intermediate in the limpets and polychaetes.

The trophic position calculations based on changing

baseline resources (in this case, SPM) showed interesting

differences between the two suspension-feeders, whereby

at some times during the year the polychaetes were more

than a full trophic position higher than the mussels

(Table 3). Taking the full year into account, the mean

trophic position of mussels was 1.9, reflecting a true pri-

mary consumer level, and the mean trophic position of

polychaetes was much higher at 2.8 (Table 3). The tro-

phic position calculations for the grazers reflected a pri-

mary consumer level (2.2 for limpets, 2.1 for sea urchins;

Table 3). We selected the primary producer baseline for

each grazer according to the closer association of Ulva sp.

to the limpets, and of Ge. pristoides to the sea urchins

(Fig. 4B), although the diets of these consumers are cer-

tainly more complex than this simple assignation (based

on the high intra-specific variability of consumer d13Cand d15N signatures; Fig. 4B). When we performed addi-

tional trophic position calculations for limpets and sea

urchins using mussels as the baseline, the mean trophic

positions were higher (2.4 � 0.3 and 2.6 � 0.5 for

A

B

Fig. 4. d15N and d13C (&) biplots of raw

data from (A) suspension-feeders (mussels

Perna perna and polychaetes Gunnarea

gaimardi) collected during August, November,

February and May, and (B) grazers (sea

urchins Parechinus angulosus and limpets

Cymbula oculus) collected during August,

October, April and June. Dotted lines are

convex hull areas; trophic enrichment factors

have been subtracted from each C and N

signature; note different x and y scales used.

NS SPM 1, nearshore suspended particulate

matter measured during the current study;

NS SPM 2, nearshore SPM from Richoux et al.

(2014b); NS ZOOP, nearshore zooplankton

from Vorwerk & Froneman (2009); Bryopsis,

Gelidium and Ulva macroalgal signatures are

from the current study, and Corallina sp. was

extracted from Allan et al. (2010).



limpets and sea urchins, respectively), and although a

temporal effect was absent in the sea urchins

(F10,30 = 1.83, P = 0.098), there remained a significant

effect of time on limpet trophic position (F10,44 = 3.69,

P = 0.001).

Isotopic niche areas (&2), calculated from data pooled

from all collection dates, were larger in the grazer com-

pared with the suspension-feeder populations (Fig. 5).

Convex hull areas were 19.9&2 and 6.6&2 for the sea

urchins and limpets, and 2.4&2 and 0.9&2 for the

polychaetes and mussels, respectively. SEAs based on the

original d13C and d15N signatures were 5.1&2, 1.4&2,

0.8&2 and 0.3&2 for the sea urchins, limpets, polychae-

tes and mussels, respectively. Convex hull areas over-

lapped between the two suspension-feeder populations

and between the two grazer populations, but the SEAs

did not overlap within either feeding guild (Fig. 5A,C).

SEAs calculated using baseline-corrected data (d15Nreplaced with trophic position and d13C replaced with

d13Ccorrected) were 0.4&2, 0.1&2, 0.1&2 and 0.06&2 for

Table 3. Trophic position (TP) of Perna perna and Gunnarea gaimardi relative to suspended particulate matter (SPM; Cymbula occulus relative to

Ulva sp. and Parechinus angulosus relative to Gelidium pristoides.

Perna perna (mussels) Gunnarea gaimardi (polychaetes) Cymbula oculus (limpets) Parechinus angulosus (sea urchins)

TP

July 2010 1.7 (0.1) 3.2 (0.0) 2.1 (0.2) 2.0

August 2.0 (0.1) 3.6 (0.3) 2.1 (0.1) 1.9 (0.2)

October 1.6 (0.0) � 2.0 (0.2) 2.0 (0.6)

November 1.7 (0.2) 2.7 (0.3) 2.3 (0.2) 2.4 (0.0)

December 1.9 (0.2) 2.5 (0.1) 2.1 (0.2) 2.3 (0.2)

January 2011 2.0 (0.2) 2.7 (0.2) 2.8 (0.3) 2.5 (0.3)

February 2.3 (0.2) 2.6 (0.1) 2.0 (0.2) 2.3 (0.5)

March 2.0 (0.2) 2.5 (0.1) 2.4 (0.2) 2.2 (0.3)

April 1.9 (0.1) 2.9 (0.2) 2.4 (0.2) 2.3 (0.4)

May 2.0 (0.1) 2.8 (0.1) 2.1 (0.2) 1.6 (0.2)

June 1.9 (0.2) 2.9 (0.1) 2.0 (0.3) 1.2 (0.2)

TP grand mean 1.9 (0.2) 2.8 (0.4) 2.2 (0.3) 2.1 (0.6)

Dd15N 1 to 6 = 5.0 0.5 to 5 = 4.5 3 to 7 = 4.0 0.2 to 12 = 11.5

DTP 1.5 to 2.5 = 1.0 2.3 to 3.8 = 1.5 1.6 to 3.2 = 1.6 1.0 to 3.1 = 2.1

Dd13C 0.2 to 2 = 1.8 0.2 to 2 = 1.8 3 to 17 = 14.0 5 to 24 = 19.0

Dd13Ccorrected 1.0 to 1.3 = 0.3 1.1 to 1.5 = 0.4 0.0 to 0.5 = 0.5 �0.2 to 0.8 = 1.0

TP values are means (�SD); D values represent the maximum change within a time series; and Ccorrected are standardized d13C values (see text for

explanation); – no data.

