Impacts of Land Use and Flood Regime on Zooplankton Egg Banks in a Large River Floodplain

Zooplankton egg banks are crucial in rebuilding zooplankton populations in temporary water bodies when favourable conditions return after dry periods. However, their abundance, viability, and hatching success depend on many factors, with changes in land use and flooding regimes being potentially crucial drivers.

Here, we studied a large‐river floodplain to compare the egg banks of soils across a land use gradient and at different elevations, the latter affecting inundation frequency and duration. We performed experimental manipulations at both the ecosystem scale (land use in the floodplain) and in the laboratory (incubation of soils of different origins). Six land use types were considered: (i) natural maple swamp, (ii) natural wet meadow, (iii) old forage cropping, (iv) recently sown forage cropping, (v) agri‐environmental corn/soybean cropping, and (vi) conventional corn/soybean cropping implemented in collaboration with agricultural producers. Egg banks were characterized through both (i) direct egg counts and (ii) by counting the individuals emerging following the incubation of soils in the laboratory, testing the effects of land use and flooding regime (duration, frequency) on hatching success.

We found no significant difference in the abundance of resting eggs of different taxa in soils across the land use gradient in frequently inundated sites at low elevation. In contrast, at high elevation, where inundation periods are less frequent and shorter, rotifer and cladoceran egg density varied across land uses, with higher abundance in old forage cropping compared to agricultural cropping. Ostracods, rotifers and copepods were the most abundant taxa that hatched in the laboratory. Their abundance varied across land use types, with natural soils supporting higher abundances than agricultural soils, and forage cropping showing intermediate abundances. Frequently inundated sites (low elevation) had higher abundances of hatchlings than the less frequently inundated (high elevation) sites. Inundation duration in incubators strongly influenced ostracod hatching only, with an overall increase after 2 weeks of inundation and a decrease thereafter for most soil types.

Different taxa dominated the egg and hatchling counts. For example, cladocerans were common in the egg bank but not in hatchling samples, while ostracods were absent from the former but dominated the latter. This apparent paradox suggests that egg viability varied along the land use gradient (e.g., due to agricultural practices during summer months), that the cues required for hatching differed across taxa, or that stress (land use) modulates the likelihood that ostracods enter dormancy as eggs versus juveniles/adults.

Natural wetland habitats in floodplains favour the resilience of zooplankton communities facing a disturbance (i.e., the dry period) via a high potential recovery through the emergence of resting stages. In contrast, alterations to these habitats due to agricultural intensification in floodplains hampers zooplankton community resilience via decreased emergence success. Agricultural practices leading to changes in soil properties, which influence the viability of resting stages, are the plausible explanations for this phenomenon. It is paramount to conserve natural wetlands and implement sustainable land use practices in floodplains, to allow egg banks to rebuild zooplankton communities in floodplains and sustain their productivity and ecosystem services.