Establishment of a Metalimnetic Oxygen Refuge for Zooplankton in a Productive Lake Ontario Embayment

Abstract

Hypolimnetic oxygen injection is a management tool used to improve water quality by preventing anoxia and associated phosphorus release from the sediments. An additional benefit would be formation of a low‐oxygen refuge for large‐bodied herbivorous zooplankton in the metalimnion. The magnitude and timing of hypolimnetic oxygenation was deliberately manipulated during summer 1997 in Irondequoit Bay, a eutrophic Lake Ontario embayment (New York, USA), to maintain metalimnetic dissolved oxygen (DO) concentrations in the range of 1–2 mg/L. We assessed the vertical distributions of fish and zooplankton in 1997, a year with controlled oxygenation, and 1996, a year with oxygenation but without a deliberate attempt to create amenable refuge conditions. After initiation of oxygenation, June through early August metalimnetic DO in 1997 was 1.8 ± 0.5 mg/L (means ± 2 se), whereas metalimnetic DO in 1996 was 3.8 ± 1.0 mg/L. Hydroacoustic surveys in 1997 and gillnet catches in both years indicated that >96% of planktivorous fish, alewives (Alosa pseudoharengus) and emerald shiners (Notropis atherinoides), were restricted to depths with DO > 4.0 mg/L. A refuge effect in 1996 was slight and evidenced only by significantly larger zooplankton in the metalimnion while densities of Daphnia galeata medotae and D. retrocurva were similar or significantly greater in the epilimnion. In 1997, the refuge effect was strong despite an almost twofold increase in planktivorous fish abundance compared to 1996. Overall zooplankton densities were similar in the epilimnion and metalimnion while zooplankton size and densities of both daphnids were significantly greater in the metalimnion. Densities of an invertebrate predator, Mesocyclops edax, were also significantly higher in the metalimnion compared to the epilimnion. Despite high fish abundance in 1997, mean summer (late May through August) chlorophyll a decreased 29% compared to 1996, consistent with expectations from grazing by large Daphnia protected in the refuge. Presence of large‐bodied zooplankton in 1997 despite high planktivore abundance demonstrated that successful creation of a low‐oxygen refuge from fish predation was possible in an open Great Lakes embayment where traditional biomanipulation via fish removal was not practical.

Vertical distribution of fish biomass in hypoxic waters on the Gulf of Mexico shelf

“…Lowoxygen bottom waters may serve as a refuge for hypoxia-tolerant zooplankton, leading to reduced for-aging efficiency of planktivorous fishes that cannot occupy the bottom hypoxic layer (Taylor & Rand 2003, Klumb et al 2004). Alternatively, both zooplankton and pelagic planktivores may be concentrated in surface waters as a result of behavioral avoidance of bottom water hypoxia, presumably leading to enhanced encounter rates and increased foraging efficiency by pelagic planktivores (Roman et al 1993, Keister et al 2000.…”

Artificial mixing to control cyanobacterial blooms: a review

“…Higher oxygen concentrations in the hypolimnion enlarge suitable habitat for a large number of oxygendependent organisms such as zooplankton and fish (Doke et al 1995;Klumb et al 2004;Skinner et al 2014) and avoid fish kills by preventing the upwelling of anoxic hypolimnetic water (Müller and Stadelmann 2004). Enhanced oxygen availability also has major effects on many biogeochemical processes.…”

Sprat Sprattus sprattus can exploit low oxygen waters for overwintering

“…On the other hand, hypoxic waters may provide refuges for prey that are more tolerant than their predators. In the absence of such hypoxic refuges, the predators can reduce or completely wipe out the prey population (Horppila et al 2000, Klumb et al 2004. Therefore, whether hypoxia shifts the balance in favor of the predator or the prey depends on their relative tolerance to hypoxia (Domenici et al 2007).…”