A B

C D

Fig. 5. Isotopic data for suspension-feeders

[mussels Perna perna (n = 48) and

polychaetes Gunnarea gaimardi (n = 46)]: (A)

d13C and d15N (&), (B) d13Ccorrected and

trophic position; and grazers [sea urchins

Parechinus angulosus (n = 41) and Cymbula

oculus (n = 55)]: (C) d13C and d15N (&), and

(D) d13Ccorrected and trophic position; data

from all dates are pooled. Dotted lines are

convex hull areas and ellipses are standard

ellipse areas (values for these are included

next to each group of points, units are &2);

note the different scales used for the axes.



the sea urchins, limpets, polychaetes and mussels, respec-

tively, and there was complete overlap of SEAs between

the sea urchins and limpets (Fig. 5B,D).

Discussion

Temporal variability in consumers

We investigated the hypothesis that isotopic signatures of

rocky shore suspension-feeders are more variable through

time relative to those of grazers owing to the greater

potential temporal variability in suspended basal food

sources relative to that of benthic algae. Our hypothesis

was partially supported, as there were consistent temporal

effects on both of the suspension-feeding species (for

both d13C and d15N signatures and trophic position), but

generally not for the grazers (except for sea urchin d13Cand trophic position for both grazers; Table 1, Fig. 2).

The trophic position of the grazers changed abruptly

between consecutive months on several occasions

(Table 3), hence leading to the significant effects of time

on trophic position. Grazer trophic position values were

calculated based on Ulva sp. and Gelidium pristoides sig-

natures in the first instance, and temporal shifts in the

macroalgal signatures were large and erratic (particularly

throughout the summer; Fig. 3), so the algae did not rep-

resent ideal baselines from which to calculate grazer tro-

phic positions. However, even when we used mussels as

the baseline, there remained a significant effect of time

on limpet trophic position. Despite any uncertainties or

large variations in basal food sources, the temporal shifts

in diet amongst the different consumers were clearly dis-

tinct for each population, and intra-population variability

during each sampling time (shown using the CV and Dcalculations) was clearly a critical influence on the statis-

tical outcomes. For example, in contrast with the large

intra-population variability in the grazers on each date,

variability within the suspension-feeders was much smal-

ler (especially for the carbon metrics), so even small shifts

from one date to the next were readily apparent. The

smaller within-date intra-population variability in the

suspension-feeders probably reflected their increased feed-

ing specificity compared with grazers, as filtration mecha-

nisms of many species exploit certain components of the

suspended particulates available (Decottignies et al. 2007;

Yokoyama & Ishihi 2007; Kang et al. 2009). One aspect

of our data that did not support our hypothesis was that

temporal variability was generally not greater in the SPM

compared with the macroalgae (Fig. 3; Table 2). In fact,

although mean within-date CVs for SPM d13C and d15Nvalues were larger than those of the macroalgae (Fig. 3),

SPM variations over the year were generally less

pronounced than those in the macroalgae (Fig. 3;

Table 2). As such, consumer feeding behaviour appeared

to be a more important driver for temporal variations in

their own diet signatures than variations within the basal

resources.

The mobility and therefore increased foraging range of

grazers compared with sessile suspension-feeders may

have contributed to the increased within-date intra-popu-

lation variability, but decreased temporal variability,

observed in the grazer guild. Increased amongst-individ-

ual foraging variability in consumers has been expressed

as an increase in population niche width in some terres-

trial consumers (Darimont et al. 2009; Semmens et al.

2009), although the opposite pattern (i.e. greater isotopic

variation in specialist consumers relative to generalists)

can also occur (e.g. Flaherty & Ben-David 2010). Cer-

tainly the spatial scale and relative heterogeneity of the

foraging areas available to consumers (e.g. a rocky inter-

tidal zone available to benthic invertebrates versus the

greater span of terrestrial habitats available to large terres-

trial vertebrates) must also be taken into consideration.

Bearhop et al. (2004) predicted that increased foraging

range could lead to increased variation in isotopic signa-

tures of consumers, and our larger within-date intra-pop-

ulation variability in the limpets and sea urchins supports

their prediction. It also makes intuitive sense that

increased intra-population variations would dampen tem-

poral effects. Woodland et al. (2012) completed a large-

scale meta-analysis of published studies that spanned

freshwater, estuarine and coastal habitats. They found an

effect of feeding group on the temporal variability of con-

sumers, with generally increased variability in grazers rel-

ative to suspension-feeders or detritivores (Woodland

et al. 2012), so our data did not fit their overall pattern.

Bearhop et al. (2004) predicted that populations with

individuals that feed from different trophic levels will

have increased isotopic variance; this prediction seems

particularly relevant to the sea urchins in our study,

which had the largest variation in trophic position of all

the consumers that we assessed (Table 3). Potential diet

items for the different consumers in our study, with

respect to how they may affect trophic position, are

described in the next two sections. The short answer to

our first hypothesis about the temporal variability of

grazers versus suspension-feeders is that grazers were less

variable through time, but probably not because their

food was more consistent through time.

