Zooplankton Community Response to Seasonal Hypoxia: A Test of Three Hypotheses

Abstract

Several hypotheses of how zooplankton communities respond to coastal hypoxia have been put forward in the literature over the past few decades. We explored three of those that are focused on how zooplankton composition or biomass is affected by seasonal hypoxia using data collected over two summers in Hood Canal, a seasonally-hypoxic sub-basin of Puget Sound, Washington. We conducted hydrographic profiles and zooplankton net tows at four stations, from a region in the south that annually experiences moderate hypoxia to a region in the north where oxygen remains above hypoxic levels. The specific hypotheses tested were that low oxygen leads to: (1) increased dominance of gelatinous relative to crustacean zooplankton, (2) increased dominance of cyclopoid copepods relative to calanoid copepods, and (3) overall decreased zooplankton abundance and biomass at hypoxic sites compared to where oxygen levels are high. Additionally, we examined whether the temporal stability of community structure was decreased by hypoxia. We found evidence of a shift toward more gelatinous zooplankton and lower total zooplankton abundance and biomass at hypoxic sites, but no clear increase in the dominance of cyclopoid relative to calanoid copepods. We also found the lowest variance in community structure at the most hypoxic site, in contrast to our prediction. Hypoxia can fundamentally alter marine ecosystems, but the impacts differ among systems.

Spatial and temporal variations of faecal indicator bacteria in Lake Bunyonyi, South-Western Uganda

“…The dissolved oxygen (DO) levels were relatively low which is attributed to a higher rate of decomposition, especially in the wet season. Nevertheless, the observed DO range do not cause stress to aquatic organisms and can rarely cause mortality and reduction of the sensitive species [16]. Similar to our study results, DO levels of 5.7 and 5.7 mg/L have been reported by Tiémoko et al [28] at Lake Taabo and Kossou, respectively.…”

Spatio-temporal variations in physicochemical water quality parameters of Lake Bunyonyi, Southwestern Uganda

“…The observed DO concentrations in the current study were > 2 mg/L. These values according to Keister et al [17] do not cause stress to aquatic organisms and ecosystems and can rarely result in changes in the communities through direct organisms mortality and reduction in the population growth of sensitive species. In the aquatic ecosystem, DO provides a broad indication of water quality and depends on many factors such as microbial activity, temperature, level of organic matter, pressure and the sampling time [10,27].…”

Unexpected food web responses to low dissolved oxygen in an estuarine fjord

“…Low dissolved oxygen affects swimming performance of prey and predators (Domenici et al 2007) and it alters food intake rates of predators (Nestlerode and Diaz 1998, Pichavant et al 2001). Finally, the zooplankton community structure (Koslow et al 2011, Keister and Tuttle 2013, Keister et al 2020) can be affected by oxygen levels, and thereby alter the availability of preferred prey. Thus, spatial overlap alone may not indicate how predator–prey linkages are altered by exposure to dissolved oxygen below biological thresholds.…”