Ecological Thresholds for Salt Marsh Nekton and Vegetation Communities

Mary-Jane James-PirriGraduate School of OceanographyUniversity of Rhode Island

Jeffrey SwansonComputer Science and StatisticsUniversity of Rhode Island

Charles RomanNational Park ServiceNorth Atlantic Coast Cooperative Ecosystem Studies Unit

Howard GinsbergUSGS Patuxent Wildlife Research Center

James HeltsheComputer Science and StatisticsUniversity of Rhode Island

Nauset Marsh, Cape Cod NS

Project Background & Objectives• Funded by the USGS Status & Trends Program.• Use existing data to define ecological thresholds for Vital Signs associated with

the National Park Service’s Inventory and Monitoring Program.• Study Focus: Vital Signs of salt marsh nekton & vegetation community

composition from NPS Monitoring Protocols.• Identify patterns in community structure and correlate patterns with landscape

gradients (e.g., degree of urbanization), if possible.

The Marshes

Sites had a range of hydrologic disturbance:• No disturbance (reference sites)• Tidal restriction• Tidal restoration• Open marsh water management (OMWM) for mosquito control

~ 50% of sites were sampled in multiple years (mostly 2001 to

2006)

~ 50% of marshes were reference sites

Edwin B. Forsythe NWR

Prime Hook NWR

Assateague Island NS

Boston Harbor Islands NPACape Cod NS

Sagamore Hill NHS

Fire Island NSGateway NRA

Colonial NHS

George Washington Birthplace NM

Rachel Carson NWR

Parker River NWR

Stewart B. McKinney NWR

Long Island Complex NWR

Narragansett Bay, RI14 sites Dartmouth, MA sites

Sachuest Point NWR

National Parks

US Fish & Wildlife Refuges

Other sites

83 individual marshes in 48 watersheds

The DatasetsVegetation Community Data (70 sites, 156 individual datasets)

• Point intercept data from 1-m2 plots• 203 cover types – live vegetation & non-vegetative cover (water, wrack)

Nekton Community Data (81 sites, 185 individual datasets)• Relative abundance data from enclosure gear (throw traps & ditch nets)• 62 species - fish and crustaceans

Landscape metrics (48 sub-watersheds)• 2000 U.S. Census data (human population density of sub-watershed)• 2001 National Land Cover Data (land use within 5 km radius) (not presented)

Multivariate techniques were used to explore patterns in community composition

Cape Cod NS, MA Gateway NRA, NY Fire Island NS, NY

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Vegetation Community – Common cover types

• Reduced dataset to 21 common cover types (present in >20% of datasets)• 76 reference and 80 disturbed marsh datasets:

• Principal Component Analyses (PCA), 1st three principal components only accounted for ~40% (1st PC: 20%);

• No clear pattern in PCA scores using common cover types;• No clear gradient of land use in PCA scores;• No correlation with watershed human population density.

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Reference sitesDisturbed sites

1st Principal Component

Gradient of Marsh Disturbance

Reference sites: reflect desired or target condition.

Disturbed sites: reflect degraded or less desired condition.

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HydrologicallyAltered (OMWM)~ 11 species

Reference sites~13 species

Restoring sites~35 species

Restricted sites~26 species

Vegetation Groups- Wetland status & salinity tolerance

• Grouped vegetation by wetland status & salinity tolerance using USDA definitions (e.g., obligate wetland species, high salinity tolerance);

• Invasive and state-listed status (e.g., endangered, threatened)• Disturbed sites 1st three PC’s accounted for 68% variation;• No correlation with watershed population density;• Pattern in vegetation groups with degree marsh disturbance;

• Shift from obligate wetland plants with high salinity tolerance to less salinity tolerant species at disturbed sites;

• Invasive species more prevalent at disturbed sites.

High

Low

None

Medium

Salinity tolerance of obligate

wetland plants

Invasive plants

Increasing marsh disturbance

Vegetation – Possible metrics to detect change

• Vegetation groups:• Wetland status (e.g., obligate, facultative);• Salinity tolerance;• Invasive species;

• Pattern with degree of marsh disturbance.

