Colossal Aggregations of Giant Alien Freshwater Fish as a Potential Biogeochemical Hotspot

The ubiquity and fascinating nature of animal aggregations are widely recognised. We report here consistent and previously undocumented occurences of aggregations of a giant alien freshwater fish, the Wels catfish (Silurus glanis). Aggregative groups were on average composed of 25 (±10 SD, ranging from 15 to 44) adults with estimated average total biomass of 651 kg (386 – 1132) and biomass density of 23 kg m−2 (14 – 40). Aggregations always occurred within the same location. No foraging, reproductive or anti-predator behaviour were observed during the aggregations. A mass-balance model estimated that these colossal aggregations of an alien species can locally release, through excretion only, up to 70 mg P m−2 h−1 and 400 mg N m−2 h−1, potentially representing the highest biogeochemical hotspots reported in freshwater ecosystems and another unexpected ecological effect of alien species

An Order of Magnitude Faster AIP1-Associated Actin Disruption than Nucleation by the Arp2/3 Complex in Lamellipodia

The mechanism of lamellipod actin turnover is still under debate. To clarify the intracellular behavior of the recently-identified actin disruption mechanism, we examined kinetics of AIP1 using fluorescent single-molecule speckle microscopy. AIP1 is thought to cap cofilin-generated actin barbed ends. Here we demonstrate a reduction in actin-associated AIP1 in lamellipodia of cells overexpressing LIM-kinase. Moreover, actin-associated AIP1 was rapidly abolished by jasplakinolide, which concurrently blocked the F-actin-cofilin interaction. Jasplakinolide also slowed dissociation of AIP1, which is analogous to the effect of this drug on capping protein. These findings provide in vivo evidence of the association of AIP1 with barbed ends generated by cofilin-catalyzed filament disruption. Single-molecule observation found distribution of F-actin-associated AIP1 throughout lamellipodia, and revealed even faster dissociation of AIP1 than capping protein. The estimated overall AIP1-associated actin disruption rate, 1.8 µM/s, was one order of magnitude faster than Arp2/3 complex-catalyzed actin nucleation in lamellipodia. This rate does not suffice the filament severing rate predicted in our previous high frequency filament severing-annealing hypothesis. Our data together with recent biochemical studies imply barbed end-preferred frequent filament disruption. Frequent generation of AIP1-associated barbed ends and subsequent release of AIP1 may be the mechanism that facilitates previously observed ubiquitous actin polymerization throughout lamellipodia

Biomass-dependent Effects of Age-0 Common Carp on Aquatic Ecosystems

Fishes play a functional role in structuring aquatic ecosystems through top-down and bottom-up processes. Adult common carp (Cyprinus carpio) are well recognized for their middle-out effects on aquatic ecosystems that can shift shallow lakes from the clear- to turbid-water stable state through benthic foraging activities. However, less is known about ecosystem effects of age-0 common carp. Age-0 common carp are planktivorous and can be highly abundant, suggesting that large year classes may also produce undesirable ecosystem effects. We evaluated the effects of four age-0 common carp densities (0, 175, 475, and 812 kg/ha) on water quality (ammonium, total phosphorus, total Kjeldahl nitrogen, and turbidity) and primary (macrophytes and phytoplankton) and secondary (zooplankton and benthic invertebrates) production. Common carp increased nutrient availability and phytoplankton production and decreased water transparency with effects increasing with carp biomass. Common carp also reduced macrophyte coverage and cladocera density (through effects on chydorus and ceriodaphnia) and body size, but effects were not density dependent. In contrast, common carp did not appear to affect copepod, rotifer, chironomid, or gastropod densities. These results suggest that even relatively low age-0 common carp densities (\u3e175 kg/ha) may have many comparable ecosystem effects as adult carp, although effects may be accrued through different pathways

The importance of excretion by Chironomus larvae on the internal loads of nitrogen and phosphorus in a small eutrophic urban reservoir

Measurements of ammonium and phosphate excretion by the Chironomus larvae were conducted in order to evaluate the importance of these chironomids for the internal loads of a small eutrophic urban reservoir. Ammonium and phosphate excretion rates by Chironomus larvae of small size (6-10 mm total length) were significantly higher than those of the Chironomids having medium (9-11 mm) and large (11-16 mm) sizes. A dependence in relation to temperature was recorded for the ammonium and phosphate excretions that was significantly higher at 25 °C than at 20 and 15 °C. Through a linear relation between biomass (dry weight) and total length and, between excretion and biomass and, data on chironomids densities, after an intense sampling in 33 sites distributed all along the reservoir bottom, the mean phosphate and ammonium excretion rates corresponded to 2,014 ± 5,134 µg.m-2/day and 1,643 ± 3,974 µg.m-2/day, respectively. Considering the mean biomass (34 mg.m-2) of Chironomus, the lake area (88,156 m²) and the mean excretion rates, the contribution of benthic chironomids to the internal loads would be 181 KgP and 147 KgN. for the sampling months (October-November 1998). These values showed that the internal loads by excretion from Chironomus larvae correspond to approximately 33% of the external loads of phosphorus in the lake and, in the case of nitrogen, to only 5%