Trophic interactions can influence the flux of energy and nutrients between donor and recipient ecosystems, thereby altering recipient food webs and communities. We investigated the potential for anadromous alewife to influence filter-feeding insects in lake outflow streams by altering seston and zooplankton export from lakes to streams through size-selective consumption of zooplankton in lakes. We compared spring and summer zooplankton biomass and body size and seston concentration in 6 lakes and their respective outflow streams (3 with alewife access, 3 without alewife access) in Maine, USA. The contribution of seston particles ≥500-µm and mean pelagic zooplankton size decreased from spring to summer in alewife lakes, but not in lakes without alewife. However, we found no seasonal change in zooplankton body size and seston ≥500-µm in the outflow streams of alewife lakes. Furthermore, zooplankton biomass and seston concentration differed between lakes and streams, a result indicating the influence of additional factors on seston movement from lakes to streams. Last, we found no relationship between filter-feeder biomass or community structure and seston quantity, a result indicating that stream communities probably were not strongly regulated by seston availability. Our results illustrate that strong trophic interactions may not be propagated from lakes to streams and suggest the importance of boundaries in modulating the extent to which trophic interactions in one ecosystem can influence the flow of energy to adjacent ecosystems.
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