Physicochemical features at multiple spatial scales (e.g., watershed, embayment, and habitat) can be important for fish community structure [1–3]. Variability in nutrient inputs, hydrology, and morphometry among and within aquatic ecosystems can shape fish communities [1, 4–6]. In turn, fish community structure influences ecosystem function, such as energy transfer and nutrient cycling [7–9] via trophic interactions and, in some cases, habitat modification [10, 11]. Consequently, fish communities are important indicators of and interactors in aquatic ecosystems.
We explored how physicochemical features shaped fish communities in Lake Ontario embayments. Great Lakes embayments are relatively shallow, inshore ecosystems located between the shorelines of the lakes and their watersheds. Embayments vary considerably in nutrient loading, hydrology, and morphometry. Additionally, embayments serve as conduits of nutrients and other materials from their watersheds [12, 13], support high fish species diversity , provide spawning and nursery habitats for both nearshore and offshore Great Lakes fishes [14, 3, 15], and are concentrated areas of human activities . These characteristics make embayments ideal systems with which to address physicochemical effects on fish community structure in the context of ecosystem function.
Great Lakes embayments range in hydrogeomorphic type, including flooded river mouths, coastal wetlands, and large, deep enclosed bays [17, 18, 15]. Embayments are connected to their watersheds by tributary inflow, surface runoff, and/or groundwater flow. While some embayments lack direct, surface water connections to the main lake, most embayments have either man-made or natural connections that can be permanent, seasonal, or ephemeral . This combination of morphometric and hydrologic variability results in physicochemical habitat conditions that differ both among and within embayments [16, 20]. For example, morphometry and water inflow from tributaries and the lake (via seiches) interact to influence water chemistry, submerged aquatic vegetation, and dissolved oxygen and temperature profiles .
We posed the question: how do watershed inputs, hydrology, and embayment morphometry affect fish community structure in eight embayments located along the southeastern coast of Lake Ontario? We expected that variation in these physicochemical factors across spatial scales would influence multiple metrics of fish community structure, including diversity, relative abundance, biomass, and size structure. At the watershed scale, watershed size, discharge and land use affect productivity, which in turn, can influence fish community structure and dynamics. We hypothesized that high nutrient inputs to embayments, from either high watershed flows (i.e., short water residence time) or high nutrient concentrations due to land use, would positively affect fish abundance and biomass [21, 22, 1] and negatively affect species diversity through loss of intolerant species [4, 23]. At the system (i.e., embayment) scale, greater surface area with a more complex depth profile can increase habitat and resource heterogeneity, which positively impact fish abundance, biomass, and diversity [24, 6].
Within systems, availability of vegetated, littoral habitat also affects fish communities by increasing habitat and resource heterogeneity [14, 25, 26]. As such, we predicted that embayments with higher habitat heterogeneity (e.g., large surface area and/or abundant, vegetated littoral habitat) would support more diverse and abundant fish communities than small or more homogeneous embayments. Morphometry also impacts fish community size structure [1, 6]. We hypothesized that a higher proportion of small-bodied than large-bodied fishes would occur in shallow embayments dominated by vegetated habitat . In contrast, large embayments having deep, open habitat would provide support for large-bodied fishes , resulting in a low proportion of small-bodied fishes due to predation [10, 27].
In this paper, we used hierarchical mixed modelling to relate differences among embayment fish communities to abiotic and biotic factors at the watershed through sampling station scales. The response variables we considered were fish community species diversity, relative biomass and abundance, and size structure. Predictor variables included total phosphorus load, embayment area, sampling station depth, percent aquatic vegetation, and percent littoral habitat and, for size structure only, piscivore relative biomass.