Large-scale (inter-annual) and small- to medium-scale

(daily to monthly) changes in the isotopic signatures of

nearshore SPM and rocky inter-tidal macroalgae occur

within different biogeographical regions of the South

African coastline (Hill et al. 2006, 2008). At some loca-

tions, mean d13C values of SPM changed by more than

4& from 1 year to the next (and by up to 3& during a



tidal cycle within 1 day), and the macroalgal signatures

(including Ulva sp. and Ge. pristoides) were largely and

erratically variable so that no temporal patterns could be

extracted and no direct links to local primary consumers

were possible (Hill et al. 2006, 2008). A clear nearshore

to offshore gradient (from the shoreline to 10 km off-

shore) of increased depletion in SPM d13C values was

apparent; thus, large- and medium-scale temporal

changes in coastal SPM signatures could arise from vari-

able nearshore oceanographic dynamics, shifts in the

position of the Agulhas Current and perhaps variable

mixing of offshore and nearshore waters (Hill et al.

2006). In these previous studies, the shifts in SPM signa-

tures were not reflected in Perna perna so there was a dis-

connect between food and consumer, and the mussels

seemed to have fairly constant diets over large time peri-

ods (i.e. months to years; Hill et al. 2006, 2008). Tempo-

ral variations in the SPM can be caused or enhanced by

periodic upwelling conditions (or any other process that

produces changes in temperature, light, resuspension

events, sources of inorganic carbon or circulation pat-

terns) or shifts in phytoplankton community composition

(Goering et al. 1990; Rolff 2000; Takai et al. 2004; Nord-

str€om et al. 2009), whereas consumer diets can be greatly

dampened by their relatively slow tissue turnover rates

(Rolff 2000; O’Reilly et al. 2002). We observed a similar

uncoupling between SPM and the suspension-feeders that

we assessed (Pe. perna and Gunnarea gaimardi), as the

SPM d13C signatures were fairly low relative to the con-

sumers (whereas our SPM values measured during previ-

ous years were higher; Richoux et al. 2014b; Fig. 4A),

and the temporal dynamics of food and consumer did

not match. All of our water collections were completed

during low tide, when d13C signatures can be most

depleted in a tidal cycle (Hill et al. 2008), whereas the

average of SPM signatures measured throughout an entire

tidal cycle were enriched probably by about 2&.

Researchers studying the rocky inter-tidal zone in a

region of the Sea of Japan demonstrated how the physical

resuspension of microphytobenthos caused significant 13C

enrichment in the suspended particulates, and in that

case this enrichment was reflected in the local suspen-

sion-feeders (Takai et al. 2004). However, mismatches

between isotopic signatures of food and benthic consum-

ers are not uncommon in the literature (e.g. Goering

et al. 1990; Lorrain et al. 2002; Nordstr€om et al. 2009)

and are an important reflection of alternative isotopic

turnover dynamics at different trophic levels.

Studies of temporal dynamics in basal food sources

and consumers have been carried out in some soft- and

hard-substrate coastal regions. Nordstr€om et al. (2009)

noted temporal changes in isotopic signatures of infaunal

invertebrates in the Baltic Sea resulting from seasonal

shifts in food availability and in consumer feeding habits,

with no time lags observed between basal resources and

consumers. These researchers showed some evidence for

potential resource partitioning amongst different species

within the same feeding guilds in a shallow subtidal

coastal community (Nordstr€om et al. 2009), and it would

be interesting to explore their data set using isotopic met-

rics. Relatively small seasonal changes were observed in

isotopic signatures of many rocky shore consumers in

Western Brittany, France, occupying several feeding guilds

(filter-feeders, grazers and carnivores; Schaal et al. 2010),

although the sampling events during that study were nei-

ther numerous nor frequent. Our own data suggest that

the temporal scale chosen for any particular study may

greatly influence the clarity regarding shifts in producers

or consumers occupying different trophic guilds, as dif-

ferent components of a food web can shift isotopically in

very different manners.

Isotopic niches of suspension-feeders

As the SPM isotopic signatures were not statistically more

temporally variable than those of the representative

attached macroalgae (Table 2), the increased temporal

variations in suspension-feeder diets were probably influ-

enced by differences in particle selection between the two

species relative to changing food availability. Some sus-

pension-feeders have exceptional abilities to select specific

types of particles from a complex seston mixture (Ward

& Shumway 2004), so regardless of the composition of

the SPM, they can regulate their diets (to a point). This

premise led us to test whether the isotopic niches of the

two suspension-feeders differed, potentially owing to dis-

parate particle selection behaviours.

Most bivalves tend to have excellent selective abilities

(Ward & Shumway 2004). Mussels such as Perna perna

use cilia located along highly folded gills to actively create

feeding currents and trap suspended particles brought in

with the incurrent flow (Riisg�ard & Larsen 2010). Perna

mussels can retain particles as small as 0.46 lm and as

large as 2 mm (Schleyer 1981; Berry & Schleyer 1983;

Wong & Levinton 2006), a range that encompasses a

large variety of organisms from bacteria to microzoo-

plankton, but they tend to preferentially select and ingest

particles between 2 and 100 lm (Riisg�ard & Larsen

2010). Through flexible selection abilities and physiologi-

cal plasticity, Pe. perna can regulate the organic content

of the food it ingests to maximize its energy gain

throughout periods of changing food supply (Suplicy

et al. 2003).

