Decline in zoobenthos densities in the profundal sediments of Lake Mendota (Wisconsin, USA)

Abstract

High densities of zoobenthos inhabited Lake Mendota's profundal zone in the early 1900s through the mid-1940s . Chaoborus punctipennis was the most abundant organism during the winter, along with moderate densities of Chironomus spp ., Pisidium sp ., oligochaetes, and Procladius sp. By the early 1950s, Chaoborus punctipennis densities had declined to 10% of former levels, while Chironomus increased significantly . However, by the mid-1960s, Chaoborus, Chironomus, and Pisidium densities had decreased to very low population levels . By 1987-89, Pisidium was no longer found . Zoobenthos that had not decreased from earlier surveys were oligochaetes and Procladius, although further sampling of oligochaetes is needed to confirm current densities . These organisms are the most tolerant of severe anoxia .Four possible reasons for this decline were evaluated : (a) decline in food availability, (b) increase in fish predation, (c) use of toxic insecticides in the drainage basin, and (d) changes in the profundal sediment environment . Based on literature information and long-term data for Lake Mendota, a change in the profundal sediment environment is the most likely explanation for the decline in the less-tolerant zoobenthos species . Although the duration and extent of anoxia in the hypolimnion have not changed since the early 1900s, hypolimnetic ammonia and hydrogen sulfide concentrations apparently have increased as Mendota became more eutrophic after the mid-1940s . However, further study is needed to determine if these higher concentrations or other factors were responsible for the dramatic decline in lake Mendota's profundal zoobenthos . 353

Profundal benthic invertebrates in an oligotrophic tropical lake: different strategies for coping with anoxia

“…For example, Chironomus (Lathrop, 1992) is commonly abundant in areas of high-quality detritus (Jiménez and Springer, 1996). The largest diatom flux (304×10 6 cells m -2 day -1 ) in Lake Alchichica occurs in February, soon after the onset of the diatom bloom (Ardiles et al, 2012).…”

“…The low number of invertebrates in the central and deepest parts of Alchichica may be due not only to their intolerance to prolonged anoxic conditions but also to the presence of high concentrations of hydrogen sulfide (Alchichica 50 m=2.66 mg· H 2 S L -1 , Alchichica 60 m=3.08 mg· H 2 S L -1 ); this gas is toxic or lethal to most invertebrates (Lathrop, 1992). The combination of anoxia and the presence of H 2 S may limit the development and survival of the profundal benthos (Corbari et al, 2005).…”