Seasonality of phytoplankton production in the Columbia River: A natural or anthropogenic pattern?

This study evaluated seasonal variability in the quantity and quality of particulate organic matter exported by the Columbia River into its estuary. Samples of suspended particulate material (SPM), collected monthly over a 1 year period (Nov ember 1995-October 1996) from a freshwater site in the Columbia River, near the head of its estuary, were analyzed for total aluminum, particulate organic carbon and nitrogen (POC and PN, respectively) and algal pigments (chlorophyll and carotenoids). High Al content, averaging 7.9% by weight throughout the year, indicated that detrital minerals accounted for the majority of SPM mass at all times. Organic matter (approximately 2 X POC) varied on a seasonal basis both in terms of its mass contribution to SPM (5-26% by weight) and its chemical composition. In winter, organic matter originated mainly from allochthonous sources, most likely from erosion of soils. Diatoms, the major phytoplankton group present as inferred from pigment analysis, bloomed in April-June 1996, resulting in increased autochthonous contribution to POC during spring and summer as well as increased organic matter content of SPM introduced to the estuary. Comparison with results for the same site in the mainstem Columbia obtained between 1990 and 1999 as part of a land-margin ecosystem research project shows that enhanced algal production in springtime is not a feature unique to our data set, at least in the Columbia River's present highly regulated state of hydrographic operation. The possibility is discussed that recent human influence, principally dam construction, has significantly altered the historical SPM concentration and the chemical composition of its associated organic matter. Copyright (C) 2001 Elsevier Science Ltd.