Sedimentation of phytoplankton during a diatom bloom : Rates and mechanisms

Phytoplankton blooms are uncoupled from grazing and are normally terminated by sedimentation. There are several potential mechanisms by which phytoplankton cells may settle out of the photic zone: sinking of individual cells or chains, coagulation of cells into aggregates with high settling velocities, settling of cells attached to marine snow aggregates formed from discarded larvacean houses or pteropod feeding webs, and packaging of cells into rapidly falling zooplankton fecal pellets. We quantified the relative significance of these different mechanisms during a diatom bloom in a temperate fjord, and evaluated their potential to control phytoplankton population dynamics. Overall specific sedimentation rates of intact phytoplankton cells were low during the 11-day study period, averaging ca. 0.1 d−1, and mass sedimentation and bloom termination did not occur. Most cells settled attached to marine snow aggregates formed from discarded larvacean houses, whereas settling of unaggregated cells was insignificant. Formation rates of phytoplankton aggregates by physical coagulation was very low, and losses by this mechanism were \u3c0.07 d−1; phytoplankton aggregates were neither recorded in the water column (by divers) nor in sediment traps. The low coagulation rates were due to a very low ‘stickiness’ of suspended particles. The dominant diatom, Thalassiosira mendiolana, that accounted for up to 75% of the phytoplankton biomass, was not sticky at all, and did not turn sticky upon nutrient depletion in culture experiments. The low particle stickiness recorded may be related to low formation rates by diatoms of transparent exopolymeric particles (TEP), that occurred in low concentrations throughout the study period. Zooplankton grazing rate did not respond to the development of the bloom and accounted for a loss term to the phytoplankton populations comparable to sinking of intact cells; fecal pellets accounted for 30–50% of settled phytoplankton and phytodetritus. While coagulation may give rise to density-dependent losses to phytoplankton populations and, hence, control blooms, neither of the other mechanisms examined worked in a density dependent manner. In the absence of significant coagulation rates, rapid mass sedimentation of this bloom did not occur

Distribution, movements and diet of nocturnal fishes on temperate reefs

We counted nocturnal fishes both day and night, and monitored the position of tagged individuals on temperate reefs in New South Wales, Australia. Pempheris affinis and P. multiradiata were the most abundant nocturnal planktivores on Sydneyrsquos rocky reefs and showed great differences in diel migration behaviour. Both species were observed in deep shelter sites during the day (5–10thinspm), and most emerged into the water column at night. P. multiradiata was found to undergo extensive vertical and horizontal migrations. In contrast, P. affinis remained within daytime depth strata, with tagged individuals often moving less than 20thinspm at night. Tagged adult P. affinis returned to tagging sites for up to 7thinspweeks, indicating high site fidelity. Dietary analysis demonstrated that small and large pempherids differed in diet and the timing of foraging, suggesting a size-based transition from diurnal to nocturnal foraging. Stratified sampling of planktonic assemblages at different depths during the day and night showed spatial variation in the availability of prey items at different times of the day. Amphipods, the main prey of large fish, were only available during the night, and concentrated in shallow water, whereas decapod larvae, consumed mainly by small fish, were abundant day and night. Large P. affinis also fed on polychaetes, which were never found in the stomachs of P. multiradiata, suggesting that these species may have different prey requirements, or that these polychaetes are only found in deep water where foraging P. affinis were abundant. We found no general model for the Pempheridae. The movements and behaviour of nocturnal fishes varied greatly by species, and this may be due to differences in body size, and/or physiological (e.g. visual ability) and ecological constraints