Coastal rivers link terrestrial and freshwater systems to oceans. River networks drain watersheds, delivering freshwater, nutrients, sediment, chemicals, and biota to estuaries and coastal ecosystems. These influences can negatively and positively affect downstream receiving water bodies. Effects of rivers on oceans depend on rates of transport and export of river-borne materials from channels versus rates of in-channel processing: degradation, storage, or biological uptake of those materials. In California, under a Mediterranean climate, biotic transformations slow and fluxes increase during the winter months, which coincide with higher river flows.
I studied effects of two river-borne substances on coastal oceans: pyrethroid insecticides and riverine algae. Pyrethroids are in widespread use in both agricultural and urban environments in California. In upstream reaches of the Salinas River, in Central California, sediment samples collected in agricultural and urban creeks were found to be toxic to a common test species, and the mass of pyrethroids present in all these sediments could explain the measured toxicity. While compositional differences in sediment pyrethroid mixtures between the agricultural an urban land uses were not dramatic, movement of pyrethroids short distances from the areas of terrestrial application to downstream waterways was confirmed.
Suspended sediments sampled from three coastal rivers during storm events showed that pyrethroids were routinely discharged from these rivers. Sediments in estuaries and downstream reaches of the rivers contained concentrations of pyrethroids above those expected to be toxic. However pyrethroid residues were not detected in bed sediments of the continental shelf or in the deep sea, presumably due to dilution or degradation.
Coastal rivers can also transport nutrients, organic matter, or biota. We collected algal drift in the Eel River as it entered the estuary of the Eel in Northern California. The major component of this drift during summer base flows was benthic macroalgae which had detached from upstream aquatic habitats, while a winter storm delivered more terrestrial material. Marine and freshwater macroalgae were fed to interstitial estuarine invertebrates, both in an experiment examining preferences of free-swimming benthic isopods and amphipods, and in an experiment examining consumption rates of enclosed amphipods on various types of algae. These experiments showed the quick consumption and preference for freshwater algae. The flux of freshwater algae into the estuary and its quick consumption suggest that fluxes of riverine algae into marine ecosystems are likely to disappear rapidly due to preferential grazing, and would be easy to overlook as potentially important trophic subsidies for estuarine or coastal marine ecosystems.