Sabellid reef-building polychaetes such as Gunnarea

gaimardi use ciliated tentacles to create water currents

and trap suspended particles, and most research has



King

Highlight

indicated that these animals feed on particles between 1

and 20 lm (Nichol 1930; Merz 1984; Mayer 1994; Ri-

isg�ard & Larsen 2010). Suspended particles in this size

range generally include items such as phytoplankton,

detritus and bacteria. In contrast with these foundation

feeding studies on suspension-feeding polychaetes, the

growing isotopic literature indicates that a variety of

polychaete species occupy relatively high level trophic

positions relative to other co-occurring suspension-feed-

ers including mussels, oysters, ascidians and sometimes

barnacles (Dubois et al. 2007; Lefebvre et al. 2009; Ric-

houx et al. 2014b). A higher trophic position would indi-

cate that the polychaetes are feeding on primary

consumers such as microzooplankton. It is difficult to

reconcile the existing divergent evidence for polychaete

feeding dynamics from traditional and isotopic methods,

but this interesting contrast deserves additional attention

so that the question about polychaete diets may be better

resolved.

Previous research on the diet of the cape reef worm

Gu. gaimardi revealed that it fed at a higher trophic level

than the brown mussel Pe. perna (trophic positions 2.6

and 2, respectively; Richoux et al. 2014b). In this earlier

study, the trophic position of 2 for Pe. perna was

assigned, and the polychaete trophic position was calcu-

lated in reference to the mussels (Richoux et al. 2014b).

In our temporal study, we calculated the trophic posi-

tion of mussels at 1.9 � 0.2SD relative to the SPM, and

the position of the polychaetes was 2.8 � 0.4 (Table 3),

further confirming a wide distinction between the feed-

ing habits of these species. In comparing the shifts in

polychaete and mussel diets through time, it appears

that during winter the polychaetes had larger contribu-

tions from zooplankton (or some other enriched food

source) in their diets; hence, their trophic positions were

highest during this period (Fig. 2; Table 3). However, in

the summer, the polychaetes approached the isotope

profiles of the mussels, which fed consistently at a pri-

mary consumer level for the entire year. As such, differ-

ential microzooplankton availability through time may

be influential on the polychaete diets, whereas mussels

consistently selected a smaller size fraction of the SPM

that was potentially more isotopically constant through-

out the year. The small intra-population variance of

polychaetes at each sampling time indicated that the

switch away from the 15N-enriched diet after August

2010 was occurring synchronously within the polychaete

population (otherwise the variation would have increased

at times of the year when different individuals were

selecting different components from the seston). Data on

the temporal variability of stable isotope signatures and

availability of microzooplankton as food for coastal con-

sumers in the study area are virtually absent; thus, future

studies involving these aspects of the rocky shore habitat

will only help to clarify the speculations that we have

made above regarding the temporal dynamics of the

polychaete diets.

Temporally fluctuating feeding niche relationships have

also been observed in co-occurring invasive suspension-

feeders, the slippersnail Crepidula fornicata and the oyster

Crassostrea gigas, in Bourgneuf Bay, France (Decottignies

et al. 2007). Species-specific differentiation in particle

selection abilities and behaviour helped to explain the

temporal variability, as the oysters were capable of

actively selecting higher quality particles during phyto-

plankton bloom periods, whereas the slippersnails fed

indiscriminately throughout the year (Decottignies et al.

2007). In that case, inter-specific competition was proba-

bly occurring only during winter and spring when food

quality was decreased in the environment (Decottignies

et al. 2007). Similar limited trophic competition between

co-occurring suspension-feeding consumers has been

observed in other studies (Riera et al. 2002; Dubois et al.

2007; Kang et al. 2009). In our study of mussels and

polychaetes, both the convex hull and SEAs for specimens

collected over the entire year showed that the isotopic

niches of the two populations were completely distinct

(Fig. 5A,B).

Isotopic niches of grazers

Temporal variability of macroalgal isotopic signatures was

greater than that of the SPM (Fig. 3, Table 2), which

may have contributed to the high within-month intra-

population variability observed in the sea urchin isotope

signatures (Table 3). Less temporal and within-month

intra-population variability occurred in the limpets rela-

tive to the sea urchins (Fig. 2, Table 1); therefore, the

limpets probably consumed a less complex and more iso-

topically consistent food source mixture. We expected

that the two grazer species consumed different sources

throughout the year, and that the isotopic niches of the

grazers would differ, potentially owing to alternative food

preferences in the consumers.

The goat’s eye limpet Cymbula oculus is a non-territo-

rial limpet that travels to and from a feeding scar and

can consume foliose macroalgae such as Ulva sp. (but

not Gelidium pristoides), coralline algae, algal spores and

benthic diatoms (Branch 1971; Whittington-Jones 1997;

Maneveldt et al. 2006). Branch (1971) described C. oculus

as having a wide dietary range. However, C. oculus can

occur on rocky shores devoid of macroalgae (sometimes

owing to their own grazing actions), so their diets can

also consist mainly of one source such as microalgal bio-

film (Maneveldt et al. 2009). Most of the metrics that we

calculated to describe the diet in the limpets showed no



temporal variability, little within-month intra-population

variability and a moderately small isotopic niche; these

findings contradict the idea of C. oculus having a wide

dietary range. High macroalgal contributions relative to

microalgae were found in the diets of two limpets typi-

cally identified as microalgal grazers in the Eastern Cape

of South Africa (Siphonaria capensis and Scutellastra gran-

ularis; Hill & McQuaid 2008), so there appears to be

regional differentiations within species based on local

food availability. An in-depth study focussing on isotopic

signatures of specific limpet species and all potential food

sources in the area would help to clarify the trophic

niches occupied by co-occurring limpets.

