Effects of small rivers on chemical properties of sediment and diets for primary consumers in estuarine tidal flats

We compared effects of particulate organic matter (POM) transported from small rivers (POMR) on chemical properties of sediment and diets of primary consumers in 3 estuarine tidal flats with different watershed characteristics in southwestern British Columbia, Canada. The source rivers (0.1 to 1.9 m(3) s(-1), annual mean) flowed through forested and urbanized areas (FR) or through agricultural areas (AG1 and AG2). The C:N ratio of POMR Was significantly higher in FR (8.6 to 10.1) than in AG1 and AG2 (6.7 to 7.0). The chlorophyll a concentration of river water was significantly lower in FR (0.4 mu g l(-1)) than in AG1 and AG2 (9.4 to 10.3 mu g l(-1)). In the tidal flat of FR, organic matter of sediment (SPOM) had relatively higher C: N ratios (FR, 17.9; AG1, 9.6 and AG2, 8.2), suggesting the accumulation of terrestrial organic matter. Mixing model (IsoSource) calculations based on stable isotope measurements showed that in the tidal flat of FR, POMR was dominant in SPOM (similar to 70%), also suggesting POMR was an important food source for some primary consumers (37 to 48%) as were benthic microalgae (35 to 45 %). In AG1 and AG2, POMR did not dominate SPOM, and there was no evidence of significant incorporation of POMR into macrobenthos diets. In the present study, POMR from the watershed with substantial forest areas appeared to be more influential on the chemical properties of sediment and macrobenthos diets in the tidal flat. This could be attributed to the low degradability and long retention of POMR in the tidal flat sediments.