Benthic fluxes in San Francisco Bay

Abstract

Measurements of benthic fluxes have been made on four occasions between February 1980 and February 198 1 at a channel station and a shoal station in South San Francisco Bay, using in situ flux chambers. On each occasion replicate measurements of easily measured substances such as radon, oxygen, ammonia, and silica showed a variability (& la) of 30% or more over distances of a few meters to tens of meters, presumably due to spatial heterogeneity in the benthic community. Fluxes of radon were greater at the shoal station than at the channel station because of greater macrofaunal irrigation at the former, but showed little seasonal variability at either station. At both stations fluxes of oxygen, carbon dioxide, ammonia, and silica were largest following the spring bloom. Fluxes measured during different seasons ranged over factors of 2-3,3,4-5, and 3-10 (respectively), due to variations in phytoplankton productivity and temperature. Fluxes of oxygen and carbon dioxide were greater at the shoal station than at the channel station because the net phytoplankton productivity is greater there and the organic matter produced must be rapidly incorporated in the sediment column. Fluxes of silica were greater at the shoal station, probably because of the greater irrigation rates there. N + N (nitrate + nitrite) fluxes were variable in magnitude and in sign. Phosphate fluxes were too small to measure accurately. Alkalinity fluxes were similar at the two stations and are attributed primarily to carbonate dissolution at the shoal station and to sulfate reduction at the channel station. The estimated average fluxes into South Bay, based on results from these two stations over the course of a year, are (in mmol mm2 d I): 02 = -27 + 6; TC02 = 23 f 6; Alkalinity = 9 f 2; N + N = -0.3 * 0.5; NH3 = 1.4 f 0.2; PO4 = 0. I f 0.4; Si = 5.6 f I. 1. These fluxes are comparable in magnitude to those in other temperate estuaries with similar productivity, although the seasonal variability is smaller, probably because the annual temperature range in San Francisco Bay is smaller.Budgets constructed for South San Francisco Bay show that large fractions of the net annual productivity of carbon (about 90%) and silica (about 65%) are recycled by the benthos. Substantial rates of simultaneous nitrification and denitrification must occur in shoal areas, apparently resulting in conversion to N2 of 55% of the particulate nitrogen reaching the sediments. In shoal areas, benthic fluxes can replace the water column standing stocks of ammonia in 2-6 days and silica in 17-34 days, indicating the importance of benthic fluxes in the maintenance of productivity.Pore water profiles of nutrients and Rn-222 show that macrofaunal irrigation is extremely important in transport of silica, ammonia, and alkalinity. Calculations of benthic fluxes from these profiles are less accurate, but yield results consistent with chamber measurements and indicate that most of the NH3, Si02, and alkalinity fluxes are sustained by reactions occurring throughout the upper 20-40 cm ...

Submarine Groundwater Discharge-Derived Nutrient Loads to San Francisco Bay: Implications to Future Ecosystem Changes

“…Previous studies focused on the impact of bioirrigation on SFB water quality (Hammond et al 1985), but many of the studies have been conducted prior to the more recent documented changes in the bay ecosystem. The current understanding of productivity in the bay is that phytoplankton blooms are limited by light caused by high turbidity in bay waters and increased grazing by introduced bivalves (Alpine and Cloern 1992;Cloern et al 2007).…”

“…An increase in water clarity could eventually transition the SFB to a nutrient-limited system that will increase ecosystem sensitivity to nutrient inputs and nutrient stoichiometry, both of which may be significantly influenced by SGD. Despite decades of water quality and ecosystem monitoring work in SFB (Baylosis et al 1997;Cloern et al 2000), few estimates (Hammond et al 1985;Spinelli et al 2002) and no direct measurements of the contribution of total SGD (i.e., including recirculated seawater, tidal pumping, and wave action) to the system exist. Such estimates are especially important in view of the current and predicted changes in sea level, rainfall, and demand on freshwater sources in the bay (Knowles and Cayan 2002).…”

“…SGD flux calculations are sensitive to the selected size of the box; for example, if excess Ra is elevated to the same level at 30 m offshore, the calculated flux would be 45 % higher. If the discharge zone extends more than 20 m offshore, our calculations will constitute a conservative estimate particularly because about 80 % of the bay is classified as shallow shoals (Hammond et al 1985). These nearshore fluxes were extrapolated to the entire bay basin (South or Central) assuming these sites are representative of similar settings along each bays' coastlines.…”

“…Previous work in San Francisco Bay (SFB) has modeled Rn benthic exchange from sediments to the overlying water column with focus on molecular diffusion and bioirrigation (Hammond and Fuller 1979;Hartman and Hammond 1984;Hammond et al 1985). Based on Rn porewater profiles, the studies demonstrate that advective processes (i.e., irrigation) must occur in addition to molecular diffusion.…”

“…Based on Rn porewater profiles, the studies demonstrate that advective processes (i.e., irrigation) must occur in addition to molecular diffusion. Porewater is exchanged to depths of >40 cm in the sediments at some locations in SFB with advective flux estimated to account for 60 % of benthic exchange when considering bioirrigation compared to diffusional fluxes (Hammond et al 1985). These studies indicate that SGD, mostly as recirculated seawater discharged through bioirrigation of sediments in SFB, is prevalent; however, other processes that may contribute to SGD and associated nutrient fluxes such as tidal pumping, wave action, salt dispersion, and seasonal oscillations were not accounted for and could be important (Michael et al 2005).…”