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The importance of dissolved silicate (DSi), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), phosphate and dissolved organic phosphorus (DOP) on algal growth is analysed for the Gulf of Riga and the adjacent open Baltic Sea. The results of three cruises (May, June, and July, 1999) along a transect across the Gulf of Riga from the entrance to the Daugava River to the open Baltic are presented. Nutrient-limitation was identified on the basis of available nutrient concentrations and stoichiometric analysis. In spring, phosphate appeared to be the algal-growth-potential-limiting nutrient at the entrance of the Daugava River, DSi in the central Gulf, and DIN at the open Baltic station. There was no correlation between limiting nutrient and spring phytoplankton community structure. Both the DIN and phosphate pools of the upper mixed layer were exhausted by mid-May, except at the river mouth. In summer there was a good correlation between phytoplankton biomass and DOP along the transect. Contrary to the situation in the open Baltic, the lower layer DIN/phosphate ratio in the Gulf of Riga significantly exceeds the Redfield ratio, and upwelling likely does not favour nitrogen-fixing species. Therefore, the upper layer DOP pool should be regarded as potentially the main source of phosphorus for nitrogen-fixing cyanobacteria in the Gulf of Riga.

Only a minor fraction of copepod defecation appears to leave the upper water column as fast-sinking fecal pellets in coastal waters. This study suggests that most egested matter from copepods is retained in the water column because (1) > 50% of fecal matter is released as small, slowsinking particles that are not surrounded by a peritrophic membrane and (2) small fecal pellets sink slowly and are degraded rapidly. The production, appearance and fate of fecal material from the calanoid copepod Acartia tonsa (fed on 2 different phytoplankton species, the cryptophyte Rhodomonas salina and the diatom Skeletonema costatum) was followed in association with the grazing activity of the copepod in a laboratory experiment. For both diets, > 50% of the defecation was released as dispersed small (<10 µm) non-pellet-bound particles. The diatom was less suitable as a food item than the flagellate and led to a 3 times higher rate of grazing and egestion. Nevertheless, specific assimilation and egg production per female were 2 times higher for the Rhodomonas diet versus the Skeletonema diet. As a result, the total egestion comprised 18% of the ingestion of Rhodomonas and 27% of the Skeletonema ingestion. In terms of vertical loss, sinking rates for both types of fecal pellets were ca. 5 m d -1 and, in terms of degradation, ca. 0.5 d -1 (18°C) in the absence of copepods. Transferring these findings to similar neritic conditions suggests that 60% of the fecal pellets from copepods will be recycled within a 15 m deep mixed layer and that > 80% of the total fecal matter can be expected to be retained when the unbound fecal material is also included.

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Liepaja harbor is a former Soviet military site. Remediation of the harbor basin, including dredging of the sediments heavily contaminated with petroleum hydrocarbons and heavy metals is necessary to convert the harbor for civilian use. Abandoned pollution sources on land still continue to leach contaminants into the harbor basin. Our long-term research objective therefore is to identify transport pathways into the harbor sediments and to assess the in-situ degradation of pollutants in the system. The Liepaja harbor is treated as a complex system, where processes in the water column interact with the sediments and the nepheloid sediment boundary layer. For this research PAHs are used as the model organic contaminant. In this project the seasonal variation in biodegradation of PAHs is measured. This analysis is coupled with total bacterial production to establish the maximum degradation rate. With complex mixing of organic contaminants and need for understanding the ground water circulation it is difficult to determine the contaminant species. For this study a thorough assessment of PAH speciation has been used to determine the variation of sources. Variation in the PAH speciation is coupled with stable carbon isotope analysis to provide additional information on the range of sources in the harbor. Radio carbon isotope analysis of CO 2 in the vadose zone from a site at the Naval Station Norfolk is introduced as an approach to survey natural attenuation in this region. The Liepaja harbor is the first contaminated harbor site to be remediated in Latvia. Knowledge gained within the project will help to develop the management of harbors and polluted sites in the country. At the same time, the Liepaja harbor can be regarded as a "typical" abandoned former Soviet army site. Experiences from the Liepaja harbor will be useful in future common efforts between the former Soviet Union and the US Navy.

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