EDITORIAL article Front. Ecol. Evol., 04 August 2023Sec. Biogeography and Macroecology Volume 11 - 2023 | https://doi.org/10.3389/fevo.2023.1258317

Estuaries worldwide are undergoing changes to patterns of aquatic productivity because of human activities that alter flow, impact sediment delivery and thus the light field, and contribute nutrients and contaminants like pesticides and metals. These changes can influence phytoplankton communities, which in turn can alter estuarine food webs. We used multiple approaches-including high-resolution water quality mapping, synoptic sampling, productivity and nitrogen uptake rates, Lagrangian parcel tracking, enclosure experiments and bottle incubations-over a short time period to take a "spatial snapshot" of conditions in the northern region of the San Francisco Estuary (California, USA) to examine how environmental drivers like light availability, nutrients, water residence time, and contaminants affect phytoplankton abundance and community attributes like size distribution, taxonomic structure, and nutrient uptake rates. Zones characterized by longer -residence time (15-60 days) had higher chlorophyll-a concentrations (9 +/- 4 mu g L-1) and were comprised primarily of small phytoplankton cells (<5 mu m, 74 +/- 8%), lower ammonium concentrations (1 +/- 0.8 mu M), higher nitrate uptake rates, and higher rates of potential carbon productivity. Conversely, zones characterized by shorter residence time (1-14 days) had higher ammonium concentration (13 +/- 5 mu M) and lower chlorophyll-a concentration (5 +/- 1 mu g L-1) with diatoms making up a larger percent contribution. Longer residence time, however, did not result in the accumulation of large (>5 mu m) cells considered important to pelagic food webs. Rather, longer residence time zones had a phytoplankton community comprised primarily of small cells, particularly picocyanobacteria that made up 38 +/- 17% of the chlorophyll-a - nearly double the concentration seen in shorter residence time zones (22 +/- 7% picocyanobacterial of chlorophyll-a). Our results suggest that water residence time in estuaries may have an effect as large or larger than that experimentally demonstrated for light, contaminants, or nutrients. Published by Elsevier B.V.

The hypoxic zone on the Louisiana Continental Shelf (LCS) forms each summer due to nutrient enhanced primary production and seasonal stratification associated with freshwater discharges from the Mississippi/Atchafalaya River Basin (MARB). Recent field studies have identified highly productive shallow nearshore waters as an important component of shelf-wide carbon production contributing to hypoxia formation. In this study we present results from a three-dimensional hydrodynamic-biogeochemical model named CGEM (Coastal Generalized Ecosystem Model) applied to quantify the spatial and temporal patterns of hypoxia, carbon production, respiration, and transport between nearshore and middle shelf regions where hypoxia is most prevalent. We first demonstrate that our simulations successfully reproduced spatial and temporal patterns of carbon production, respiration, and bottom-water oxygen gradients compared to field observations. We then used interannual simulations to identify transport of particulate organic carbon (POC) from nearshore areas where riverine organic matter and phytoplankton carbon production are greatest. The spatial disconnect between carbon production and respiration in our simulations was driven by westward and offshore POC flux, a pattern that supported heterotrophic respiration on the middle shelf where hypoxia is frequently observed. These results validate the importance of offshore carbon flux to hypoxia formation, particularly on the west shelf where hypoxic conditions are more variable.

Benthic invertebrate community composition was surveyed across the salinity gradient of the Pensacola Bay Estuary in Florida during summer 2016. Macrofauna densities ranged from 1000 to 9300 individuals m(-2), with highest densities occurring at the upper estuary and the lowest in the mid- and lower estuary. Taxonomic richness and Shannon diversity were lowest in the upper estuary and increased along the salinity gradient. Small-bodied, near-surface infaunal polychaete species (e.g., Mediomastus ambiseta and Paraprionospio alata) dominated the macrofaunal community in fine sediment areas. We calculated the Gulf of Mexico Benthic Index of Biological Integrity for each site and compared the index scores with those from Environmental Monitoring and Assessment Program - Estuaries, an earlier benthic assessment model. Condition evaluations by the different models did not match across all sites in this study; however, scores consistently indicated that most sites were at or near degraded levels, implying that Pensacola Bay represents a marginal habitat for a "healthy" benthic macrofauna community. This study provided new information about the benthic communities and sediments in the Pensacola Bay estuary. Integr Environ Assess Manag 2020;16:245-256. Published 2019. This article is a US Government work and is in the public domain in the USA.

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 C-14-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.

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 d-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 d-1 and sharply increasing at the channel site from 93.4 to 197.4 mmol O2 m-2 d-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 d-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 d-1 in spring to 86.7 mmol O2 m-2 d-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.

