Monitoring the Milwaukee Harbor breakwater : an Engineering With Nature® (EWN®) demonstration project

The US Army Corps of Engineers (USACE) maintains breakwaters in Milwaukee Harbor. USACE’s Engineering With Nature® (EWN®) breakwater demonstration project created rocky aquatic habitat with cobbles (10–20 cm) covering boulders (6–8 metric tons) along a 152 m section. A prolific population of Hemimysis anomala, an introduced Pontocaspian mysid and important food source for local pelagic fishes, was significantly (p < .05) more abundant on cobbles versus boulders. Food-habits data of alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax) provided evidence that H. anomala were a common prey item. Night surveys and gill netting confirmed O. mordax preferred foraging on the cobbles (p < .05) and consumed more H. anomala than at the reference site (p < .05). H. anomala comprised a significant portion of the diets of young-of-the-year (YOY) yellow perch (Perca flavescens), YOY largemouth bass (Micropterus salmoides), and juvenile rock bass (Ambloplites rupestris) caught on the breakwater. The natural features’ construction on the breakwater increased the available habitat for this benthopelagic macroinvertebrate and created a novel ecosystem benefiting forage fish and a nursery habitat benefiting nearshore game fish juveniles. These data will encourage the application of EWN concepts during structural repairs at other built navigation infrastructure.

The quick and the dead: copepods dominate as cladocerans decline following invasion by Hemimysis anomala

The North American invasion of a non-native mysid, Hemimysis anomala, has been expanding since 2006, with the first inland lake invasions detected for Oneida Lake in 2009 and Seneca Lake in 2010. Although we know that Hemimysis primarily consumes zooplankton, our ability to predict the community-level impact of an invasion is hampered by a lack of information on taxon-specific effects. To investigate the effects of Hemimysis on native zooplankton, we conducted two mesocosm experiments that compared composition between communities with and without Hemimysis and studied how the effects of this predator on zooplankton species composition varied across a natural gradient of low to high invader densities (0.01–0.1 individuals·L−1). Our first experiment found that Hemimysis primarily affected cladocerans, and particularly Daphnia, shifting communities towards dominance by copepods. Our second experiment showed that Hemimysis invasions may do little to suppress Daphnia abundances until between 0.067–0.11 individuals·L−1 or higher. Cladocerans are important links in freshwater trophic transfer and the nutrient cycle, and disruption of these linkages following Hemimysis invasion could result in both bottom-up and top-down impacts in nearshore food webs. However, Hemimysis can also fill a similar trophic role as the zooplankton they consume, and longer-term experiments are required to better assess their eventual impacts on native communities.