Measuring spatial variation in secondary production and food quality using a common consumer approach in Lake Erie

Abstract

Lake Erie is a large lake straddling the border of the USA and Canada that has become increasingly eutrophic in recent years. Eutrophication is particularly focused in the shallow western basin. The western basin of Lake Erie is hydrodynamically similar to a large estuary, with riverine inputs from the Detroit and Maumee Rivers mixing together and creating gradients in chemical and physical conditions. This study was driven by two questions: (1) How does secondary production and food quality for consumers vary across this large mixing zone? and (2) Are there correlations between cyanobacterial abundance and secondary production or food quality for consumers? Measuring spatial and temporal variation in secondary production and food quality is difficult for a variety of logistical reasons, so here a common consumer approach was used. In a common consumer approach, individuals of a single species are raised under similar conditions until placed in the field across environmental gradients of interest. After some period of exposure, the response of that common consumer is measured to provide an index of spatial variation in conditions. Here, a freshwater mussel (Lampsilis siliquoidea) was deployed at 32 locations that spanned habitat types and a gradient in cyanobacterial abundance in the western basin of Lake Erie to measure spatial variation in growth (an index of secondary production) and fatty acid (FA) content (an index of food quality). We found secondary production was highest within the Maumee river mouth and lowest in the open waters of the lake. Mussel tissues in the Maumee river mouth also included more eicosapentaenoic and docosapentaenoic fatty acids (EPA and DPA, respectively), but fewer bacterial FAs, suggesting more algae at the base of the food web in the Maumee river mouth compared to open lake sites. The satellite-derived estimate of cyanobacterial abundance was not correlated to secondary production, but was positively related to EPA and DPA content in the mussels, suggesting more of these important FAs in locations with more cyanobacteria. These results suggest that growth of secondary consumers and the availability of important fatty acids in the western basin are centered on the Maumee river mouth.

Associations between cyanobacteria and indices of secondary production in the western basin of Lake Erie

“…Length is the maximum anterior‐to‐posterior dimension of the shell measured roughly parallel to the hinge; width is the maximum left–right dimension with both valves appressed; height is the maximum dorsal‐ventral dimension of the shell measured roughly perpendicular to the hinge. As previously reported, in 2013 sometimes mussels grew in atypical shapes due to crowding in the cages (Larson et al ), this did not occur in other years. For this reason, we did not use length as our index of growth, but instead report change in nominal volume (length × width × height) on a per‐day basis.…”

“…Mussel cages used in this study are identical to those previously reported in Larson et al () and Larson et al (). Briefly, mussel cages were made using plastic‐coated mesh with diamond‐shaped openings that have a maximum width of 1.4 cm.…”

“…Because cyanobacteria are poor quality food resources, we would predict (1) negative associations between our indices of secondary production and cyanobacterial abundance. Although we did not see this relationship in a previous study (Larson et al ), this study expands the available data from 1 yr to 3 yr and includes direct phytoplankton biovolume (BV) measurements at a subset of sites. We also predict (2) negative associations between our indices of secondary production and direct measurements of total microcystin.…”

“…Within a particular year, only mussels from a single generational cohort were used. The origins of the cohorts used in 2013 and 2014 are described in previous publications (Larson et al 2016a,b, respectively). L. siliquoidea used in 2015 were from the same cohort used in 2014, except that only males were used to avoid distributing any potentially gravid females into the study areas.…”

“…Instead of direct measurements, here we use indices of secondary production. Specifically, we deployed ecological process monitoring stations (EPS) that included a common consumer and a standardized invertebrate sampler (Hester and Dendy ; Larson et al ). In the common consumer method, members of a single taxon (and preferably of a single cohort) are deployed across an environmental gradient, given time to react to those locations and associations between the reactions and the gradient are examined.…”