Observed bio-optical water quality data collected from 2009 to 2011 in Pensacola Bay, Florida were used to develop empirical remote sensing retrieval algorithms for chlorophyll a (Chla), colored dissolved organic matter (CDOM), and suspended particulate matter (SPM). Time-series of the three bio-optical water quality variables were generated from MEdium Resolution Imaging Spectrometer (MERIS) observations from 2003 to 2011. Bio-optical water quality in this estuary exhibited spatial and temporal variations that were correlated to river discharge and wind. Both annual mean and monthly mean bio-optical water quality variables were positively correlated to river discharge. Monthly mean bio-optical water quality variables were also positively correlated to wind speed and wind density (defined by the number of days with daily mean wind speed > 3 m s(-1) in a month) over this estuary. These results indicate that bio-optical water quality dynamics in this estuary are vulnerable to changes in river discharge and river constituent loads and local weather conditions such as winter storms and hurricanes. (C) 2016 Elsevier Ltd. All rights reserved.

Nutrient inputs to the Louisiana continental shelf (LCS) from lateral ocean boundaries can be significant, but the effect of these nutrients on LCS primary production has not been examined. Herein, we apply a three-dimensional physical-biogeochemical model to calculate nitrogen and phosphorus mass balances on the LCS and quantify the contributions of riverine and offshore nutrient inputs to primary production. A model sensitivity analysis to different offshore nutrient concentrations indicated that modeled primary production was most sensitive to boundary nitrogen concentrations, whereas changing boundary phosphorus concentrations had little effect. The primary production response also varied spatially and temporally, with its greatest response being to changing boundary nitrogen concentrations in areas furthest from the river plume, and during the late summer for all regions of the shelf when Mississippi River discharge approaches its annual minimum. These results indicate that even for river-dominated shelves like the LCS, uncertain boundary condition nutrient concentrations are likely to contribute significantly to uncertainty in modeled primary production. The modeling study highlights the need for further observational studies to understand the sources and variability of nutrients at LCS offshore boundaries and the impacts to LCS primary production. Published by Elsevier B.V.

In aquatic systems, time series of dissolved oxygen (DO) have been used to compute estimates of ecosystem metabolism. Central to this open-water method is the assumption that the DO time series is a Lagrangian specification of the flow field. However, most DO time series are collected at fixed locations, such that changes in DO are assumed to reflect metabolism and that effects of advection or mixing are negligible. A weighted regression model was applied to remove variability in DO time series from tides, thereby helping to partially relax this assumption and improve metabolism estimates. The method offers a distinct advantage over traditional deconvulution methods by targeting the periodicity of the tidal component while preserving the true biological signal. The model was first applied to simulated DO time series with specified biological and physical characteristics, and then applied to 1 yr of continuous monitoring data from four stations within the National Estuarine Research Reserve System. The correlation of DO and metabolism estimates with tides was greatly reduced after using weighted regression. The model was especially effective when the magnitude of tidal influence was high and correlations between tidal change and the solar cycle were low at the time scales of interest. The model was less robust when tides and the solar cycle were correlated for protracted periods. By reducing the effects of physical transport on metabolism estimates, there may be increased potential to empirically relate metabolic rates to causal factors on timescales of several days to several weeks.

The Louisiana shelf, in the northern Gulf of Mexico, receives large amounts of freshwater and nutrients from the Mississippi–Atchafalaya river system. These river inputs contribute to widespread bottom-water hypoxia every summer. In this study, we use a physical–biogeochemical model that explicitly simulates oxygen sources and sinks on the Louisiana shelf to identify the key mechanisms controlling hypoxia development. First, we validate the model simulation against observed dissolved oxygen concentrations, primary production, water column respiration, and sediment oxygen consumption. In the model simulation, heterotrophy is prevalent in shelf waters throughout the year, except near the mouths of the Mississippi and Atchafalaya rivers, where primary production exceeds respiratory oxygen consumption during June and July. During this time, efflux of oxygen to the atmosphere, driven by photosynthesis and surface warming, becomes a significant oxygen sink. A substantial fraction of primary production occurs below the pycnocline in summer. We investigate whether this primary production below the pycnocline is mitigating the development of hypoxic conditions with the help of a sensitivity experiment where we disable biological processes in the water column (i.e., primary production and water column respiration). With this experiment we show that below-pycnocline primary production reduces the spatial extent of hypoxic bottom waters only slightly. Our results suggest that the combination of physical processes (advection and vertical diffusion) and sediment oxygen consumption largely determine the spatial extent and dynamics of hypoxia on the Louisiana shelf.

River-dominated continental shelf environments are active sites of air-sea CO2 exchange. We conducted 13 cruises in the northern Gulf of Mexico, a region strongly influenced by fresh water and nutrients delivered from the Mississippi and Atchafalaya River system. The sea surface partial pressure of carbon dioxide (pCO(2)) was measured, and the air-sea CO2 flux was calculated. Results show that CO2 exchange exhibited a distinct seasonality: the study area was a net sink of atmospheric CO2 during spring and early summer, and it was neutral or a weak source of CO2 to the atmosphere during midsummer, fall, and winter. Along the salinity gradient, across the shelf, the sea surface shifted from a source of CO2 in low-salinity zones (0S<17) to a strong CO2 sink in the middle-to-high-salinity zones (17S<33), and finally was a near-neutral state in the high-salinity areas (33S<35) and in the open gulf (S35). High pCO(2) values were only observed in narrow regions near freshwater sources, and the distribution of undersaturated pCO(2) generally reflected the influence of freshwater inputs along the shelf. Systematic analyses of pCO(2) variation demonstrated the importance of riverine nitrogen export; that is, riverine nitrogen-enhanced biological removal, along with mixing processes, dominated pCO(2) variation along the salinity gradient. In addition, extreme or unusual weather events were observed to alter the alongshore pCO(2) distribution and to affect regional air-sea CO2 flux estimates. Overall, the study region acted as a net CO2 sink of 0.963.7 mol m(-2) yr(-1) (1.154.4 Tg C yr(-1)).

