A physical context for gelatinous zooplankton aggregations: a review

The magnitude and extent of jellyfish blooms are influenced not only by the biology and behavior of the animal, but also by the geographic setting and physical environment. Hydrography alone is often thought to cause or favor gelatinous zooplankton aggregations, however, it is clear that interactions between biology of the animal and physics of the water are very important sources of population variations, especially at local scales. We summarize the role of physical processes and phenomena that promote aggregations of gelatinous zooplankton. We have identified and discussed a suite of physical gradients that can be perceived by gelatinous zooplankton. These include light, gravity, temperature, salinity, pressure and turbulence. A recurring theme is accumulation of jellyfish around physical discontinuities such as fronts (shelf-break, upwelling, tidal and estuarine) and pycnoclines (thermoclines and haloclines). Interestingly, there are few data to suggest that large-scale, quasi-stationary features, such as the largest oceanic fronts, serve to physically aggregate gelatinous animals at a similar scale. Rather, examples of local aggregations appear to dominate the literature. We also discuss various jellyfish behaviors that are theorized to promote aggregation, feeding and reproduction in relation to physical discontinuities.     

The Environmental Control of Near-Surface Thermoclines in Boreal Lakes

We report on the effect of lake size, water transparency, and wind on the frequency of transient near-surface thermoclines in 39 boreal lakes from the Experimental Lakes Area (ELA) and Northwest Ontario Lake Size Series (NOLSS). This study was based on more than 3000 archived temperature profiles amassed over a 25-year period for lakes ranging from 2 ha to 8 million ha in surface area. The incidence of transient thermoclines decreased with increasing lake size from 90% of all summer days in small lakes (less than 4 ha) to 40% or less in the larger NOLSS lakes (up to 34,700 ha). No transient near-surface thermoclines were detected in Lake Superior. Forest fires and climatic variability were also found to affect the frequency of near-surface thermoclines. Long-term trends indicate an increase in average annual wind velocity in the area, possibly as the result of extensive forest fires and clearcutting. The subsequent decrease in the frequency of shallow secondary thermoclines in aquatic ecosystems has possible consequences for the lake biota, as the result of changes in radiation, turbulence, and the nutrient regime. Received 8 November 2000; accepted 30 April 2001.