John H. Chafee National Wildlife Refuge, RI

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Nekton Community – all species vs common species

• All species • Only 16% of the variability explained by 1st three principal components;• PC’s loaded primarily on rare species;

• 17 Common species, species present at >10% of marshes• Accounted for 34% of total variability;• No separation of sites;

• Conclusion: Individual species would not be a good indicator to detect change in nekton communities.

1st Principal Component

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Reference sitesDisturbed sites

Common Nekton Species

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Nekton Groups – based on life history characteristics

Reference sitesDisturbed sites

1st Principal Component

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Grass Shrimp Dominated (Palaemonetes spp.) Resident Fish

Dominated

Transient Fishpresence

• Nekton Groups: Fish and crustaceans grouped by resident or transient species;

• 85% of variation explained by 1st three PCs;• PC1 showed negative correlation between resident fish & shrimp; • PC3 loaded on transient fish;

• Can use PC1 & PC3 to distinguish between sites with high proportions of resident fish, resident shrimp, & transient fish.

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Watershed Population Density

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• First PC correlated with watershed population density for reference sites• First PC correlated with watershed population density;

• Sites in watersheds with higher population densities tended to be dominated by grass shrimp & sites in less dense watershed tended to be fish dominated.

1st Principal Component

Shrimp dominated

communities

Fish dominated communities

Pearson correlation coefficient: -0.64, p<0.0001, adj. r2: 0.40

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Nekton Groups & Population Density

1st Principal Component

3rd P

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Population of Surrounding watershed

Resident Shrimp(Palaemonidae spp.)

Resident fish(killifish spp.)

Transient fish(e.g., flounder,

eel)

1,000 to 4,9995,000 to 9,99910,000 to 19,99920,000 & above

Small watersheds surrounded by dense populations inNarragansett Bay, RI

• Sites in watersheds with high population densities were dominated by Palaemonidae grass shrimp;• As population density decreases, resident fish increase in abundance;• Transient fish are associated with lower population density watersheds sites.

Nekton– Possible metrics to detect change

• Nekton groups life history characteristics:• Resident fish;• Resident shrimp (Palaemonidae species);• Transient fish;

• Pattern with watershed population density.

•Although not strongly correlated in our analyses, these parameters are important:• Exotic species presence (exotic species can threaten native species);• Total species richness (generally, low richness is a sign of a degraded

community).

Striped killifish (resident fish) Young-of-the-year winter flounder (transient fish)

Grass shrimp Palaemonetes spp. (resident shrimp)

Estimate of Condition – Salt Marsh Vegetation

Vegetation Metric Condition Estimator (% cover)Good Moderate Poor

High salinity tolerant, wetland obligate >55% 40-55% <40%Low salinity tolerant, wetland obligate <2% 2-20% >20%Invasive plants <1% 1-20% >20%

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Reference & OMWM marshes Restricted & Restoring marshes

Data graphed are:• 50th percentile (point) • 75th & 25% percentiles (box)• 90th & 10th percentiles (bars)

Data graphed are:• 50th percentile (point) • 75th & 25% percentiles (box)• 90th & 10th percentiles (bars)

Resident fish

Resident shrimp

Transient fish

Resident fish

Resident shrimp

Transient fish

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Low population watersheds High population watersheds

Resident fish

Resident shrimp

Transientfish

Resident fish

Resident shrimp

Transientfish

Estimate of Condition – Salt Marsh Nekton

Nekton Metric Condition EstimatorGood Moderate Poor

Resident fish (percent of catch) >60% 30-60% <30%

Resident shrimp (percent of catch) <15% 15-50% >50%

Transient fish (percent of catch) >15% 4-15% <4%

Exotic/Invasive species absent - present

Species richness ≥10 6-9 ≤5Breakpoints based on quartile values for reference marshes in watersheds with differing population densities.