Depending on their location and the food availability,

sea urchins can have a highly varied diet and may act as

drift trappers or direct grazers of macro- or microalgae

(Anderson & Velimirov 1982; Day & Branch 2002). On

the west coast of South Africa where expansive kelp for-

ests occur, drift trapping behaviour in Parechinus angulo-

sus has been emphasized (Day & Branch 2002), and these

urchins can show preferential selection of specific macro-

algae under experimental situations (Anderson & Velimi-

rov 1982). Less is known about the diet preference of

Pa. angulosus in oligotrophic or mesotrophic regions that

do not support kelp, such as the Eastern Cape of South

Africa (our study region). The large within-month intra-

population variability observed in sea urchin isotopic sig-

natures (especially d13C), which dampened potential tem-

poral effects (Fig. 2; Table 1), supported the idea that the

sea urchins feed on a wide variety of food items in their

environment. Similar high amongst-individual variation

in the sea urchin Phyllacanthus irregularis was observed

on the west coast of Australia, most likely arising from

their consumption of a variety of food sources or the

consumption of changing proportions of the same food

sources (Hyndes et al. 2013). The wide dietary range of

the sea urchins at our study site was further reflected in

the largest SEAs of all of the consumers in our study,

regardless of whether raw or standardized values were

examined (Fig. 5). However, the form of data used (raw

or standardized) affected the level of niche overlap

between the sea urchins and limpets, with no overlap

occurring using the raw data, and complete overlap with

the standardized data. These contrasting results indicate

how large an effect data treatment can have on an analy-

sis of isotopic niches. The apparent isotopic niche overlap

between the grazer populations (Fig. 5D) was largely

affected by the wide range of d13C basal signatures incor-

porated into the correction factors (Ulva sp. signatures

were very low relative to the limpets, and Corallina sp.

signatures were very high; Fig. 4B); thus, the main limpet

diet was probably something with an intermediate isoto-

pic signature, more similar to the limpets. Although it is

technically possible that individual limpets were all feed-

ing on equal parts of widely dissimilar food sources such

as Ulva sp. and Corallina sp., this scenario is highly unli-

kely considering the small within-month variations in the

population. Examination of the literature published on

macroalgal signatures along the southeast coast of South

Africa revealed few candidates for the primary food of

the limpets (i.e. with d13C values approximating �11&;

Fig. 4B), as published values of different species were

either too high (e.g. Corallina sp. at �7&, biofilm at

�4.3&, Hill & McQuaid 2008; Allan et al. 2010) or too

low (various green and brown algae all less than �14&;

Allan et al. 2010). Caution must be used when consider-

ing published isotopic values of food sources potentially

collected at limited temporal scales, although one brown

alga (Sargassum incisifolium, with mean d13C values of

�12 � 0.9& and d15N values of 6.3 � 0.2&; Hill &

McQuaid 2008) could be a potential food candidate for

the limpets to assess in future investigations as these sig-

natures match those of the limpets very closely (C:

�11 � 1&; N: 6.7 � 0.5&) after subtracting fraction-

ation factors. Biofilm is not a likely food option for

C. oculus as the d15N signatures are too low (2.4 �0.4&) and the d13C signatures too high (�4.3 � 0.6&;

signatures were derived from material collected from a

rocky inter-tidal location only a few km from our study

location; Hill & McQuaid 2008). However, detailed

small-scale spatial analysis of rocky shore microphytoben-

thos in the Sea of Japan has shown that large variations

in community structure and isotopic signatures can occur

(Takai et al. 2004); therefore, a detailed spatial (and also

temporal) study on South African microphytobenthic

communities could be very illuminating. Either way, the

potential niche overlap between sea urchins and limpets

is probably much smaller than that indicated in Fig. 5D,

and no niche overlap (Fig. 5C) is more ecologically realis-

tic. Once we increase the spatial and temporal resolution

of isotopic signatures for all potential food sources and

consumers in the rocky inter-tidal, we can convert the

data that we have reported here in d-space to p-space

(i.e. calculate dietary proportions of different foods for

each consumer; Newsome et al. 2007) to further refine

the sources of isotopic variability in the consumers.

Conclusions

Temporally related questions about consumer diets and

food web relationships are becoming more prevalent in

the aquatic literature, and particular attention has been

focussed on aquacultured or invasive animals. Informa-

tion on natural rocky inter-tidal communities remains

relatively scarce, and studies lacking in frequent sampling

over at least an annual cycle certainly have the potential



to miss ecologically important fluctuations, even to the

point that consumer relationships appear to be seasonally

stable. For those researchers who wish to address tempo-

ral hypotheses, we strongly suggest a sampling regime of

greater than quarterly collections per year – preferentially

at least monthly collections. Our own data indicate that

this kind of detailed approach is necessary to describe the

potentially intricate temporal variability in basal resources

and consumers. We strongly agree with the conclusion by

Dethier et al. (2013) that further exploration of natural

variations in biomarkers will provide meaningful ecologi-

cal insights into coastal food webs.

