Seasonal patterns of benthic-pelagic coupling in oyster habitats

Abstract

Oysters enhance benthic-pelagic coupling in coastal systems by moving large quantities of suspended particulates to the sediments, stimulating biogeochemical processes. Recent research efforts have focused on quantifying the impact of oysters on coastal biogeochemical cycling, yet there is little consensus on how oysters influence processes across systems. A potential driver of this variance is availability of organic material suspended in the water column and subsequent loading to sediment by oysters. Here, we measured fluxes of sediment di-nitrogen (N2-N), ammonium (NH4+), combined nitrate-nitrite (NOx), and phosphate (PO43-) in spring, summer, and fall at 2 oyster reefs and 1 farm in a temperate estuary (Narragansett Bay, Rhode Island). We then linked these fluxes with patterns of water column primary production. Nitrogen removal from the system was highest in spring, when we detected net sediment denitrification (48.8 µmol N2-N m-2 h-1) following a winter-spring diatom bloom. In contrast, we measured sediment N2 fixation in fall (-44.8 µmol N2-N m-2 h-1) at rates nearly equivalent to spring denitrification. In the summer, we measured a nearly net zero sediment N2-N flux (-2.7 µmol N2-N m-2 h-1). Recycling of nitrogen to the water column was consistent across seasons, composed almost exclusively of NH4+. These results demonstrate that sediment nitrogen cycling in oyster habitats is dynamic and can change rapidly based on seasonal patterns of productivity. At carrying capacity, the impact of oysters on nitrogen cycling is large and should be considered during efforts to increase oyster populations through aquaculture or reef restoration.

Negligible Greenhouse Gas Release from Sediments in Oyster Habitats

“…CO 2 release was unexpectedly low in the fall when sediment organic matter was again high (3.65 ± 1.22%) and temperature was higher than that in the spring (17.9 ± 0.6 °C). This observation may possibly be due to organic matter of a relatively lower quality in fall than that in the spring . Neither sediment organic matter ( p = 0.756, R 2 = 0.002, and df = 41) nor temperature ( p = 0.319, R 2 = 0.021, and df = 48) was a good predictor of sediment CO 2 release in the seasonal study.…”

“…The seasonal patterns of CO 2 and N 2 O fluxes we observed are consistent within the context of seasonal patterns of phytoplankton productivity in Narragansett Bay, which has historically been characterized by a large winter–spring diatom bloom and lower productivity during the summer. , If oysters drive sediment GHG production by increasing organic matter deposition to the sediments during filter-feeding, it might be expected that the highest GHG fluxes will occur following large phytoplankton blooms. We were able to test this hypothesis in our seasonal incubations as the spring sediment core incubation was conducted shortly after the winter–spring bloom, summer core incubations were conducted following a period of low productivity, and fall core incubations were conducted following a smaller fall bloom . This water-column production impacted the sediments as we measured the highest sediment % organic matter in the spring (3.97 ± 1.00%) followed by the fall (3.65 ± 1.22%) and lowest % organic matter in the summer (2.74 ± 0.57%).…”

“…Upon return to the lab (within 6 h of core collection), cores were moved to a water bath in an environmental chamber set to the same temperature as the field site. We uncapped the cores and carboys and bubbled them gently overnight with aquarium pumps to maintain oxygenation in the overlying water. , …”

“…Two exetainers were reserved for the analysis of CH 4 and N 2 O concentrations and flux, and the other two were reserved for the analysis of O 2 concentration (and estimation of CO 2 flux). The five sample time points were spaced to ensure the O 2 concentration in each core (except the water control) dropped at least 2.0 mg L –1 without becoming hypoxic (O 2 concentration ≤ 2.0 mg L –1 ). , The incubation chamber was kept dark throughout the incubation, except for brief periods to collect samples and check dissolved oxygen concentration. Temperature in the chamber remained constant from the time cores were returned to the lab until incubations had ended.…”

A Conflict between the Legacy of Eutrophication and Cultural Oligotrophication in Hiroshima Bay

“…However, the oyster production in the bay is in a crisis due to the lack of feed phytoplankton, and recently, it has been observed that the oyster larvae cannot survive due to the lack of small-sized phytoplankton (<5 µm in diameter) suitable for their feed [43]. In contrast, oysters, through their feeding activity, stimulate biogeochemical processes in the sediments by increasing the sedimentation of organic matter from the water column [44,45]. Therefore, the sediment quality is muddy and contains vast amounts of refractory organic matter which have been historically deposited from the oyster culture via both oyster feces and dead oyster meat, and organisms attached on the shells sink to the bottom.…”