Because lake sturgeon Acipenser fulvescens, like other sturgeon species, are threatened or endangered in many aquatic ecosystems, it is imperative that we increase our understanding of their role in food webs. Our main objective was to determine the carbon sources for lake sturgeon in Lake Winnebago, Wisconsin, which contains one of the largest populations of lake sturgeon in North America. Gut content analysis revealed that gizzard shad Dorosoma cepedianum and Chironomus plumosus larvae (56% and 33% by gut content mass, respectively) were the primary prey items for lake sturgeon in the winter. Larger lake sturgeon were more piscivorous than smaller individuals. A mixing model using d 13 C and d 15 N suggests that Chironomus contributes 49% and gizzard shad 37% to the carbon assimilated by lake sturgeon. We estimated the carbon half-life in lake sturgeon to be about 0.6-3.0 years based on a model incorporating metabolism and growth. Thus, the stable isotope results integrate over a considerably longer time period than the gut content analysis. Our results provide critical baseline information about the carbon sources for lake sturgeon that can be used to assess how their role in food webs may change after perturbations such as the introduction of exotic species and changes in land use.
Macrophyte beds have been shown to influence organic matter retention and nutrient processing in streams. Less is known about the extent to which plant beds contribute to abundance, biomass, and diversity of macroinvertebrate assemblages in low-order streams. We measured aquatic invertebrate abundance, biomass, and diversity associated with plant beds and sand/gravel patches in a low-gradient second-order stream in the Central Sand Plains of Wisconsin, USA from March to October. Invertebrate abundance and biomass were higher on average in plant beds (2,552 m -2 and 1,575 mg m -2 ) than in sand/gravel patches (893 m -2 and 486 mg m -2 ). Although sand/gravel habitat was over three times more abundant than plant beds in the study reach, plant beds and sand/gravel patches contributed similarly to invertebrate abundance and biomass at the wholereach scale. The abundance and biomass of invertebrates associated with plant beds decreased from spring to autumn. Non-insect invertebrates in the plant beds increased in relative abundance as the year progressed. Shannon-Weiner diversity and taxa richness of invertebrates were higher in the plant beds than in the sand/gravel habitat. Our results suggest that plant beds can represent hot spots for invertebrate abundance and production in low-gradient streams, and have implications for stream management and restoration in these types of ecosystems.
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