Effect of a size-selective biomanipulation on nutrient release by gizzard shad in Florida (USA) lakes

Although fish removal for biomanipulation is often highly size-selective, our understanding of the nutrient cycling effects of this size-selection is poor. To better understand these effects, we measured nutrient excretion by gizzard shad (Dorosoma cepedianum) of differing sizes from four central Florida (USA) lakes and combined these measures with gillnet biomass and size-structure data to compare lake-wide effects among lakes and years. Direct removal of P in fish tissue ranged from 0.16−1.00 kg·P·ha-1·yr-1. The estimated reduction in P excretion due to harvest ranged from 30.8−202.5 g·P·ha-1·month-1, with effects strongly tied to the biomass and size structure harvested. The amount of P release prevented per kg of fish removed was lower in previously unharvested lakes, due to the initial removal of larger fish with lower mass-specific excretion rates. Gill net mesh size impacted the size distribution of harvested fish, with smaller fish that excrete more P per gram being more vulnerable to smaller mesh sizes. In Lake Apopka, decreasing the mesh size by 1.3 cm yielded P excretion reductions that were 10.7−15.1% larger. Fish harvesting to reduce internal nutrient cycling can be most effective by increasing total harvest and by harvesting smaller size classes over multiple years

Maximum growth rates and possible life strategies of different bacterioplankton groups in relation to phosphorus availability in a freshwater reservoir

We investigated net growth rates of distinct bacterioplankton groups and heterotrophic nanoflagellate (HNF) communities in relation to phosphorus availability by analysing eight in situ manipulation experiments, conducted between 1997 and 2003, in the canyon-shaped Rimov reservoir (Czech Republic). Water samples were size-fractionated and incubated in dialysis bags at the sampling site or transplanted into an area of the reservoir, which differed in phosphorus limitation (range of soluble reactive phosphorus concentrations - SRP, 0.7-96 mu g l(-1)). Using five different rRNA-targeted oligonucleotide probes, net growth rates of the probe-defined bacterial groups and HNF assemblages were estimated and related to SRP using Monod kinetics, yielding growth rate constants specific for each bacterial group. We found highly significant differences among their maximum growth rates while insignificant differences were detected in the saturation constants. However, the latter constants represent only tentative estimates mainly due to insufficient sensitivity of the method used at low in situ SRP concentrations. Interestingly, in these same experiments HNF assemblages grew significantly faster than any bacterial group studied except for a small, but abundant cluster of Betaproteobacteria (targeted by the R-BT065 probe). Potential ecological implications of different growth capabilities for possible life strategies of different bacterial phylogenetic lineages are discussed

Contribution of grazing in phytoplankton overall losses in a shallow French lake

Based on data for 14C-primary production and biomass changes in the small, shallow Creteil Lake, overall phytoplankton losses were calculated through an annual cycle (November 1985-October 1986). Summer 1986 is compared with summer periods in 1985 and 1980, these two years corresponding to extreme levels of algal biomass. Independent from the trophic state of the lake, phytoplankton populations were dominated by small-sized species (<20 μm); their high growth rate (maximal in May and June: 0.8 day-1) was characteristic of nanoplanktonic natural populations. The positive correlation between phytoplankton losses and production indicates a close coupling between growth and loss processes. With a high filtering rate (0.22 day-1 as an annual average), zooplankton impact is considerable at any time of the year but especially in late summer, when grazing losses exceeded primary production. The persistence of small algae throughout the year implies that a great part of the phytoplankton production was harvested by grazers which led to a recycling of organic matter within the water column. -from AuthorsSCOPUS: NotDefined.jFLWNAinfo:eu-repo/semantics/publishe