1Institute for Great Lakes Research and Department of Biology, Central Michigan University
Throughout the Laurentian Great Lakes, lacustrine wetlands form where coastal geomorphology provides protection from hydrologic energy and where sediments are conducive to macrophyte growth. Great Lakes coastal wetlands (GLCWs) are important habitat for fish, amphibians, reptiles, wading birds, and waterfowl. Invertebrates make up a large component of the diets of these animals—linking algal and detrital energy sources to higher trophic levels. Therefore, identifying the natural and anthropogenic drivers of GLCW invertebrate communities has ecosystem-level implications given the central role of invertebrates in wetland food webs.
The structure of GLCW invertebrate communities has been related to vegetation zonation (Cardinale et al. 1997), fetch and wave exposure (Burton et al. 2004), benthic substrate (Cooper et al. 2007), water levels (Gathman and Burton 2011), water quality and surrounding land use (King and Brazner 1999), invasive plants (Holomuzki and Klarer 2010), and habitat fragmentation (Cooper et al. 2012). This growing body of literature has revealed important drivers of community structure, though nearly all studies have been conducted over relatively limited geographic extents and very few basin-scale analyses have been undertaken. Additionally, most studies of GLCW invertebrate communities have been conducted over just one or two growing seasons, yielding only snapshots of community structure.
Our goal was to extend both our spatial and temporal understanding of these communities. We predicted that community structure across the basin would correlate with watershed agriculture and/or developed land use, in situ water quality, and hydrologic exposure (i.e., wave action). We also evaluated temporal patterns in community structure from 1997 to 2012 at three wetlands of Saginaw Bay, Lake Huron. We predicted that these communities would track one another through time and show a common response to the 1-m water level decline that began in 1997.
Invertebrates were collected during the summer of 2002 at 67 coastal wetlands spanning all five Great Lakes (Figure 1). Triplicate samples were collected within each emergent vegetation zone at each wetland using 0.5-mm mesh, D-frame dip nets following Great Lakes Coastal Wetlands Consortium protocols (Uzarski et al. 2004). Specimens were identified to lowest operational taxonomic unit (typically genus) in the laboratory. The same sampling methodology was also used at the three ‘temporal’ wetlands of Saginaw Bay.
We calculated an index of hydrologic exposure based on fetch measurements along eight azimuths corresponding to the four cardinal and four sub-cardinal directions (Cooper et al. 2013). We measured six water quality variables (soluble reactive phosphorus, nitrate, ammonium, chloride, turbidity, and conductivity) at multiple locations within each wetland to reflect potential anthropogenic impacts associated with urban and agricultural runoff. Finally, percent agriculture and developed land uses were calculated for the watersheds associated with each wetland. The fetch index, water quality, and land use data were all used to interpret potential responses of invertebrate communities to abiotic divers.
Results and Discussion
We identified 215 taxa across the 67 wetlands sampled basin-wide. The vast majority of taxa were rare, leading to a very “hollow” species-abundance curve (Figure 2). Genera richness ranged from 16 to 86 (mean±SE: 41±3) and was highest in wetlands of northern Lake Michigan. Insects made up a large percentage of the genera collected, ranging from 50 to 83% of genera collected per wetland (mean±SE: 66±2%). Crustaceans were also quite common across the basin (mean relative abundance: 29±2%). Multivariate analyses revealed that at the basin-scale, invertebrate community structure correlated most strongly with the fetch index (i.e., hydrologic exposure) and watershed agriculture. The influence of these two variables on community structure far outweighed all other drivers based on multiple analytical approaches.
Invertebrate community structure at the three Saginaw Bay wetlands sampled from 1997 to 2012 appeared markedly correlated with one another through time. In the years following the 1-m water level decline, invertebrate community structure changed considerably at all three wetlands (Figure 3). From 2002 to 2004, gastropod relative abundance increased dramatically at all three wetlands. Over about the same period, insects—especially chironomids—declined substantially at all three wetlands. Crustaceans declined at all three wetlands beginning in 1999 and reached minima in 2002 to 2004. One crustacean, Hyalella, then rebounded dramatically after 2004, driving total crustacean abundance back up to pre-1999 levels.
Our results suggest that at the basin scale, wetland invertebrate communities respond to both the suite of impacts brought about by agricultural runoff (e.g., eutrophication, sediment loading, agro-chemicals, hydrologic alteration) and to wave exposure. These drivers appeared important enough to transcend other potential effects, even ambient water quality. Specific mechanisms linking fetch and invertebrate communities are unclear; however, a combination of physical disturbance of organisms by wave energy, the influence of hydrologic energy on sediment organic matter, and the effect of wave-induced turbidity on visual predators are likely all important.
We hypothesize that the relationship between invertebrate community structure and watershed agriculture is a function of nutrients and suspended solids (i.e., turbidity) influencing the production and distribution of basal resources (e.g., attached vs. planktonic algae) in wetlands receiving agricultural runoff. This seems highly plausible given that aquatic invertebrate communities are generally influenced by these basal resources. Our results build upon previous studies demonstrating the link between watershed agriculture and wetland faunal community structure but expands this relationship to the basin scale.
The general co-variation of communities through time suggests a response to broad-scale environmental drivers. Water level is a potential lake-scale driver and we found that the most pronounced community shifts occurred during the years immediately following a 1-m decline in Lake Huron spring-summer water levels. While we certainly acknowledge the potential for other lake-scale drivers to have influenced the observed community shifts, correlations between community metrics and the prior year’s water level are consistent with a lagged response of communities to the water level decline.
Across the Great Lakes basin, coastal wetland invertebrate communities correlated with fetch and watershed agriculture. However, in light of the substantial variation in community structure that we found in our 15-year time series, caution must be exercised when interpreting community patterns from short-term (e.g., one or two year) datasets. Our 2002 basin-wide survey serves as a case in point; for example, the community attributes that made Saginaw Bay communities unique in 2002 (e.g., high insect richness and abundance, low crustacean and gastropod abundance) changed dramatically in subsequent years (Figure 2). While a more thorough understanding of GLCW invertebrate communities is beginning to emerge, additional research is needed and should be directed at (1) identifying mechanistic linkages between community structure and environmental drivers, (2) identifying long-term drivers of community structure, including the influence of climate and land use change, and (3) understanding metacommunity dynamics such as the influence of dispersal vs. environmental filtering. Since invertebrates form critical linkages between basal resources and higher trophic levels, understanding how these communities are structured has ecosystem level implications. Watershed land use, wave exposure, and water levels appear to be principal drivers of GLCW invertebrate communities at broad scales.
Full study published in the Journal of Great Lakes Research, Issue 40 (Supplement 1): 168-182. Additional authors include: Donald G. Uzarski (Central Michigan University) and Gary A. Lamberti (University of Notre Dame).
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Featured Image: Lake Michigan and Lake Huron, as seen from the International Space Station. (Credit: NASA)