Offshore Zooplankton Research Articles

Seasonal Cycle of Zooplankton off Southwestern British Columbia: 1979–89

Average seasonal cycles of zooplankton biomass, species composition, and environmental conditions are estimated from 1979–89 samples collected off the southwest coast of British Columbia. Average total dry weight biomass ranges from a winter minimum <1.5 g/m2 to a late spring maximum >8 g/m2. Three subregions within the study area can be distinguished based on bathymetry and current pattern: the inner shelf banks, the shelf break and slope, and a gyre off the mouth of Juan de Fuca Strait. Total zooplankton biomass, species composition, and seasonality differ sharply between subregions, as do temperature, salinity, and nutrient and phytoplankton distributions. From spring through autumn, there are large inputs of upwelled water (and nutrients) to the continental shelf. Phytoplankton biomass is high, especially in mid- to late summer, and is ample to support growth of herbivorous zooplankton. Despite high food availability and relatively low predation pressure, surface layer zooplankton populations decline on the continental shelf from late spring through autumn, while offshore zooplankton populations increase slowly. A high washout rate due to upwelling and subsequent seaward and alongshore transport is the most plausible explanation. For the continental shelf in summer, the advective component of population turnover is probably larger than local predation mortality.

Changes in the Nearshore and Offshore Zooplankton Communities in Lake Ontario: 1981–88

We examined trends and factors influencing changes in nearshore and offshore zooplankton abundance and composition in Lake Ontario between 1981 and 1988. In the nearshore (southshore and eastern basin), zooplankton abundance decreased and shifts occurred in the relative abundances of Bosmina longirostris and Daphnia retrocurva (eastern basin) and Daphnia retrocurva and Daphnia galeata mendotae (southshore). These changes could have resulted from increased vertebrate predation or reduced food resources which intensified the effects of predation. In the offshore, the first appearance (FA) of the larger, less common cladoceran species occurred earlier in the season as of 1985. FA was correlated with cumulative epilimnetic temperature (CET) and the catch per unit effort (CPUE) of alewife (Alosa pseudoharengus) [Formula: see text] caught in U.S. waters in the spring. In 1987, when CET was high and CPUE of alewife [Formula: see text] was low, large populations of these cladocerans developed in June and July. Bythotrephes cederstroemi, a recent invader in the Great Lakes, was abundant only in 1987 when the CPUE of alewife was lowest. Changes in zooplankton abundance, development, and composition along the nearshore–offshore gradient reflected effects of temperature, habitat, and planktivory on the community.

Surface zooplankton dynamics and community structure in the Gulf of Aqaba (Eilat), Red Sea

Winter and summer zooplankton maxima were observed on both near-reef and offshore sampling sites in the northern part of the Gulf of Aqaba, with summer maxima smaller than those of winter and more characterized by larval forms. Near-reef zooplankton biomass was generally several times greater than that observed 2 km offshore. During 1987, a near-reef maximum of 155 ind. or 12.2 g wet biomass m−3 was observed in March, while 103 ind. or 8.5 g wet biomass m−3 was observed in July. In the same year, 2 km offshore a maximum of 53 ind. or 2.5 g wet biomass m−3 was observed in February, while a maximum of 33 ind. or 0.5 g wet biomass m−3 was noted in July. The following year, 1988, the near-reef zooplankton abundances were little changed, but offshore zooplankton abundances were much higher (317 m−3). During 1987, the dominant winter (March) forms near the reef were gammarid amphipods, at maximum concentrations of 100 ind. m−3, where the summer (July) maximum was composed primarily of mysids (34 m−3), gammarid amphipods (30 m−3), and fish eggs (24 m−3). The offshore winter zooplankton fauna was characterized by copepods and appendicularians, each at a maximum concentrations of ca 13 ind. m−3, while the summer maximum was dominated by brachyuran zoea (31 m−3). Though the 1988, winter near-reef zooplankton community compositions were similar to those of 1987, the offshore zooplankton fauna was dominated by ostracods, which were relatively rare in previous years. Preliminary data suggests that holoplanktonic forms like chaetognaths, copepods and appendicularians, at an offshore site exhibit different patterns of vertical migration than those near the reef. This different behavior may result from different species compositions of these taxa or from high concentrations of “pseudoplanktonic” bentho-neritic peracarid crustaceans.

A study of the offshore zooplankton of the tropical western Atlantic near Barbados

Abstract A series of 95 zooplankton samples from 400 m to the surface and 94 samples at the surface were collected over a period of 28 months at an offshore station near Barbados, West Indies, latitude BOI5'N, longitude 59°42'W. Average zooplankton wet volume of the oblique samples was 0.020 ± 0.001 ml per m3, dry weight 2.180 ± 0.143 mg per m3, and density 155 ± 14 per m3, Positive correlation coefficients of the oblique and surface samples were 0.379 for volume and 0.627 for dry weight. Seasonal variation of plankton biomass were not observed but the average amount of plankton from January to August, 1968.was significantly greater than at other times, probably caused by enrichment of the euphotic zone. The number of zooplankton species observed was 245 and the estimated number about 300. Copepods accounted for about half of the number of animals in most samples. 141 species of copepods were identified: 92 Calanoida, 38 Cyclopoida, 9 Harpactacoida and 2 Caligoida. Twelve species accounted for about 75% o...

The elemental composition of plankton

Phytoplankton samples, collected in Monterey Bay, California, were analyzed for their Pb, Hg, Cd, Co, Ag, Cr, Ti, V, Mn, Ni, Cu, Fe, Zn, AI, Mo, Ba, Sr, K, Ca, Mg, Na and SiO 2 content. The results of these analyses were categorized on a chemical basis and the sample data were placed in three groups: Group I, Ti not detected; Group II, Ti detected; and Group III, Sr concentrators present. Levels of most elements were higher in Groups II and III for a variety of reasons that are discussed in the text. The siliceous frustules, remaining after organic-matter digestion, were also analyzed for the elements listed above. Significant amounts of Al, Ti, Fe, Mn, Cu and Zn were found. Zooplankton and microplankton samples, collected in Monterey Bay, California; off the coast of Oregon; and on a transect between Hawaii and Monterey, were also analyzed for the elements listed above (except Si). In general, element levels in the inshore and offshore zoo-plankton were similar; however, the microplankton samples, in which strontium was highly concentrated, were almost always higher in Pb, Hg, Cu, Fe and Zn.

MERCURY IN A MARINE PELAGIC FOOD CHAIN1

Phytoplankton, zooplankton, and anchovies collected in Monterey Bay, California, over 10‐month period were analyzed for total mercury. In general, mercury levels were low and no evidence of food chain amplification was observed. Temporal variations of Hg concentrations in phytoplankton were greater than those for zooplankton; however, no seasonal trends were observed for either group. Plankton samples were also collected on a transect between the Hawaiian Islands and Monterey. Mercury levels were approximately equal in inshore and offshore zooplankton. The highest average mercury concentration was found in phytoplankton‐net samples from the open ocean that contained radiolarians and other small zooplankton forms as well as diatoms.