Stuck between a rock and a hard place: zooplankton vertical distribution and hypoxia in the Gulf of Finland, Baltic Sea

Abstract

exhibit a preparatory antioxidant response, protecting them from oxidative damage during migrations. The acoustic data showed peaks of crustacean zooplankton biomass in hypoxic (<2 mL L −1 ) and low oxygen (2-4 mL L −1 ) concentrations (depth >75 m), whereas fish shoals and A. aurita medusae were found in normoxic (5-6 mL L −1 ) upper water layers (<40 m), with individual fish in deeper water excepting that rule. Mysid shrimp from areas with hypoxia had significantly enhanced antioxidant potential compared with conspecifics from areas with no hypoxia and had no significant indications of oxidative damage. We conclude that mysids can protect themselves from oxidative damage, enabling them to inhabit hypoxic water. Our data suggest that hypoxic and low oxygen zones (up to 4 mL L −1 ) may provide some zooplankton species with a refuge from visual predators such as fish.

Fatty acid composition and lipid content in the copepod Limnocalanus macrurus during summer in the southern Bothnian Sea

“…Nevertheless, the fatty acid composition can indicate trophic relationships at least at major taxonomic levels as has been shown for other omnivorous and carnivorous zooplankton species [see e.g., 26,52,53]. In July-September, adult L. macrurus were located in deeper water levels and, based on the low number of oil sacs in them, were probably unable to migrate to upper water layers, as suggested by Lindqvist [3] and Webster et al [37]. Therefore, it is possible that in July-September L. macrurus fed on ciliates and phytoplankton, sinking from the upper water layers or, alternatively, preyed on organisms inhabiting the same water layers.…”

“…The parallel trends between the abundance of adults and the frequency of oil sacs among them suggests that adults suffered from starvation and, as a result, became eliminated from the population. This idea of high mortality due to starvation is supported by Webster et al [37], who suggested that high predation pressure and increasing temperature at sea surface force adults downwards, with the result that the population concentrates in a smaller space where competition for food resources increases. If only a part of the adults survives and reproduce in these conditions, the summer season could, therefore, act as a bottleneck for the population growth of L. macrurus, despite it is the main production period of its food resources [25].…”

“…For a cold-stenothermic species, moving down to lower temperatures is understandable but could also be influenced by the presence of planktivores such as the Baltic herring (Clupea harengus membras), which in the Bothnian Sea preys heavily on adult L. macrurus during May and June [36]. In order to avoid the visually foraging fish, adults move from surface to deep water, but as a trade-off, they may suffer from a scarcity of food because their prey organisms occur at higher abundances closer to the surface [37,38].…”

Interactive Effects of Hypoxia and Temperature on Coastal Pelagic Zooplankton and Fish

“…These predicted differences in oxygen supply and demand provide a useful way to assess how low oxygen waters directly determine habitat availability for estuarine and coastal zooplankton. Field surveys have shown that zooplankton generally avoid the warmer low DO waters of the Gulf of Mexico (e.g., Qureshi and Rabalais, 2001;Roman et al, 2012) but reside in the colder low oxygen waters of the Baltic Sea for significant portions of the day, affording the zooplankton a potential refuge from predation (e.g., Appeltans et al, 2003;Webster et al, 2015). Because of the higher oxygen demand by coastal zooplankton and fish in warm tropical and subtropical waters, the loss of habitat space due to low oxygen waters is expected to be more severe in these regions.…”

“…Low DO bottom waters can influence the spatial overlap and trophic coupling of pelagic zooplankton and fish populations. For example, more hypoxia-tolerant zooplankton may use low DO bottom waters as a refuge from fish predation (Appeltans et al, 2003;Taylor and Rand, 2003;Webster et al, 2015; Figure 9). In contrast, zooplankton avoidance of hypoxic bottom waters can result in prey aggregations at oxyclines where fish and invertebrate predators may aggregate (Qureshi and Rabalais, 2001;Taylor and Rand, 2003;Roman et al, 2012).…”