Showing papers in "Journal of Plankton Research in 1986"

Nutrient levels and the development of diatom and blue-green algal blooms in a shallow Australian estuary

Abstract: In the Peel-Harvey estuary system, Western Australia, some 90% of riverflow and nutrient loading occurs in three winter months Diatom blooms follow riverflow, but are replaced by blooms of the blue-green Nodularia spumigena Mert, especially in Harvey Estuary By analysis of time series data from 1977-1983, it is shown that the magnitude of the Nodularia bloom in summer is related to the minimum salinity of the estuary (and hence total river flow), maximum phosphate concentration and total riverine phosphorus loading, in the previous winter The relationships have a predictive capacity It is argued that diatom blooms trap phosphorus, which is sedimented largely as faecal pellets; the phosphorus is recycled and supports Nodularia growth under warmer conditions, and the amount available determines Nodularia biomass Nodularia blooms collapse when summer salinities reach 30‰

Predator-limited population growth of the copepod Pseudocalanus sp.

Abstract: Author(s): Ohman, MD | Abstract: The impact of predators on population growth of Pseudocalanus sp. was investigated in Dabob Bay, Washington. Mortality of Pseudocalanus sp. was determined from stage-specific survivorship, from seasonal changes in mortality rates of adult males and females and from incidence of injuries to adult copepods. The principal predators of adult Pseudocalanus were identified as the predatory copepod Euchaeta elongata, the omnivorous euphausiid Euphausia pacifica and the chaetognath Sagirta elegans. Predator attack rates - and prey mortality rates - are highly density-dependent and thus sensitive to prey dispersion in the water column, particularly to layering in the vertical plane. Predation rates by the three principal predators exceeded 100% of the recruitment rate to adult Pseudocalanus sp. beginning in early summer, thus restricting population growth. Planktivorous fish predation (by adult three-spine stickleback, Gasterosteus aculeatus, and juvenile chum salmon, Oncorhynchus keta) on Pseudocalanus sp. adults was estimated to be two orders of magnitude lower than consumption rates by predatory zooplankton, at a deep water station in July. Analysis of seasonal changes in prey ingested by Sagitta elegans revealed that Pseudocalanus sp. was the major prey item of S. elegans in April (61.0% of prey) and in June (67.0% of prey), thereafter declining seasonally in importance. Predation by S. elegans varied seasonally with changes in chaetognath stage structure, vertical distribution and diapause, not size structure alone. Although chaetognath recruitment and population growth appear to be directly coupled to the abundance of Pseudocalanus sp., predation by S. elegans has little reciprocal impact on Pseudocalanus sp. population growth; hence asymmetries may occur in the in teraction of planktonic prey and predators. © 1986 IRL Press Limited.

The effect of sedimentation and microbial nitrogen regeneration in a plankton community: a simulation investigation

Abstract: Examen du developpement et du declin d'une proliferation de phytoplancton pour verifier que le modele propose est realiste

Shape, size and density of daytime surface swarms of the euphausiid Meganyctiphanes norvegica in the Bay of Fundy

Abstract: Daytime surface swarms of Meganyctiphanes norvegica in the Bay of Fundy were examined us- ing a variety of techniques to provide measurements of their shapes, sizes and densities. Shapes and sizes were determined from two aerial photographs; swarms were spherical, ribbon-like or amorphous, were up to 28 6 m long and ranged in area from 0.4 — 111 7 m2. Densities were measured by a bag-sampling device which gave figures of up to 41 000 animals m~3. by photographic methods which gave figures of up to 770 000 animals m~3 and by a plankton net which gave maximum values of six animals m~3. Using the photographic method, the maximum euphausiid biomass was estimated to be 154 kg m~ 3 within swarms and the largest swarm measured was estimated to contain up to 2 1 tonnes of M. norvegica. Mean patch biomass estimates for the two aerial photographs ranged from 77.8-778 g m~ 3 and 15 6- 155.6 g m" 3 which are similar to figures obtained by other authors using integrating sampling techniques at depth.