The importance of riverine algae in organic matter export to the Eel River estuary Northern California

Background/Question/Methods

Coastal rivers link terrestrial and freshwater ecosystems to oceans. These river networks drain watersheds, delivering freshwater, nutrients, sediment, chemicals, and biota to downstream estuaries and coastal systems. These influences can both negatively and positively affect downstream receiving water bodies. The effects of rivers on oceans depend on the rates of transport and export of river-borne materials from channels, versus the rates of in-channel processes, like degradation, storage, or biological uptake. In California, under a Mediterranean climate, biotic transformations slow and fluxes downstream increase during the winter months, which coincide with higher river flows. However, coastal rivers can transport nutrients, organic matter, or biota from their places of origin and affect downstream organisms throughout the year. To study the transport of organic material, we collected coarse organic drift in the Eel River as it entered the estuary of the Eel in Northern California. 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.

Results/Conclusions

The major component of the coarse organic matter drift during summer base flows was benthic macroalgae which had detached from upstream aquatic habitats. Winter storms resulted in increased fluxes of organic material and delivered more terrestrial material. Feeding experiments showed the quick consumption and preference for freshwater algae by estuarine benthic invertebrates. 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.