The Exclusion of Limnetic Cladocera from Turbid Glacier‐Meltwater Lakes

Abstract

Glacial lakes, turbid (@>5 nephleometric turbidity units) with suspended particles (1—30 @mm), have both lower May—November levels of chlorophyll a and temperatures compared to nonglacial systems. Macro—zooplankton densities are also lower and dominated by Cyclops and Diaptomus. Extensive surveys showed that, regardless of the presence or absence of planktivorous fish, filter—feeding caldocerans (e.g., Bosmina, Daphnia, and Holopedium) were only absent from the limnetic zooplankton community of the glacial lakes. Both laboratory and in situ biochamber experiments demonstrated that turbidity reduced Daphnia survival and recruitment. Nondiscriminating filter feeders ingest glacial silt (average diameter of °11 @mm) because the size—range overlaps that of the phytoplankton. We speculate that such an inefficient foraging strategy, especially when silt levels are high and algal numbers low, lowers energy extractable from ingested food below maintenance levels. Thus, the limnetic macro—zooplankton community of most Alaskan glacial lakes is restricted to either the selective herbivore Diaptomus and the raptorial feeding Cyclops, or to just Cyclops.

Type of suspended clay influences zooplankton response to phosphorus loading

“…Furthermore, the comparative success of Diaphanosoma in the clay treatments runs counter to the general hypothesis that mineral turbidity will favor rotifers over cladocerans. Considering our results and those of others (Threlkeld 1986a;Hart 1988;Koenings et al 1990;Shuman 1990), we suggest that the working hypothesis be altered to state that mineral turbidity will tend to favor copepods and rotifers over most cladocerans with the important excention of the clay-tolerant genera, Diaphanosoma and Moina.…”

“…Some reported abiogenic turbidities are: 70 NTU for Lake le Roux in Africa (Hart 1986a), 150 NTU for several glacial lakes in Alaska (Koenings et al 1990), and 80-350 NTU in the lakes on the Darling River system in Australia (Shiel 1985). Lakes in which mineral turbidity often causes Secchi depths of 3 5 cm or less include Tuttle Creek Reservoir in Kansas (Shuman 1990), Lake Texoma in Oklahoma (Threlkeld 19863), and reservoirs in North Carolina (Smith 1990).…”

“…Although most of the work on turbid systems has been done in warm climates, the general trend of cladoceran replacement by copepods has also been seen for waters of the arctic that are turbid with glacial flour (Koenings et al 1990) and to a lesser degree in Lake Michigan during "whitings" of calcite precipitation (Vanderploeg et al 1987).…”

“…Calanoid copepods, and the cladocerans Diaphanosoma and Moina appear better adapted to mineral 566 turbidity than other crustacean zooplankton (Threlkeld 1986a;Hart 1988;Koenings et al 1990;Shuman 1990). Kirk and Gilbert (1990) suggested that rotifers should outcompete cladocerans in turbid systems, because rotifers selectively can ingest algae instead of clay (Kirk 199 1).…”

“…Some reports associate reduced zooplankton densities with turbid conditions (Shiel 1985;Hart 1987;Koenings et al 1990). Others have found turbid systems with zooplankton populations typical of clear lakes, despite a paucity of algal food (McCabe and O'Brien 1983;Arruda et al 1983).…”