Acknowledgements

We thank J. A. Iitembu and E. S. Antonio for assistance

in the field, and G. Schaal for assistance with the map fig-

ure. This research was funded by the National Research

Foundation of South Africa (NRF) and Rhodes University

(RU), with bursary funding to R.T.N. provided by the

NRF and RU. Stable isotope analysis was carried out by

S. Kaehler at IsoEnvironmental cc, RU.

References

Aguilera M.A., Navarrete S.A., Broitman B.R. (2013)

Differential effects of grazer species on periphyton of a

temperate rocky shore. Marine Ecology Progress Series, 484,

63–78.

Airoldi L., Fabiano M., Cinelli F. (1996) Sediment deposition

and movement over a turf assemblage in a shallow rocky

coastal area of the Ligurian Sea. Marine Ecology Progress

Series, 133, 241–251.

Allan E.L., Ambrose S.T., Richoux N.B., Froneman P.W.

(2010) Determining spatial changes in the diet of nearshore

suspension-feeders along the South African coastline: stable

isotope and fatty acid signatures. Estuarine, Coastal and

Shelf Science, 87, 463–471.

Anderson R.J., Velimirov B. (1982) An experimental

investigation of the palatability of kelp bed algae to the sea

urchin Parechinus angulosus LESKE. Marine Ecology, 3,

357–373.

Bearhop S., Adams C.E., Waldron S., Fuller R.A., Macleod H.

(2004) Determining trophic niche width: a novel approach

using stable isotope analysis. Journal of Animal Ecology, 73,

1007–1012.

Berry P.F., Schleyer M.H. (1983) The brown mussel Perna

perna on the Natal coast, South Africa: utilization of

available food and energy budget. Marine Ecology Progress

Series, 13, 201–210.

Bode A., Alvarez-Ossorio M.T., Varela M. (2006)

Phytoplankton and macrophyte contributions to littoral

food webs in the galician upwelling estimated from stable

isotopes. Marine Ecology Progress Series, 318, 89–102.

Branch G.M. (1971) The ecology of Patella Linnaeus from the

Cape Peninsula, South Africa: zonation, movements and

feeding. Zoologica Africana, 6, 1–38.

Bustamante R.H., Branch G.M., Eekhout S. (1995) Maintenance

of an exceptional intertidal grazer biomass in South Africa:

subsidy by subtidal kelps. Ecology, 76, 2314–2329.

Cabana G., Rasmussen J.B. (1996) Comparison of aquatic food

chains using nitrogen isotopes. Proceedings of the National

Academy of Sciences, 93, 10844–10847.

Camus P.A., Daroch K., Opazo L.F. (2008) Potential for

omnivory and apparent intraguild predation in rocky

intertidal herbivore assemblages from northern Chile.

Marine Ecology Progress Series, 361, 35–45.

Darimont C.T., Paquet P.C., Reimchen T.E. (2009) Landscape

heterogeneity and marine subsidy generate extensive

intrapopulation niche diversity in a large terrestrial

vertebrate. Journal of Animal Ecology, 78, 126–133.

Day E., Branch G.M. (2002) Effects of sea urchins (Parechinus

angulosus) in recruits and juveniles of abalone (Haliotis

midae). Ecological Monographs, 72, 133–149.

Decottignies P., Beninger P.G., Rinc�e Y., Riera P. (2007)

Trophic interactions between two introduced

suspension-feeders, Crepidula fornicata and Crassostrea gigas,

are influenced by seasonal effects and qualitative selection

capacity. Journal of Experimental Marine Biology and

Ecology, 342, 231–241.

Dethier M.N., Sosik E., Galloway A.W.E., Duggins D.O.,

Simenstad C.A. (2013) Addressing assumptions: variation in

stable isotopes and fatty acids of marine macrophytes can

confound conclusions of food web studies. Marine Ecology

Progress Series, 478, 1–14.

Dubois S., Orvain F., Marin-L�eal J., Ropert M., Lefebvre S.

(2007) Small-scale spatial variability of food partitioning

between cultivated oysters and associated suspension-feeding

species, as revealed by stable isotopes. Marine Ecology


Flaherty E.A., Ben-David M. (2010) Overlap and partitioning

of the ecological and isotopic niches. Oikos, 119, 1409–1416.

Fukumori K., Oi M., Doi H., Takahashi D., Okuda N., Miller

T., Kuwae M., Miyasaka H., Genkai-Kato M., Koizumi Y.,

Omori K., Takeoka H. (2008) Bivalve tissue as a carbon and

nitrogen isotope baseline indicator in coastal ecosystems.

Estuarine, Coastal and Shelf Science, 79, 45–50.

Goering J., Alexander V., Haubenstock N. (1990) Seasonal

variability of stable carbon and nitrogen isotope ratios of

organisms in a North Pacific bay. Estuarine Coastal and


Grange N., Whitfield A.K., De Villiers C.J., Allanson B.R.

(2000) The response of two South African east estuaries to

altered river flow regimes. Aquatic Conservation: Marine and

Freshwater Ecosystems, 10, 155–177.

Hill J.M., McQuaid C.D. (2008) d13C and d15N biogeographic

trends in rocky intertidal communities along the coast of

South Africa: evidence of strong environmental signatures.

Estuarine, Coastal and Shelf Science, 80, 261–268.



Hill J.M., McQuaid C.D., Kaehler S. (2006) Biogeographic and

nearshore-offshore trends in isotope ratios of intertidal

mussels and their food sources around the coast of southern

Africa. Marine Ecology Progress Series, 318, 63–73.

