Hydrobiological Bulletin

Data are presented from several campaigns concerning to the residual transport of water, salt and suspended particulate matter (SPM) at different depths and at different longitudinal and cross-sectional locations in the Elbe Estuary, Northern Germany. The intratidal behaviour of SPM is analysed and explained in terms of cyclic settling and resuspension. Superimposed on these local processes are advective horizontal transports of water and SPM in different vertical layers. Tidally averaged horizontal fluxes of salinity and SPM have been calculated at different depths. For the most part, they show upstream directed transport in the bottom layers, while in the upper water column SPM is carried towards the sea. Examples of the influence of vertical salinity gradients on SPM dynamics, of the lateral variation of residual transport of water and SPM, and the effects of a runoff peak are also presented.

In The Netherlands the process of eutrophication often results in hypertrophy, the aspects of which are presented in an outline. Emphasis is given to the evaluation of existing trophic situations in relation to species diversity, biomass, and effect of accelerated eutrophication compared to natural eutrophication. The possible role of potassium in eutrophication is mentioned.

The combined effects of grazing and nutrient regeneration by Daphnia and Eudiaptomus on the growth of Rhodomonas and heterotrophic bacteria was assessed experimentally. The responses of Rhodomonas and bacteria to the grazers were measured as net specific growth rate over the entire experimental periods, as well as production and specific production at the end of the experiments. Both zooplankton species had a negative effect on Rhodomonas net specific growth rate due to grazing and a positive effect on specific primary production due to nutrient regeneration. Daphnia had no effect on bacterial net specific growth rate, bacterial production or specific bacterial production in one of two experiments. In the other experiment, however, both bacterial growth rate and production decreased as a result of grazing. Furthermore, Daphnia had a negative effect on specific bacterial production, but Eudiaptomus had a positive effect on all bacterial parameters due to nutrient regeneration, probably of phosphorus. Positive effects of copepods on bacterial growth has previously been attributed to trophic cascades via protozoa. However, the present experiments show that regeneration of nutrients, especially phosphorus, may account for a large part of the stimulation of bacterial growth.

Marine and freshwater ecosystems differ in persistence, size, population connectivity, and the variance in physical and biotic conditions they experience. These differences may select for differing reproductive modes, life histories, dispersal strategies, and chemically cued recruitment behaviors. In marine systems, adults are commonly less mobile, while larvae spend hours to weeks to months dispersing in the plankton and may move over great distances. It is these immature larval stages that must select appropriate recruitment sites in marine environments. In freshwater systems, the fully developed adults more commonly disperse over greater distances, and it is usually adults that determine juvenile recruitment sites via their placement of larvae or fertilized eggs. Thus, in terms of large-scale habitat choices involving chemical cuing, adult stages should be selected to detect and react to habitat cues among most freshwater species, while juveniles should play this role among most marine species. Few studies assess this hypothesis, but adults of freshwater organisms as different as mosquitoes and frogs do key on chemical cues to select sites for depositing eggs or larvae, while chemical cuing of recruitment in marine systems occurs primarily among the larval stages of the numerous fishes and marine invertebrates investigated to date. Cues to general habitat features, to predators or competitors, and to specific prey or hosts have all been shown to affect recruitment. Here, we review chemically mediated recruitment in marine versus freshwater systems, summarizing what is known and suggesting unknowns that may be productive to investigate.

During five 28-hours measurements in 1981, the oxygen production and consumption in an eelgrass community in saline Lake Grevelingen were investigated using light plexiglass enclosures. Applying a conversion factor of 0.29 the amount of carbon fixed and the amount of organic carbon mineralized were estimated. Gross and net production were estimated over 24-hours periods. There appeared to be a good correlation between production and insolation on the water surface. For every measurement period the production as a function of light and aboveground eelgrass biomass in the enclosure were calculated. This showed a maximum of 5.10−6 mg C.J.−1 g dry weight−1 in April and minimum of 1.4.10−6 mg C.J.−1 g−1 in August. Using the calculated production coefficients, the insolation and the eelgrass biomass the gross production, net production and consumption during the growing season of 1976 were calculated. Gross production amounted to 340 gC.m−2, and net production came to 130 g C.m−2. Approximately 60 gC.m−2 was respired by the eelgrass plants while the remaining 150 gC.m−2 was consumed or mineralized by other organisms on the sampling spot. Approximately 120 g C.m−2.yr−1 was transported by wind and wave action towards the eastern part of the lake where it became anaerobically degraded. This resulted in the formation of sulfide and methane.

