Influences of flood and ebb tides on nutrient fluxes and chlorophyll on an intertidal flat

Abstract

Intertidal estuarine sediment processes produce organics and nutrients and are influenced by river outflow and tidal cycles. Hence the sediments act as a source or sink of nutrients into or out of the water column. In most studies, a chamber incubation approach is used to study fluxes of nutrients into and out of the sediment; the chamber isolates the sediments from the hydrodynamics occurring in the overlying water column. Therefore, field data over an entire intertidal flat are lacking. Here, we present a unique case in which an intertidal flat is semi-enclosed, usually inundated for 12 to 14 h during a tidal cycle, and hence can be treated like an incubation chamber during this period of submergence. The intertidal flat on Sturgeon Bank, British Columbia, was directly exposed to domestic sewage effluents from the Iona Sewage Treatment Plant between 1967 and 1989. Since 1989, the sewage effluent has been diverted and discharged into deep water in the Strait of Georgia. The objective of the present study was to determine the effects of tidal cycles on the flux of nutrients from the sediment into the water column. We expanded the spatial scale of our study site by treating the semi-enclosed tidal flat as a large natural incubation chamber during the 12 to 14 h period of submergence. Water samples at 10 stations covering Sturgeon Bank were taken during late flood tides and middle ebb tides, 12 to 13 h later. Salinity, temperature, chl a, nutrients (NH4, NO3 and PO4), suspended load, and particulate carbon (PC) and nitrogen (PN) at the surface were measured or analyzed. Based on nearly 2 yr observation between 1994 and 1995, we found that water coming onto Sturgeon Bank was a mixture of different origins since salinity was not correlated with temperature or any other variables except for particulate C/N. Chl a was higher during flood tides than during ebb tides, indicating a loss of chl a from the water column, likely due to feeding by benthic organisms. Ammonium concentrations were almost always higher during ebb tides than flood tides, indicating a release of NH4 from the sediment. Therefore, the intertidal flat of Sturgeon Bank appears to be a sink for chl a and a source of NH4 to the Strait of Georgia. Particulate C/N ratios were always higher than 7, indicating a predominantly terrestrial origin of particulate organics. Particulate C/N ratios were also found to be higher during ebb tides than flood tides, indicating preferential utilization of PN when organic matter was deposited onto the sediment. Our study has shown that it is possible to treat an intertidal flat as an incubation chamber and follow fluxes of nutrients and particulates into and out of the water column during flood and ebb tides. With this approach, we concluded from NH4 concentrations and fluxes that the intertidal flat at Sturgeon Bank showed no signs of eutrophication due to previous sewage effluent contamination.