Application to Resource ManagementAssign scores to each condition estimator, with high scores for better or more desired condition

• Good = 5, Moderate = 3, Poor = 1; • Presence / absence: absent=5; present = 1;• Sum the scores to obtain an overall value.• Set a range based on number of metrics,example for 3 metrics:

• Good: 12-15• Moderate: 7-11• Poor: 3-6

Assateague NS

Metric Condition Estimate

Score

Metric A Poor 1

Metric B Good 5

Metric C Moderate 3

Total score Moderate 9

Application to Resource Management

Vegetation data from Sachuest restricted & reference marshesVegetation metric Sachuest restricted

(before restoration, 1996)Sachuest restricted

(after restoration, 2002)Sachuest reference

(2002)

High salinity plants 38% (poor, 1) 77% (good, 5) 70% (good, 5)

Low salinity plants 25% (poor, 1) 25% (poor, 1) 1% (good, 5)

Invasive plants Present (poor, 1) Present (poor, 1) Absent (good, 5)

Total score POOR (3) MODERATE (7) GOOD (15)

Condition criteria applied to vegetation data from Sachuest Point NWR, RIA tidally restricted, degraded marsh that was tidally restored in 1997

Condition criteria applied to nekton data from Jamaica Bay, GATE

Nekton data from Big Egg & Elders marshes, GATE

Nekton metric Big Egg 2005 Big Egg 2009 Elders Lower 2009

Resident fish 46% (moderate, 3) 81% (good, 5) 39% (moderate, 3)

Resident shrimp 48% (moderate, 3) 7% (good, 5) 5% (good, 5)

Transient fish 1% (poor, 1) 8% (moderate, 3) 51% (good, 5)

Exotic species Present (1) Present (1) Absent (5)

Species richness 8 (moderate, 3) 10 (good, 5) 5 (moderate, 3)

Total score POOR (11) MODERATE (19) GOOD (21)

Conclusions

Cape Cod NS, MA

• Estimates were derived from a large regional multi-year database with a gradient of marsh disturbance from reference to severely degraded condition.

• Able to estimate condition criteria for salt marsh vegetation and nekton using key indicator metrics.

• Metrics were able to detect change in both vegetation and nekton communities when applied to new data.

Nekton Guilds

Salt Marsh Residents

Fish

Shrimp

Crabs

Salt Marsh Transients

Fish Crustaceans KillifishMinnows

Grass shrimp

Green crabsFiddler crabs

EelFlounder

Silversides

Blue crabsBay shrimp

Guilds based on life history characteristics associated with salt marshesResidents: salt marshes are critical to their life historyTransients: found in marshes, but not critical to their life history

Vegetation Groups - Surrounding Land use (5 km radius

• Canonical Correspondence Analyses (CCA);• Relationship among vegetation guilds and surrounding land use;• Vegetation groups from disturbed marshes were correlated with land use;

• Disturbed sites: 1st three CCA axes accounted for 67% of the variation (reference sites: 47% of variation);

• Higher proportions of invasive plants and low salinity tolerant species associated with developed lands.

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Developed land (high, medium, low intensity) & open developed land

Wetlands & deciduous forest

Mixed forest & shrubWater & barren land

High salinity

Low salinity

Medium salinity

Salinity tolerance of wetland

obligate plants

Invasive plants

Disturbed sites

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Axis 1

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2Nekton Groups – Surrounding Land use (5 km radius)

• Canonical Correspondence Analyses (CCA).• Nekton guilds from reference marshes were correlated with land use;o 1st two CCA axes accounted for 43% of the variation;o Higher proportions of resident shrimp (grass shrimp) were associated with

developed lands;o Higher proportions resident & transient fish were associated undeveloped lands.

Developed land (high, medium, low intensity & open)

Wetlands & mixed forest

Water & barren land, herbaceous vegetation

Crop land, shrub, deciduous forest

Resident fish

Resident shrimp

Transient fish

Nekton guilds

Resident crabs

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Can we estimate thresholds of change?Maybe for nekton….

Transient fish Resident fish Resident shrimp (Palaemonetes spp.)

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Watershed Population

If low population sites represent a more “natural” or “desired target” condition:>60% resident fish~15% transient fish~15% resident shrimp

If high population sites represent a more “stressed” condition :10% resident fish4% transient fish85% resident shrimp

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Low pop Impacted sites, n=75

What is happening at impacted sites?At lower population watersheds (<15,000 people) shrimp are replacing the fish at the impacted marshes

Resident fish reference line: 68%

Shrimp & transient fish reference line: ~15%

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Watershed Population size

Impacted sites, n=75