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The protozoan parasite Toxoplasma gondii infects a large proportion of threatened California sea otters (Enhydra lutris nereis), and is an important waterborne pathogen in humans. Contamination of coastal waters with T. gondii is thought to occur through delivery of environmentally resistant oocysts to nearshore regions via overland runoff. The objectives of this study were to evaluate whether T. gondii oocysts and surrogate microspheres attach to aggregates ($ 0.5 mm), and whether the magnitude of aggregation depends on water type, specifically salinity. Laboratory aggregation studies were conducted by adding T. gondii oocysts and surrogate microspheres to riverine, estuarine, and marine waters, and quantifying the proportion of oocysts and surrogates in aggregate-rich and aggregate-free water fractions. Attachment of oocysts and surrogates to aggregates occurred in all water types, but was greater in estuarine and marine waters, with concentrations of T. gondii in aggregates enriched 3–4 orders of magnitude. Aquatic aggregates may, thus, significantly influence waterborne transport of terrestrially derived pathogens, both through enhanced settling and subsequent concentration in the benthos, as well as by facilitating ingestion by invertebrate vectors that can transmit pathogens to susceptible hosts, including sea otters and humans.

Bacterioplankton abundance and metabolic characteristics were observed in northern San Francisco Bay, California, during spring and summer 1996 at three sites: Central Bay, Suisun Bay, and the Sacramento River. These sites spanned a salinity gradient from marine to freshwater, and sampling occurred during a period of seasonally declining river flow. The microbial measures included radio-labeled amino acid uptake ( L-leucine, L-proline, Lserine), ectoenzyme activity (aminopeptidase and b-D-glucosidase), and bacterial abundance using 1-mm filters to separate free from particle-associated bacteria. A seasonal decline in all bacterial metabolic measures was observed at all stations, suggesting that a system-wide variable may be important in controlling bacterial activity. One such variable is freshwater flow into the Bay (as a proxy for organic matter flux), which positively covaried with all metabolic measures. A sharp decline in particle-associated bacteria was also observed in Suisun Bay and the Sacramento River between July and August. This decline may have been due to combined effects of declining nutritive value of the aging particles and increasing grazing pressure by benthic filter feeders. Aminopeptidase activity was positively related with increasing salinity, and b-D-glucosidase was negatively correlated with increasing salinity, indicating a gradient in the relative quality of organic matter from carbohydrate-rich riverine to proteinrich oceanic material. Overall, Suisun Bay had the highest mean proportion of particle-associated bacteria (49%), followed by Sacramento River (36%) and Central Bay (11%). Particles were the sites of enhanced ectoenzyme activity but not amino acid incorporation. Bacteria may be actively dissolving the particulate organic matter, but their growth rates on particles are not significantly enhanced.

By using a remotely operated vehicle (ROV), we studied large mucous aggregates produced by the larvacean Bathochordaeus in Monterey Bay, California. These fragile structures or “houses,” cannot be sampled by typical water or net collecting devices, and likely represent a class of cosmopolitan, widely spaced aggregates in subsurface waters. ROV samples allowed us to determine the contribution of the aggregate-associated communities to populations of microorganisms at depths of 100–500 m. Because the giant houses only average ˜1 per 100 m3, they harbor

The mucus feeding structures or “houses” of the giant larvacean Bathochordaeus provide abundant material for the study of deep-sea detrital communities, particularly their poorly known zooplankton associates. We sampled houses between 100 and 500 m in Monterey Bay with a submersible ROV (remotely operated vehicle) and surveyed houses for metazoans by database search of video footage taken from the ROV. Up to an order of magnitude more metazoans were found on houses than in surrounding waters. On average, copepods constituted as much as 96% of the assemblage on houses, and many of the species possess benthiclike morphology and feeding strategies. Poecilostomatoid copepods (genus Oncaea) averaged as many as 64.6 house-‘, and scarcely known calanoid copepods (genus Scopalatum) occurred in 56% of the samples. Higher numbers of metazoans occurred on shallower houses (lo&300 m), likely due to a difference in the species of larvacean present and (or) to reduce oxygen levels at greater depths. At least one copepod species, Scopalatum vorax, occurred on houses more frequently during the nonupwelling season, possibly due to the lack of other food. Our results suggest that midwater detritus contains a unique invertebrate community that has been largely undetected, mostly due to sampling difficulties. The houses also provide benthiclike habitats for midwater zooplankton and serve as feeding centers. These particle-associated zooplankton may therefore contribute to remineralization of particulate organic carbon at depth.

Indicator assemblages can define the habitat of a planktonic species. Diatom assemblages that co-occur with Salpa fusiformis, a common planktonic herbivore, suggest a constancy in the environment of this tunicate: S. fusiformis is found with a suite of diatoms whose relative abundances are predictable to a degree. Preliminary results indicate that swarm formation by S. fusiformis occurs in waters different from that of other salp species commonly found in the California Current.