Title: Sink or float: Fate of mangrove leaves in a Brazilian river mangrove.

Supervisors: Prof. Martin Zimmer and Dr. Lucy Gwen Gillis

Location: Ajuruteua peninsula, North Brazil

Time: March 2015 (Rainy) and October 2015 (Dry).

Keywords: POM, transportation, mangrove forest, outwelling, nutrients, leaves

Questions

I. How does the rainy season affect particulate nutrient transport of leaves from mangrove

forests?

II. Does high density mangrove roots retain more leaves than low density mangrove roots?

III. Are fully degraded leaves more likely to be trapped than semi-degraded or fresh leaves?

IV. How does location of the leaf affect the movement of that leaf out of a mangrove ecosystem?

Introduction

The outwelling hypotheses states that mangrove derived nutrients can support organisms and adjacent

ecosystems (Odum 1968, Lee 1995). This form of support can occur via particular or dissolved

nutrients being transported out of the forest and becoming available for other ecosystems or

organisms. In fact mangrove derived carbon has been found to be important for the coastal ocean.

Alongi (2014) estimated that mangroves contribute up to 15 % of carbon to the coastal sediment and

10-11 % of POM carbon to the coastal ocean (Alongi 2014).

Outwelling particulate organic material such as leaves can be transformed into dissolved

nutrients via degradation by micro and macro organisms (Boto and Wellington 1988, Ewel et al.

1998, Adame and Lovelock 2011). One of the most and easily transported forms of POM is

mangroves leaves (Lee 1995, Bouillon and Connolly 2009, Davis et al. 2009). Once senescent leaves

fall from the trees they will be moved by local hydrodynamics and organisms (i.e. crabs). However,

factors such as root density, location of felled leaves and the degradation state of the leaves will alter

the usability of the leaves being transported (Van Stocken et al. 2013, Gillis et al 2014a, Gillis et al.

unpublished).

Recent research has shown that mangrove forest can play a dual role in being a source

(Alongi 2014) or a sink (Gillis et al. 2014b) of nutrients. An important dynamic is that the landscape

physical aspects of the mangrove forest will be important in determining in their role as a sink or

source (Adame and Lovelock 2011, Gilli etl a. 2014b). However, local conditions will also play a part

in establishing how mangrove derived POM is outwelling. Von Stocken et al. (2013) and Gillis et al.

(2014) showed that root density, state of leaves, weather and hydrodynamic conditions will have

important considerations in the trapping capacity of leaves and therefore the potential of the forest to

outwell. These studies have only investigated these conditions in a laboratory flume and not in-situ.

In this experiment we would like to establish how factors identified by previous studies will

impact on transportation of leaves in-situ. We will test how the degradation state of the leaf vs. the

initial location of the leaf vs. density of roots will affect how quickly a leaf is moved out of riverine

mangrove system. In addition we will repeat this experiment once in the dry season and once in the

wet season to determine how seasonal changes will affect transportation rates.

Methodology

The study will be located on the Ajuruteua peninsula, the western bank of the Caete´ river estuary in

North Brazil (Figure 1). Field work will be performed in the intertidal zone near the large tidal

channel, Furo Grande (46º38’W 0º50’S) (Figure 1). The region has semidiurnal tides with amplitudes

of 2 to 5 m. During spring tide days the study site is flooded twice a day, while during neap tide days

tidal heights are too low to inundate the whole mangrove forest (Püllmans et al. 2014). The forest is

dominated by the mangrove tree Rhizophora mangle but Avicennia germinans and Laguncularia

racemosa are also present (Püllmans et al. 2014).

Figure 1 Showing location and vegetation spatial distribution of the study site (Püllmans et al. 2014)

For the experiment two channels will be chosen, the first channel will be in the inner

mangrove where it is hoped that mangrove root density is high. The second channel will be in the

fringing forest which should have lower density mangrove roots. Using the two channels will allow us

to answer the question that leaves will be less likely to be trapped in lower density mangrove roots. In

these channels two nets will be set-up (Figure 2) at either ends of the channel, these nets will stretch

from the high water mark to the bottom of the channel. The mesh size of the nets will be smaller

enough to trap whole mangrove leaves.

Figure 2 A. Showing location of nets at either end of channel. B. Conceptual diagram of the location

of the transects indicating positioning of points for location of leaves.

To test the degradation state of leaves and how this will alter leaf transportation, we will use

three different weighted mimic leaves. The first will represent fresh leaves and will be weighted so

that the mimic leaf can float on the surface of the water, the second mimic leaf will represent semi-

degraded leaves and should be able to float above the sediment bottom but below the water surface.

The third mimic leaf will represent the degraded leaves and this should sink to the sediment bottom.

In this experiment we would like to establish how far away from the channel do leaves

become trapped either in low or high density mangroves. Using a tape measure we will establish a

semi-permanent transect going from the low water mark in the channel to 30-50 m inside the forest

(Figure 2 B). Along this transect we will place each of the types of leaves (degraded, semi-degraded,

fresh) every 5-10 m (Figure 3 B).

Once each leaf type has been placed along the transect at low tide, we will allow one tidal

cycle to complete. Then the transect will be checked for leaf movement. If any of the leaves have

become entangled in the net at either ends of the channels then we will consider these leaves have

been transported from the system. However if leaves are found within the forest or the channel banks,

we will take a note of their location and check them again after the next full tidal cycle and thereafter.

The leaves will be monitored until firstly they have become trapped in the nets or when we have

deemed them to be trapped indefinitely (after approx. 6-7 tidal cycles). This protocol will be repeated

for both low and high density mangrove roots and again for the rainy and dry season. From this

experiment we hope to be answer the key questions.

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