We measured ammonium and nitrate plus nitrite fluxes from 14 common sponge species on a Florida Keys reef (Conch Reef) using a combination of incubation experiments and an in situ method that requires no manipulation of the sponge. On a 600-m 2 section of Conch Reef, species-specific biomass for all nonencrusting sponges was measured. The biomass data combined with species-specific dissolved inorganic nitrogen (DIN) flux rates yielded the benthic DIN flux from 14 species, and allowed us to extrapolate these data to the total nonencrusting sponge community. The species for which we measured DIN fluxes represented 85% of the nonencrusting sponge biomass in the study area and released a combined 480 6 93 mmol m 22 h 21 of nitrate plus nitrite, and 57 6 73 mmol m 22 h 21 of ammonium. Approximately 73% of the measured DIN flux was produced by Xestospongia muta, a massive barrel sponge. Of the 14 species studied, 10 hosted active nitrifying communities, and 8 hosted photosynthetic microbial associates. However, the presence of these microbial communities had no apparent effect on the magnitude of the total DIN flux. We estimate that the DIN flux for the entire nonencrusting sponge community is 640 6 130 mmol m 22 h 21 .

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We present measurements of flows and fluxes of phytoplankton to Conch Reef, Florida, a Caribbean reef dominated by sponges and soft corals, located in 15 m of water offshore of Key Largo. Vertical profiles of chlorophyll a, a proxy for phytoplankton biomass, showed a near-bed depletion, indicating the existence of concentration boundary layers. Along with simultaneous measurements of velocity profiles, near-bed turbulence, and temperature stratification, these profiles were used to compute a, the mass transfer velocity of phytoplankton to the bed (i.e., the flux to the bed normalized by near-bed concentration). The a value ranged from 240 to +130 m d 21 , with a significant linear positive relationship with shear velocity. The median value of a 5 48 6 20 m d 21 is larger than would be expected, given the observed population of filter-feeding sponges. Nonetheless, these large values of a are consistent with values found recently for another coral reef as well as for a soft bottom estuarine community. Taken as a whole, these measurements indicate that reefs with large roughness and/or energetic currents should be able to support higher biomasses of benthic organisms than would low relief reefs or reefs in sluggish waters.

A series of laboratory-based and field experiments was conducted to address the effects of sunlight-exposed resuspended sediments on dissolved nutrient fluxes in two different water bodies. In suspensions of tidal creek sediments in 0.2 lm-filtered creek water, measurable increases in dissolved nutrients occurred after only 2 h of exposure to simulated sunlight. During a 6-h irradiation, nutrient release rates for total dissolved nitrogen (TDN) and phosphate were 2.2 ± 0.5 (standard error; S.E.) lmol g -1 h -1 and 0.09 ± 0.005 lmol g -1 h -1 (S.E.), compared to no significant changes in dark controls. The majority of nitrogen was released as dissolved organic nitrogen (87% on average) with lesser amounts of ammonium (13%). Continental shelf sediments resuspended in unfiltered seawater also released phosphate and TDN when exposed to sunlight, suggesting that this process can occur in a variety of marine and estuarine environments. The source material for inorganic nutrients appears to be associated with sediments rather than dissolved organic matter, as no significant changes in nutrient concentrations occurred in experiments with 0.2 lm-filtered creek water or seawater alone. Results suggest that photoproduction of dissolved nutrients from resuspended sediments could be an episodically significant and previously unrecognized source of dissolved organic and inorganic nutrients to coastal ecosystems. This process may be especially important for continental margins where episodic resuspension events occur, as well as in regions experiencing high riverine sediment fluxes resulting from erosion associated with deforestation and desertification.

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debated [11,12,14,15], but less research has been devoted to the effect of oyster reefs on the surrounding sediment. Reefs act to attenuate wave energy, possibly facilitating deposition of fine sediment [8]; this process may work in concert with oyster filtration to increase light penetration that may then shift ecosystems towards more benthic primary producers [6]. Finer particles and much higher organic matter (OM) content in oyster-associated sediments suggests a substantial role for carbon and nutrient removal by burial [8,17] and benthic algal uptake where light penetration is sufficient [6]. However, mesocosm experiments show that physical factors such as bottom shear can influence sediment resuspension and benthic micro-algal biomass, making the system more complex and the likelihood of OM burial versus remineralization more difficult to predict [7]. Several recent studies suggest that sediments associated with natural and restored oyster reefs have high rates of denitrification and may thus represent important sites for long term nitrogen removal [17][18][19][20]. Whole-creek studies and some mesocosm studies do not parse the contributions of the oysters themselves versus the associated sediments but rather consider the reef-sediment system as a whole [3,5,17]. Indeed, it is difficult to separate these effects because the presence of the reef will likely alter the depositional environment and ultimately the biogeochemistry of the surrounding sediment.

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