A Lagrangian study of plankton trophodynamics over a diel cycle in a eutrophic estuary under upwelling influence

A Lagrangian study was conducted in a eutrophic estuary (Guanabara Bay, Brazil) to investigatein situplankton trophodynamics under the influence of the cold, nutrient-rich South Atlantic Coastal Water in a short-term temporal variability (scale of hours). We tested the hypothesis that the base of the plankton food web is composed of small cells and that microzooplankton is the main consumer of this assemblage. Samples of pico-, nano- and microplankton, as well as copepods, were collected during spring, when the entry of upwelling water in the Bay is commonly observed, and near the surface every 3 h during the 1-day sampling period. Potential predation of dinoflagellates, ciliates, copepod nauplii, copepodites and adult copepods was estimated based on predator-prey size relationships. The main trophic links in the Guanabara Bay food web for the period analysed were nanophytoplankton-copepods, nanophytoplankton-ciliates, and autotrophic dinoflagellates-heterotrophic dinoflagellates. According to microphytoplankton availability, adult copepods could not satisfy their food requirement, and nanophytoplankton represented an important supplementary food source. In fact, diel variations of nano- and microplankton biomass were opposite to that of copepods suggesting predation control by the latter on the former. The trophodynamics of Guanabara Bay, under the influence of upwelling water, resulted in marked differences from other eutrophic estuaries around the world.

Predation control of zooplankton dynamics: a review of observations and models

Daewel, U., Hjøllo, S. S., Huret, M., Ji, R., Maar, M., Niiranen, S., Travers-Trolet, M., Peck, M. A., van de Wolfshaar, K. E. 2014. Predation control of zooplankton dynamics: a review of observations and models. – ICES Journal of Marine Science, 71: 254–271. We performed a literature review to examine to what degree the zooplankton dynamics in different regional marine ecosystems across the Atlantic Ocean is driven by predation mortality and how the latter is addressed in available modelling approaches. In general, we found that predation on zooplankton plays an important role in all the six considered ecosystems, but the impacts are differently strong and occur at different spatial and temporal scales. In ecosystems with extreme environmental conditions (e.g. low temperature, ice cover, large seasonal amplitudes) and low species diversity, the overall impact of top-down processes on zooplankton dynamics is stronger than for ecosystems having moderate environmental conditions and high species diversity. In those ecosystems, predation mortality was found to structure the zooplankton mainly on local spatial and seasonal time scales. Modelling methods used to parameterize zooplankton mortality range from simplified approaches with fixed mortality rates to complex coupled multispecies models. The applicability of a specific method depends on both the observed state of the ecosystem and the spatial and temporal scales considered. Modelling constraints such as parameter uncertainties and computational costs need to be balanced with the ecosystem-specific demand for a consistent, spatial-temporal dynamic implementation of predation mortality on the zooplankton compartment.

Restoration of a sustainable wild grey partridge shoot in eastern England

Eastern England has been a stronghold for grey partridges Perdix perdix, but in common with the rest of Britain, numbers declined from the 1950s onwards. Partridges within a 40 km2 study area in the county of Norfolk have been monitored in conjunction with the Game and Wildlife Conservation Trust (GWCT) since the 1950s. Since 2001 a programme of habitat creation, supplementary feeding and predation control was undertaken by the landowner, farmers and gamekeepers to restore partridges. Numbers increased from 4.7 pairs/km2 in March 2001 to 54 pairs/km2 in March 2011. These densities are comparable with those before the national decline in grey partridge stock. In the last three winters, between 13 and 74 birds/km2 were harvested and spring stocks continue to increase.