Hill J.M., McQuaid C.D., Kaehler S. (2008) Temporal and

spatial variability in stable isotope ratios of SPM link to

local hydrography and longer term SPM averages suggest

heavy dependence of mussels on nearshore production.

Marine Biology, 154, 899–909.

Hoeinghaus D., Zeug S. (2008) Can stable isotope ratios

provide for community-wide measures of trophic structure?

comment. Ecology, 89, 2353–2357.

Hyndes G.A., Hanson C.E., Vanderklift M.A. (2013) The

magnitude of spatial and temporal variation in d15N and

d13C differs between taxonomic groups: implications for

food web studies. Estuarine, Coastal and Shelf Science, 119,

176–187.

Jackson A.L., Inger R., Parnell A.C., Bearhop S. (2011)

Comparing isotopic niche widths among and within

communities: SIBER – Stable Isotope Bayesian Ellipses in R.

Journal of Animal Ecology, 80, 595–602.

Kang C., Choy E.J., Hur Y., Myeong J. (2009) Isotopic

evidence of particle size-dependent food partitioning in

cocultured sea squirt Halocynthia roretzi and Pacific oyster

Crassostrea gigas. Aquatic Biology, 6, 289–302.

Kinney E.H., Roman C.T. (1998) Response of primary

producers to nutrient enrichment in a shallow estuary.


Layman C.A., Ara�ujo M.S., Boucek R., Hammerschlag-Peyer

C.M., Harrison E., Jud Z.R., Matich P., Rosenblatt A.E.,

Vaudo J.J., Yeager L.A., Post D.M., Bearhop S. (2012)

Applying stable isotopes to examine food-web structure: an

overview of analytical tools. Biological Reviews, 87, 545–562.

Lefebvre S., Mar�ın Leal J.C., Dubois S., Orvain F., Blin J.-L.,

Bataill�e M.-P., Ourry A., Galois R. (2009) Seasonal

dynamics of trophic relationships among co-occurring

suspension-feeders in two shellfish culture dominated

ecosystems. Estuarine, Coastal and Shelf Science, 82, 415–

425.

Lorrain A., Paulet Y.-M., Chauvaud L., Savoye N., Donval A.,

Saout C. (2002) Differential d13C and d15N signatures

among scallop tissues: implications for ecology and

physiology. Journal of Experimental Marine Biology and

Ecology, 275, 47–61.

Lutjeharms J.R.E. (2005) The coastal oceans of south-eastern

Africa. In: Robinson A.R., Brink K. (Eds), The Sea. Chicago

University Press, Chicago: 781–832.

Maneveldt G.W., Wilby D., Potgieter M., Hendricks M.G.J.

(2006) The role of encrusting coralline algae in the diets of

selected intertidal herbivores. Journal of Applied Phycology,

18, 619–627.

Maneveldt G.W., Eager R.C., Bassier A. (2009) Effects of

long-term exclusion of the limpet Cymbula oculus (Born) on

the distribution of intertidal organisms on a rocky shore.

African Journal of Marine Science, 31, 171–179.

Matthews B., Mazumder A. (2004) A critical evaluation of

intrapopulation variation of d13C and isotopic evidence of

individual specialization. Oecologia, 140, 361–371.

Mayer S. (1994) Particle capture in the crown of the ciliary

suspension feeding polychaete Sabella penicillus: videotape

recordings and interpretations. Marine Biology, 119, 571–

582.

McCutchan J. Jr, Lewis W. Jr, Kendall C., McGrath C. (2003)

Variation in trophic shift for stable isotope ratios of carbon,

nitrogen, and sulfur. Oikos, 102, 378–390.

Menge B.A., Daley B.A., Lubchenco J., Sanford E., Dahlhoff E.,

Halpin P.M., Hudson G., Burnaford J.L. (1999) Top-down

and bottom-up regulation of New Zealand rocky intertidal

communities. Ecological Monographs, 69, 297–330.

Merz R.A. (1984) Self-generated versus environmentally

produced feeding currents: a comparison for the sabellid

polychaete Eudistylia vancouveri. The Biological Bulletin, 167,

200–209.

Newsome S.D., Martinez del Rio C., Bearhop S., Phillips D.L.

(2007) A niche for isotopic ecology. Frontiers in Ecology and

the Environment, 5, 429–436.

Nichol E.A.T. (1930) The feeding mechanism, formation of

the tube, and physiology of digestion in Sabella pavonina.

Transactions of the Royal Society of Edinburgh, 56, 537–598.

Nordstr€om M., Aarnio K., Bonsdorff E. (2009) Temporal

variability of a benthic food web: patterns and processes in

a low-diversity system. Marine Ecology Progress Series, 378,

13–26.

Olsson K., Stenroth P., Nystr€om P., Gran�eli W. (2009)

Invasions and niche width: does niche width of an

introduced crayfish differ from a native crayfish? Freshwater

Biology, 54, 1731–1740.

O’Reilly C.M., Hecky R.E., Cohen A.S., Plisnier P.-D. (2002)

Interpreting stable isotopes in food webs: Recognizing the

role of time averaging at different trophic levels. Limnology

and Oceanography, 47, 306–309.

Parnell A.C., Inger R., Bearhop S., Jackson A.L. (2010) Source

partitioning using stable isotopes: coping with too much

variation. PLoS ONE, 5, e9672.

