Mikael Gyllström

Dormancy in freshwater zooplankton: Induction, termination and the importance of benthic-pelagic coupling

Abstract.For a short-lived organism, such as a freshwater zooplankter, the ways of coping with years of local recruitment failure are either to disperse between habitats and recolonise or to disperse in time through diapause. Diapause is common among freshwater zooplankton and is generally seen as a way to escape periods of harsh environmental conditions. The egg-bank or pool of diapausing copepodites in lake sediments resulting from the production of diapausing stages has several implications for zooplankton ecology, genetics, and evolution which we outline in this review. The presence of a benthic dormant stage also creates a coupling between the benthic habitat and the pelagic, and we argue that zooplankton phenology is a result of selective forces in both habitats. The spatial distribution of diapausing eggs appears to be governed by random resuspension dynamics coupled with higher hatching rates in shallow waters. For diapausing copepodites, however, an active choice of where and how deep to enter the sediment may affect their distribution. In a reanalysis of published data, we found a size-dependent bathymetric distribution and vertical distribution in the sediment of diapausing cyclopoid copepodites. Our review of published laboratory studies showed that predictors of seasonal change such as photoperiod and temperature were the only type of cues used for the termination of diapause. We also found a relation between generation length and the type of cue used for diapause induction: copepods mainly used seasonal cues from the abiota, rotifers mainly used cues from the biotic environment, and cladocerans used a mix of both types. We describe patterns in emergence timing and contribution to population dynamics from studies using in situ estimation of emergence, and conclude that hatching from dormant stages may qualitatively and quantitatively affect zooplankton population dynamics and seasonal succession.

The distribution of chydorids (Branchiopoda, Anomopoda) in European shallow lakes and its application to ecological quality monitoring

This study describes the chydorid (Branchiopoda, Anomopoda) assemblages from 66 European shallow lakes, and presents data relating the assemblages to lake type and ecological quality. Forty species, out of a total recorded European fauna of 60 species, were found in the study sites. No significant differences were found between chydorid assemblages associated with rock and plant substrata. Patterns of distribution were best explained primarily by latitude and pH. Chlorophyll-a, total phosphorus, water temperature and Secchi depth were also correlated with assemblage descriptors. Alonopsis elongata, Alona rectangula, Alonella excisa and Pleuroxus uncinatus were shown to have higher prevalence in certain lake types. The dominance of Chydorus sphaericus in a third of the study sites was linked to eutrophication and high levels of chlorophyll-a. The relationship between chydorids and lake ecological quality was more apparent at species rather than community level. This study identifies important typological factors affecting chydorid distribution, and confirms that patterns of chydorid distribution previously reported from regional studies hold true across Europe.

Induction and termination of diapause in a freshwater zooplankton community

I examined production of, and emergence from, diapausing eggs, together with planktonic dynamics of three cladoceran genera (Bosmina, Ceriodaphnia, and Daphnia) for 16 months in a shallow lake. In a complementary field experiment I manipulated environmental variables potentially important as cues for induction and termination of diapause. Ceriodaphnia and Daphnia, but not Bosmina, were dependent on emergence to initiate planktonic populations in spring. During the rest of the season, emergence was unimportant for population and community dynamics. The timing of diapausing-egg production differed between genera, but tended to coincide with peaks in abundance. No hatching was found in any of the experimental treatments, but diapausing-egg production was found in high nutrient treatments with no fish. Field study and experiment together imply that the types of environmental cues involved in diapause induction are different from the ones stimulating its termination. Seasonal cues such as photoperiod appear to govern the hatching of diapausing eggs. In comparison, the induction of diapausing-egg production seems less dependent on season and more on biotic factors.

Bioturbation as Driver of Zooplankton Recruitment, Biodiversity and Community Composition in Aquatic Ecosystems

In an experimental study we assessed if benthic bioturbating invertebrates affect the recruitment (hatching) of zooplankton from the sediment, and if this effect persists as differences in the zooplankton community in the water column, that is, if bioturbation quantitatively stimulates benthic–pelagic coupling. We investigated the effects of four different benthic invertebrates (Asellus aquaticus, Chironomus plumosus, Tubifex tubifex in the presence or absence of the predator Sialis lutaria). In total, 45 zooplankton taxa hatched from the sediment and the hatching success of some of these was dependent on the species identity of the bioturbating invertebrate. The predator Sialis reduced the abundance of all three invertebrate species, but tended to positively influence the zooplankton recruitment rates, possibly through increasing the activity of the bioturbating invertebrates. The most striking effect of bioturbation on the hatching and pelagic zooplankton community properties was that, on average, 11% more species hatched in the Asellus treatment than in any other treatment. This was also mirrored in the zooplankton water column community where, on average, 7% more species established a viable population in treatments with Asellus as bioturbator. In a complementary field survey, Asellus was more common in littoral than in profundal sediments. Because Asellus strongly affected recruitment of zooplankton in our experiment, we argue that bioturbation may partly explain why recruitment of resting stages of both phyto- and zooplankton is generally higher in littoral than in profundal areas.