Effect of nutrient loading on biogeochemical and microbial processes in a New England salt marsh

Coastal marshes represent an important transitional zone between uplands and estuaries. One important function of marshes is to assimilate nutrient inputs from uplands, thus providing a buffer for anthropogenic nutrient loads. We examined the effects of nitrogen (N) and phosphorus (P) fertilization on biogeochemical and microbial processes during the summer growing season in a Spartina patens (Aiton (Muhl.) marsh in the Narragansett Bay National Estuarine Research Reserve on Prudence Island (RI). Quadruplicate 1 m² plots were fertilized with N and P additions, N-only, P-only, or no additions. N-only addition significant stimulated bacterial production and increased pore water NH₄⁺ and NO₃⁻ concentrations. Denitrification rates ranged from 0 to 8 mmol m⁻² day⁻¹ . Fertilization had no apparent effect on soil oxygen consumption or denitrification measured in the summer in intact cores due to high core-to-core variation. P fertilization led to increased pore water dissolved inorganic phosphorus (DIP) concentrations and increased DIP release from soils. In contrast the control and N-only treatments had significant DIP uptake across the soil-water interface. The results suggest that in the summer fertilization has no apparent effect on denitrification rates, stimulates bacterial productivity, enhances pore water nutrient concentrations and alters some nutrient fluxes across the marsh surface.Journal Articl

Effects of irradiance on benthic and water column processes in a Gulf of Mexico estuary: Pensacola Bay, Florida, USA

We examined the effect of light on water column and benthic fluxes in the Pensacola Bay estuary, a riverdominated system in the northeastern Gulf of Mexico. Measurements were made during the summers of 2003 and 2004 on 16 dates distributed along depth and salinity gradients. Dissolved oxygen fluxes were measured on replicate sediment and water column samples exposed to a gradient of photosynthetically active radiation. Sediment inorganic nutrient (NH₄⁺, NO₃⁻, PO₄⁻³) fluxes were measured. The response of dissolved oxygen fluxes to variation in light was fit to a photosynthesis–irradiance model and the parameter estimates were used to calculate daily integrated production in the water column and the benthos. The results suggest that shoal environments supported substantial benthic productivity, averaging 13.6 ± 4.7 mmol O₂ m⁻² d⁻¹, whereas channel environments supported low benthic productivity, averaging 0.5 ± 0.3 mmol O₂ m⁻² d⁻¹ (±SE). Estimates of baywide microphytobenthic productivity ranged from 8.1 to 16.5 mmol O₂ m⁻² d⁻¹, comprising about 16–32% of total system productivity. Benthic and water column dark respiration averaged 15.2 ± 3.2 and 33.6 ± 3.7 mmol O₂ m⁻² d⁻¹, respectively. Inorganic nutrient fluxes were generally low compared to relevant estuarine literature values, and responded minimally to light exposure. Across all stations, nutrient fluxes from sediments to the water column averaged 1.11 ± 0.98 mmol m⁻² d⁻¹ for NH₄⁺, 0.58 ± 1.08 mmol m⁻² d⁻¹ for NO₃⁻, 0.01 ± 0.09 mmol m⁻² d⁻¹ for PO₄⁻³. The results of this study illustrate how light reaching the sediments is an important modulator of benthic nutrient and oxygen dynamics in shallow estuarine systems.Journal ArticleFinal article publishe

Benthic nutrient flux in a small estuary in northwest Florida (USA)