Pedersen M., Borum J. (1996) Nutrient control of algal growth

in estuarine waters. Nutrient limitation and the importance

of nitrogen requirements and nitrogen storage among

phytoplankton and species of macroalgae. Marine Ecology


Qian Y., Jochens A.E., Kennicutt M.C. II, Biggs D.C. (2003)

Spatial and temporal variability of phytoplankton biomass

and community structure over the continental margin of

the northeast Gulf of Mexico based on pigment analysis.

Continental Shelf Research, 23, 1–17.

Richoux N.B., Vermeulen I., Froneman P.W. (2014a) Fatty acid

profiles reveal temporal and spatial differentiation in diets

within and among syntopic rocky shore suspension-feeders.


Richoux N.B., Vermeulen I., Froneman P.W. (2014b) Stable

isotope ratios indicate differential omnivory among



syntopic rocky shore suspension-feeders. Marine Biology,

161, 971–984.

Riera P., Richard P. (1996) Isotopic determination of food

sources of Crassostrea gigas along a trophic gradient in the

estuarine bay of Marennes Ol�eron. Estuarine Coastal and


Riera P., Stal L.J., Nieuwenhuize J. (2002) d13C versus d15Nof co-occurring molluscs within a community dominated

by Crassostrea gigas and Crepidula fornicata (Oosterschelde,

The Netherlands). Marine Ecology Progress Series, 240, 291–

295.

Riisg�ard H.U., Larsen P.S. (2010) Particle capture mechanisms

in suspension-feeding invertebrates. Marine Ecology Progress

Series, 418, 255–293.

Rolff C. (2000) Seasonal variation in d13C and d15N of

size-fractionated plankton at a coastal station in the

northern Baltic proper. Marine Ecology Progress Series, 203,

47–65.

Sato T., Miyajima T., Ogawa H., Umezawa Y., Koike I. (2006)

Temporal variability of stable carbon and nitrogen isotopic

composition of size-fractionated particulte organic matter in

the hypertrophic Suymida River Estuary of Tokyo Bay,

Japan. Estuarine, Coastal and Shelf Science, 68, 245–258.

Schaal G., Riera P., Leroux C., Grall J. (2010) A seasonal stable

isotope survey of the food web associated to a peri-urban

rocky shore. Marine Biology, 157, 283–294.

Schleyer M.H. (1981) Microorganisms and detritus in the

water column of a subtidal reef of Natal. Marine Ecology


Semmens B.X., Ward E.J., Moore J.W., Darimont C.T. (2009)

Quantifying inter- and intra-population niche variability

using hierarchical Bayesian stable isotope mixing models.

PLoS ONE, 4, e6187.

Steinarsd�ottir M.B., Ing�olfsson A., �Olafsson E. (2009) Trophic

relationships on a fucoid shore in south-western Iceland as

revealed by stable isotope analyses, laboratory experiments,

field observations and gut analyses. Journal of Sea Research,

61, 206–215.

Suplicy F.M., Schmitt J.F., Moltschaniwskyj N.A., Ferreira J.F.

(2003) Modelling of filter-feeding behaviour in brown

mussel Perna perna (L.): exposed to natural variation of

seston availability in Santa Catarina. Brazil. Journal of

Shellfish Reaserch, 22, 125–134.

Takai N., Yorozu A., Tanimoto T., Hoshika A., Yoshihara K.

(2004) Transport pathways of microphytobenthos-

originating organic carbon in the food web of an exposed

hard bottom shore in the Seto Inland Sea, Japan. Marine

Ecology Progress Series, 284, 97–108.

Thompson R.C., Norton T.A., Hawkins S.J. (2004) Physical

stress and biological control regulate the producer-consumer

balance in intertidal biofilms. Ecology, 85, 1372–1382.

Vander Zanden M., Rasmussen J. (1999) Primary consumer

d13C and d15N and the trophic position of aquatic

consumers. Ecology, 80, 1395–1404.

Vanderklift M.A., Wernberg T. (2010) Stable isotpes reveal a

consistent consumer-diet relationship across hundreds of

kilometres. Marine Ecology Progress Series, 403, 53–61.

Vorwerk P.D., Froneman P.W. (2009) The importance of

estuarine-derived carbon for the nearshore marine

environment: studies on two contrasting South African

estuaries. African Journal of Aquatic Science, 34, 137–146.

Ward J.E., Shumway S.E. (2004) Separating the grain from the

chaff: particle selection in suspension- and deposit-feeding

bivalves. Journal of Experimental Marine Biology and Ecology,

300, 83–130.

Whittington-Jones K.J. (1997) Ecological interactions on a

rocky shore: the control of macroalgal distribution by

intertidal grazers. MSc thesis, Rhodes University,

Grahamstown.

Wong W.H., Levinton J.S. (2006) The trophic linkage between

zooplankton and benthic suspension feeders: direct evidence

from analyses of bivalve faecal pellets. Marine Biology, 148,

799–805.

Woodland R.J., Magnan P., Gl�emet H., Rodr�ıguez M.A.,

Cabana G. (2012) Variability and directionality of temporal

changes in d13C and d15N of aquatic invertebrate primary

consumers. Oecologia, 169, 199–209.

Yokoyama H., Ishihi Y. (2007) Variation in food sources of

the macrobenthos along a land-sea transect: a stable isotope

study. Marine Ecology Progress Series, 346, 127–141.



Documents

Temporal shifts in the fatty acid profiles of rocky intertidal invertebrates