Gregory S. Peterson

Water Quality in Great Lakes Coastal Wetlands: Basin-wide Patterns and Responses to an Anthropogenic Disturbance Gradient

ABSTRACT We present water quality data from 58 coastal wetlands, sampled as part of a larger effort investigating effects of nutrient enrichment and habitat disruption in the Laurentian Great Lakes. Our sampling design selected sites from across a gradient of agricultural intensity within combinations of biogeographic ecoprovince and wetland hydromorphic type and captured a large range in water quality. Levels of total nutrients (N and P), and various measures of particulate concentration, water clarity, and ionic strength were strongly associated with agricultural intensity in the watershed, and could be effectively aggregated into an overall principal component-based water quality descriptor. Lake Erie wetlands had the highest nutrient levels and lowest water clarity, while wetlands in Lakes Superior and Huron had the lowest nutrient levels and clearest water. Lake Ontario wetlands had clearer water than would be expected from their nutrient levels and position on the agricultural intensity gradient. Dissolved oxygen, silica, pH, and dissolved organic carbon (DOC) were independent of agricultural intensity but DOC was responsible for low water clarity in some Lake Superior wetlands. Simple classification by hydromorphic type (riverine or protected) did not explain water quality differences among wetlands exposed to similar agricultural intensity levels, so finer hydrologic classification may be desirable. Results are used as a basis for discussing research and information needs underlying development of water quality criteria and monitoring programs for coastal wetlands of the Great Lakes.

Stable Nitrogen Isotopes of Plankton and Benthos Reflect a Landscape-level Influence on Great Lakes Coastal Ecosystems

ABSTRACT As populations and human activities increase in coastal watersheds, an understanding of the connections of aquatic ecosystems to the adjacent terrestrial landscape is necessary to identify, monitor, and protect vulnerable coastal habitats. This study investigates the relationships between land-use patterns and δ15N values of aquatic organisms in coastal ecosystems, across a defined watershed gradient for the U.S. portion of the Great Lakes shoreline. δ15N measured in plankton and benthic invertebrates reflects a range of basin wide land-use gradients and demonstrates a strong connection between watershed-based anthropogenic activities and exposure in aquatic biota. For example, benthos δ15N values range over 12‰ across sites in our study, but regression analyses suggest that over 50% of the variability is explained by the regional landscape. Further, multiple taxa at comparable trophic position showed similar patterns in relation to watershed-scale land use. Our results suggest that within the coastal environment, the expression of landscape in aquatic biota is stronger in habitats such as embayments and wetlands than open nearshore. These results support the use of δ15N in Great Lakes coastal biota as an exposure indicator of watershed-scale N loading.

Hydrology and nutrient effects on food-web structure in ten lake superior coastal wetlands

We examined the effects of hydrology and nutrients on the food webs of ten coastal wetlands on Lake Superior, using published stable isotope food web data for three wetlands and original data from seven additional systems in order to span regional hydrologic and nutrient enrichment gradients. We used a dual-source isotope mixing model to estimate the proportion of carbon in fish that originated from planktonic versus periphytic invertebrates, and we related carbon source to 1) nutrient enrichment, 2) hydraulic residence time, and 3) an index of nutrient loading that incorporates residence time and nutrient concentrations. There was no relationship between nutrient enrichment and the proportion of planktonic versus periphytic C in fish. Proportion of planktonic C in fish increased significantly with hydraulic residence time (F = 5.68, R2 = 0.42, p = 0.044). Riverine wetlands generally had lowest proportions of planktonic C in fish, dendritic wetlands were intermediate, and lagoon wetlands had highest proportions. A regression between the loading index and planktonic C in fish was an improvement over the residence time regression (F = 11.7, R2 = 0.59, p = 0.009). We conclude that coastal wetland food webs are strongly affected by hydrology and further by nutrient enrichment. This work has implications for the development of food web-based ecological indicators of nutrient enrichment and the use of hydrology as a classification factor in the prediction of nutrient effects on food webs.

Status of Non-Indigenous Benthic Invertebrates in the Duluth-Superior Harbor and the Role of Sampling Methods in Their Detection

ABSTRACT As part of a study to develop recommendations for non-indigenous species (NIS) monitoring in Great Lakes areas at risk of invasion, we conducted intensive sampling in the Duluth-Superior Harbor and lower St. Louis River in 2005 and 2006. Of the ∼240 benthic invertebrate taxa identified, 19 were non-indigenous, including 8 first detection records for this system: New Zealand mud snail Potamopyrgus antipodarum; African/Asianorigin cladoceran Daphnia lumholtzi; Eurasian-origin amphipod Echinogammarus ischnus; Eurasian-origin bivalves Dreissena bugensis, Pisidium henslowanum and Pisidium supinum; and possibly range expanding oligochaetes Paranais frici and Pristina acuminata. Dreissenids were by far the most abundant NIS. Several other NIS were also common, but others were detected in only a few of the >200 samples taken. Non-indigenous amphipods and Dreissena were most frequently detected in sweep net and colonization plate samples of littoral vegetation, while NIS oligochaetes, gastropods, and non-dreissenid bivalves were most frequently detected in ponar and bottom sled samples of sediments. Our findings confirm that this major shipping port remains a NIS “hotspot” and emphasize that regular surveys covering a range of habitats with multiple sampling gears and thorough taxonomic effort are needed to detect and monitor non-indigenous species.