Benthic nutrient fluxes of ammonium (NH₄⁺), nitrite/nitrate (NO₂⁻ + NO₃⁻), phosphate (PO₄⁻³), and dissolved silica (DSi) were measured in Escambia Bay, an estuary within the larger Pensacola Bay system of northwestern Florida (USA). Our study occurred during a severe drought which reduced riverine inputs to Escambia Bay. Laboratory incubations of field-collected cores were conducted on 8 dates between June and October 2000 to estimate nutrient flux, and cores were collected from locations exhibiting a range of sediment organic matter content. NH₄⁺ flux ranged from – 48.1 to 110.4 μmol m⁻² h⁻¹, but the mean flux was 14.6 μmol m⁻² h⁻¹. Dissolved silica (DSi) fluxes were also variable (– 109. 3 to 145.3 μmol m⁻² h⁻¹), but the mean net flux (9.3 μmol m⁻² h⁻¹) was from the sediment to the water column. Bay sediment fluxes for NO₂⁻ + NO₃⁻ and PO₄⁻³ were less variable during this period (– 7.93 to 28.73 and – 1.74 to 3.29 μmol m⁻² h⁻¹ for NO₂⁻ + NO₃⁻ and PO₄⁻³, respectively). Low NH₄⁺ fluxes were similar to published estimates from lagoonal Gulf of Mexico (GOM) estuaries, possibly due to the reduced freshwater input. Diminished regeneration of phosphate relative to inorganic nitrogen observed during the study period was consistent with previous research in Pensacola Bay suggesting phytoplankton phosphorus limitation. Finally, the estimated residence time of Escambia Bay and the mean turnover times for NH₄⁺ and NO₂⁻ + NO₃⁻ suggested that benthic flux significantly influenced nitrogen concentrations in overlying water.Journal ArticleFinal article publishe

Patterns in phytoplankton and benthic production on the shallow continental shelf in the northeastern Gulf of Mexico

Shallow continental shelves support productive pelagic and benthic communities. This study examined primary productivity at a shallow shelf region in the northeastern Gulf of Mexico focusing on the effect of light on water column and benthic productivity at water depths between 12 and 17 m. Measurements were made between November 2015 and September 2016. Dissolved oxygen fluxes were measured using benthic chambers with four different light levels and used to calculate gross primary production and respiration. Phytoplankton productivity was measured using ¹⁴C-uptake incubations in a laboratory photosynthetron. Organic matter production by benthic microalgae is substantial in this region of northeastern Gulf of Mexico with daily production rates ranging from 0.1 to 0.8 g C m⁻² d⁻¹ in this study. Maximum rates of phytoplankton production up to 2.7 g C m⁻² d⁻¹ occurred in spring. This peak productivity followed wind conditions favorable to upwelling and occurred when bottom water NO₃⁻ concentrations were 11 times greater than on any other sample date during the study. At these shallow depths, benthic microalgae made a significant contribution to total shelf production, averaging about 14% of total production. These results helped characterize benthic and water column production rates prior to planned habitat alterations caused by placement of numerous artificial reef structures in the region.Journal Articl

Seasonal oxygen dynamics in a warm temperate estuary: Effects of hydrologic variability on measurements of primary production, respiration, and net metabolism

Seasonal responses in estuarine metabolism (primary production, respiration, and net metabolism) were examined using two complementary approaches. Total ecosystem metabolism rates were calculated from dissolved oxygen time series using Odum’s open water method. Water column rates were calculated from oxygen-based bottle experiments. The study was conducted over a spring-summer season in the Pensacola Bay estuary at a shallow seagrass-dominated site and a deeper bare-bottomed site. Water column integrated gross production rates more than doubled (58.7 to 130.9 mmol O₂ m⁻² day⁻¹) from spring to summer, coinciding with a sharp increase in water column chlorophyll-a, and a decrease in surface salinity. As expected, ecosystem gross production rates were consistently higher than water column rates but showed a different spring-summer pattern, decreasing at the shoal site from 197 to 168 mmol O₂ m⁻² day⁻¹ and sharply increasing at the channel site from 93.4 to 197.4 mmol O₂ m⁻² day⁻¹. The consistency among approaches was evaluated by calculating residual metabolism rates (ecosystem − water column). At the shoal site, residual gross production rates decreased from spring to summer from 176.8 to 99.1 mmol O₂ m⁻² day⁻¹ but were generally consistent with expectations for seagrass environments, indicating that the open water method captured both water column and benthic processes. However, at the channel site, where benthic production was strongly light-limited, residual gross production varied from 15.7 mmol O₂ m⁻² day⁻¹ in spring to 86.7 mmol O₂ m⁻² day⁻¹ in summer. The summer rates were much higher than could be realistically attributed to benthic processes and likely reflected a violation of the open water method due to water column stratification. While the use of sensors for estimating complex ecosystem processes holds promise for coastal monitoring programs, careful attention to the sampling design, and to the underlying assumptions of the methods, is critical for correctly interpreting the results. This study demonstrated how using a combination of approaches yielded a fuller understanding of the ecosystem response to hydrologic and seasonal variability.Journal Articl