Introduced pumpkinseed Lepomis gibbosus sampled from four habitat zones (fluvial pelagic, fluvial littoral, lacustrine pelagic and lacustrine littoral) in three Portuguese reservoirs were used to test the hypotheses that habitats with the least similar characteristics will show the most differentiation, and that morphological differences will relate to functional adaptations to flow and trophic habitats. Results from discriminant function analysis and ANCOVA showed that there were significant differences in external morphology in pumpkinseed captured from the four habitat zones in all three reservoirs. Littoral and pelagic differentiation was stronger than fluvial and lacustrine differentiation in all of the reservoirs, and the most significant variable that differentiated pumpkinseed from the littoral and pelagic habitats was body depth. The illustration of external morphological differentiation in pumpkinseed along both habitat dimensions demonstrates the high degree of morphological plasticity of this introduced species.

We examined patterns of fish community composition and the relation to environmental variation in a drowned river mouth system of Lake Michigan. Direct hydrological connections to the Muskegon River and Lake Michigan contribute to a unique ecosystem with large‐lake and riverine influences, thus making Muskegon Lake of particular interest for assessing fish community structure across spatiotemporal scales. Using overnight fyke‐net sets, we sampled the fish community at four littoral sites in Muskegon Lake during spring, summer, and fall of 2003–2009; we also measured a suite of physicochemical variables at each sampling location and event. Among the most abundant species captured were the yellow perch Perca flavescens, round goby Neogobius melanostomus, and bluntnose minnow Pimephales notatus. Results from nonmetric multidimensional scaling showed that fish assemblages varied strongly across seasons and across sites but that annual variation was weak. Canonical correspondence analysis by season showed that temperature was the most important variable in spring, whereas turbidity, dissolved oxygen, and depth were more influential in structuring fish communities during summer. Macrophyte cover played a significant role in explaining community composition in the fall. We observed patterns of seasonal temporal variation in species–environment relationships, similar to patterns observed in purely lacustrine systems. We also observed patterns of fine‐scale spatial variation, which are typically noted in purely riverine systems. As such, our results have implications for predicting and determining patterns in other drowned river mouth lakes and reservoirs that are heavily influenced by both riverine and large‐lake dynamics.

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Introduced pumpkinseed in Iberian reservoirs display marked external morphological differentiation along two simultaneous dimensions of flow and trophic structure. We assessed the degree of internal morphological differentiation using gill rakers, pharyngeal jaws and the levator posterior muscle among pumpkinseeds occupying four different habitats and determined whether prey consumption accounted for any discernible differences in feeding structures among ecomorphs. Results showed significant differentiation by habitat based on pharyngeal muscle and jaw dimensions in all study reservoirs, with pelagic pumpkinseeds having smaller jaws than littoral pumpkinseeds in four of the five reservoirs. Gill rakers, however, differentiated morphs in only three of the five reservoirs, corresponding to differences in zooplankton consumption among pelagic and littoral individuals in those reservoirs. Based on all internal morphological traits, greater divergence was seen along the littoral-pelagic trophic axis in the lacustrine zones of reservoirs compared to the fluvial zone. Overall differences noted in internal morphology are likely the result of phenotypic plasticity; the ability of this species to readily adapt to changing physical environments may explain the success of the pumpkinseed in its introduced range.