Large-scale hypoxia regularly develops during the summer on the Louisiana continental shelf. Traditionally, hypoxia has been linked to the vast winter and spring nutrient inputs from the Mississippi River and its distributary, the Atchafalaya River. However, recent studies indicate that much of the shelf ecosystem is heterotrophic. We used data from five late July shelfwide cruises from 2006 to 2010 to examine carbon and oxygen production and identify net autotrophic areas of phytoplankton growth on the Louisiana shelf. During these summer times of moderate river flows, shelfwide pH and particulate organic carbon (POC) consistently showed strong signals for net autotrophy in low salinity (<25) waters near the river mouths. There was substantial POC removal via grazing and sedimentation in near-river regions, with 66–85 % of POC lost from surface waters in the low and mid-salinity ranges without producing strong respiration signals in surface waters. This POC removal in nearshore environments indicates highly efficient algal retention by the shelf ecosystem. Updated carbon export calculations for local estuaries and a preliminary shelfwide carbon budget agree with older concepts that offshore hypoxia is linked strongly to nutrient loading from the Mississippi River, but a new emphasis on cross-shelf dynamics emerged in this research. Cross-shelf transects indicated that river-influenced nearshore waters <15 m deep are strong sources of net carbon production, with currents and wave-induced resuspension likely transporting this POC offshore to fuel hypoxia in adjacent mid-shelf bottom waters.

A monthly time series of remotely sensed chlorophyll-a (Chla(rs)) over the Louisiana continental shelf (LCS) was developed and examined for its relationship to river discharge, nitrate concentration, total phosphorus concentration, photosynthetically available radiation (PAR), wind speed, and interannual variation in hypoxic area size. A new algorithm for Chla(rs), tuned separately for clear and turbid waters, was developed using field-observed chlorophyll-a (Chla(obs)) collected during 12 cruises from 2002 to 2007. The new algorithm reproduced Chla(obs), with approximate to 40% and approximate to 60% uncertainties at satellite pixel level for clear offshore waters and turbid nearshore waters, respectively. The algorithm was then applied to SeaWiFS and MODIS images to calculate long-term (1998-2013) monthly mean Chla(rs) estimates at 1 km resolution across the LCS. Correlation and multiple stepwise regression analyses were used to relate the Chla(rs) estimates to key environmental drivers expected to influence phytoplankton variability. The Chla(rs) time series covaried with river discharge and nutrient concentration, PAR, and wind speed, and there were spatial differences in how these environmental drivers influenced Chla(rs). The main axis of spatial variability occurred in a cross-shelf direction with highest Chla(rs) observed on the inner shelf. Both inner (<10 m depth) and middle-shelf (10-50 m depth) Chla(rs) were observed to covary with interannual variations in the size of the hypoxic (O-2<63 mmol m(-3)) area, and they explained approximate to 70 and approximate to 50% variability in interannual hypoxia size, respectively.Key Points A new hybrid Chla algorithm was developed Driving factors of phytoplankton dynamics exhibited spatial differences Phytoplankton dynamics had significant relationship with hypoxia variation

To investigate the relative importance of microphytobenthos (MPB) oxygen (O-2) production on a river-dominated shelf, we made sediment core incubation measurements of MPB O-2 production and sediment O-2 consumption, and compared these to water-column measures of primary production and respiration during one spring and two summer cruises. Sites were located across a light attenuation (Kd) gradient on the inner Louisiana shelf (<25 m depth) with three sites on the eastern shelf and one site on the western shelf. MPB production rates were highest (range = 0.7-33.4 mmol O-2 m(-2) d(-1)) at the western site where light attenuation was lowest (range = 0.13-0.16 m(-1)). Near the river plumes, where Kd was higher (range = 0.17-0.64 m(-1)), MPB production was lower (range = 0-2.0 mmol m(-2) d(-1)) MPB production influenced the magnitude and direction of sediment O-2 exchanges at the western site and was observed to be a function of light availability. Overall, though, in comparison with integrated bottom layer production and respiration rates, MPB photosynthetic O-2 production as a percentage was small (range of means = 0%-2%) at the eastern shelf sites. At the western site, MPB production was larger, and more variable, 39% (SE 34%) of bottom layer O-2 production. Though based on a small data set, results suggest distinct differences between eastern and western shelf Kd and MPB contributions to bottom water O-2 due to proximity to the freshwater discharges of the Mississippi and Atchafalaya rivers.