An integrated approach to assessing multiple stressors for coastal Lake Superior

Biological indicators can be used both to estimate ecological condition and to suggest plausible causes of ecosystem degradation across the U.S. Great Lakes coastal region. Here we use data on breeding bird, diatom, fish, invertebrate, and wetland plant communities to develop robust indicators of ecological condition of the U.S. Lake Superior coastal zone. Sites were selected as part of a larger, stratified random design for the entire U.S. Great Lakes coastal region, covering gradients of anthropogenic stress defined by over 200 stressor variables (e.g. agriculture, altered land cover, human populations, and point source pollution). A total of 89 locations in Lake Superior were sampled between 2001 and 2004 including 31 sites for stable isotope analysis of benthic macroinvertebrates, 62 sites for birds, 35 for diatoms, 32 for fish and macroinvertebrates, and 26 for wetland vegetation. A relationship between watershed disturbance metrics and 15N levels in coastal macroinvertebrates confirmed that watershed-based stressor gradients are expressed across Lake Superiors coastal ecosystems, increasing confidence in ascribing causes of biological responses to some landscape activities. Several landscape metrics in particular—agriculture, urbanization, human population density, and road density—strongly influenced the responses of indicator species assemblages. Conditions were generally good in Lake Superior, but in some areas watershed stressors produced degraded conditions that were similar to those in the southern and eastern U.S. Great Lakes. The following indicators were developed based on biotic responses to stress in Lake Superior in the context of all the Great Lakes: (1) an index of ecological condition for breeding bird communities, (2) diatom-based nutrient and solids indicators, (3) fish and macroinvertebrate indicators for coastal wetlands, and (4) a non-metric multidimensional scaling for wetland plants corresponding to a cumulative stress index. These biotic measures serve as useful indicators of the ecological condition of the Lake Superior coast; collectively, they provide a baseline assessment of selected biological conditions for the U.S. Lake Superior coastal region and prescribe a means to detect change over time.

Patterns in habitat and fish assemblages within Great Lakes coastal wetlands and implications for sampling design

Discerning fish–habitat associations at a variety of spatial scales is relevant to evaluating biotic conditions and stressor responses in Great Lakes coastal wetlands. Ordination analyses identified strong, geographically organized associations among anthropogenic stressors and water clarity, vegetation structure, and fish composition at both whole-wetland and within-wetland spatial scales. Lacustrine-protected wetlands were generally internally homogeneous in fish composition, whereas riverine or barrier-beach lagoon wetlands could be more heterogeneous, especially if they had large tributaries and complex morphology or if the mouth area was more directly exposed to the adjacent lake than were other areas. A tendency towards more turbidity-tolerant fish but fewer vegetation spawners, nest guarders, or game and panfish differentiated both more-disturbed from less-disturbed wetlands and open-water from vegetated areas within wetlands. Variation in vegetation structure related to wetland hydromorphology and...

Rapid stable isotope turnover of larval fish in a Lake Superior coastal wetland: Implications for diet and life history studies

Trophic linkages of larval fish in Lake Superior coastal habitats can be identified using naturally occurring differences in the stable isotope ratios of nitrogen (15N:14N, δ15N) and carbon (13C:12C, δ13C). We measured 13C and 15N values in common fish larvae weekly during spring run-off (late-April to mid-July) in the hydrologically complex drowned river mouth of the St. Louis River, the second largest tributary to Lake Superior. For all species, δ13C was increasingly negative with increasing weight as the fish developed from the yolk-sac stage, during which they possess a maternally-derived isotopic signature, to an exogenously feeding larvae. Trends in δ15N with increasing weight varied among species; an increase, decrease, and no change in δ15N were observed. A weight-based stable isotope turnover function modeled well the observed changes in δ13C and δ15N. In general, fish obtained a constant signature after a 10-fold gain in body mass, implying their tissue was at isotopic equilibrium with their diet. Difference between yolk-sac and larvae δ13C and δ15N revealed distinct patterns in larval origin and settlement. Based on the species analyzed, we identify two specific Lake Superior coastal wetland-dependent fish early life histories that incorporate habitat use, movement, and trophic dynamics. This study thus provides a methodological approach that can potentially help resolve interactions between watershed character, coastal productivity, and Lake Superior that are of significance to the lakes fisheries.