Enzymes released extracellularly by micro-organisms have major functions in nutrient acquisition and organic matter degradation. Clay particles, common in many surface waters, can modify enzyme activity. Clay minerals are known to form aggregates with organic molecules, and the formation of enzyme-clay complexes could alter the level of activity. Montmorillonite clay and clay extracted from Elledge Lake (Tuscaloosa, Alabama) basin soil were combined with alkaline phosphatase, glucosidase, protease, and xylosidase solutions to assess adsorption and the effect of this adsorption on enzyme activity. Adsorption to Elledge Lake basin clay decreased alkaline phosphatase activity, and adsorption to montmorillonite was observed for all four enzymes with reductions in enzyme activities. Adsorption of substrate onto clay surfaces resulted in a concentration effect and increased enzyme activity associated with the particles. When enzyme-clay complexes were exposed to natural sunlight there was a decrease in enzyme activity, but this decrease was usually not significantly different from the adsorption only treatment. The formation of enzyme-clay complexes may serve to protect the enzymes from natural in situ photodegradation. The results indicate the complex interactive effects adsorption of enzymes to clay particles can have on the availability and capability of hydrolysis – reduction of enzyme reactivity, storage attached to clay particles with changes in transport and distribution, and protection from photodegradation.

Aquatic invertebrate data were collected from 49 erosional, Jamaican,rock pools between 1989 and 1998 and used to describe temporal patterns ofspecies diversity. This unique series of pools on the north coast of Jamaica,classified as either brackish (31) or freshwater (18), was used to determinehowdiversity changes over time, whether there was a difference between poolclassifications, and the impacts of environmental variables. Mean communitymetrics (richness, diversity, evenness, abundance) were not significantlydifferent between freshwater and brackish pools. However, there weresignificantdifferences among the eight sampling dates and differences over time dependedonpool classification. Measures of diversity for freshwater pools were relativelyconstant over time, implying little change at the community level. Brackishpools showed significant differences over time in species richness, totalabundance, and evenness implying that community composition and structure werenot static but changed in response to either environmental or biotic changes(possibly initiated by environmental change).Some temporal changes in community metrics could be linked to temporal changesin environmental variables. In brackish water pools, a significant increase inpool salinity between January 1991 and January 1992 corresponded to an increasein species richness, likely due to an increase in marine fauna. Similarly,changes in abundance and evenness corresponded to changes in temperature,dissolved oxygen, and pH. In addition, physicochemical variables used in thisstudy were shown to affect community metrics and those relationships dependedonpool classification. Most relationships between community metrics andenvironmental variables were negative with the exception of Simpson's diversityindex for which positive relationships were found. This may indicate that, aspool conditions become less favorable, a few species flourish and dominate thecommunity.

The impact of eutrophication on the biogeochemical cycle of silicate in the Rhine delta was analysed by 1) comparing the seasonal variation in river water, stagnant fresh water and coastal seawater and 2) observations in well-controlled experimental reservois subjected to different regimes of phosphate precipitation. The high input of dissolved sliciate from Rhine water was rapidly depleted in receiving waater systems through vigorous phytoplankton (datom) growth. In reservoirsca. 50% of the silicate input was retained over a seven years' observation period. Regeneration of silicate immobilized by diatoms was accelerated by very dense blooms of phytoplankton in reservoirs and in Lake IJssel that increased the pH value over 9. The prevailling lowratios of Si/N and Si/P in Rhine water entall the risk of massive blooms of flagellates or cyanobacteria in receiving waters. The 50% reduction in nitrogen and phosphate emission agreed upon in the Rhine Action Plan may reduce butnot eliminate this risk.