Seasonal Dynamics of Benthic Foraminiferal Biocoenosis in the Tropical Saquarema Lagoonal System (Brazil) In the original article, João Marcelo Ballalai was inadvertently omitted as an author of this article. The author list is correct as reflected here.

Fatty Acids to Quantify Phytoplankton Functional Groups and Their Spatiotemporal Dynamics in a Highly Turbid Estuary Abstract Phytoplankton community composition expresses estuarine functionality and its assessment can be improved by implementing novel quantitative fatty acid–based procedures. Fatty acids have similar potential to pigments for quantifying phytoplankton functional groups but have been far less applied. A recently created dataset containing vast information on fatty acids of phytoplankton taxonomic groups was used as reference to quantify phytoplankton functional groups in the yet undescribed Guadalquivir River Estuary. Twelve phytoplankton groups were quantitatively distinguished by iterative matrix factor analysis of seston fatty acid signatures in this turbid estuary. Those phytoplankton groups including species unfeasible for visual identification (coccoid or microflagellated cells) could be quantified when using fatty acids. Conducting monthly matrix factor analyses over a period of 2 years and the full salinity range of the estuary indicated that diatoms dominated about half of the phytoplankton community spatiotemporally. The abundance of Cyanobacteria and Chlorophytes was inversely related to salinity and little affected by seasonality. Euglenophytes were also more abundant at lower salinity, increasing their presence in autumn–winter. Coccolithophores and Dinophytes contributed more to phytoplankton community at higher salinity and remained little affected by seasonality. Multivariate canonical analysis indicated that the structure of the estuarine phytoplankton community was most influenced by salinity; secondly influenced by water temperature, irradiance, and river flow; and unaffected by nutrients. Fatty acids are especially suited for phytoplankton community research in high turbid estuaries and generate outcomes in synergy with those derived from classical pigment assessments.

Instantaneous Effects of Sediment Resuspension on Inorganic and Organic Benthic Nutrient Fluxes at a Shallow Water Coastal Site in the Gulf of Finland, Baltic Sea Abstract Climate change is leading to harsher resuspension events in shallow coastal environments influencing benthic nutrient fluxes. However, we lack information on the quantitative connection between these fluxes and the physical forces. Two identical experiments that were carried out both in May and August provided novel knowledge on the instantaneous effects of resuspension with known intensity on the benthic dissolved inorganic (phosphate: DIP, ammonium: NH4+, nitrite+nitrate: NOx, silicate, DSi) and organic nutrient (phosphorus: DOP, nitrogen: DON, carbon: DOC) fluxes in the shallow soft bottoms of the archipelago of Gulf of Finland (GoF), Baltic Sea. Resuspension treatments, as 2 times the critical shear stress, induced effluxes of one to two orders of magnitude higher than the diffusive fluxes from the studied oxic bottoms. The presence of oxygen resulted in newly formed iron oxyhydroxides and the subsequent precipitation/adsorption of the redox-dependent nutrients (DIP, DSi, organic nutrients) affecting their fluxes. Resuspension-induced NH4+ and NOx fluxes were associated with the organic content of sediments showing the highest values at the organic rich sites. NH4+ showed the strongest responses to resuspension treatments in August, but NOx at the time of high oxygen concentrations in near-bottom water in May. Foreseen increases in the frequency and intensity of resuspension events due to climate change will most likely enhance the internal nutrient loading of the studied coastal areas. The fluxes presented here, connected to known current velocities, can be utilized in modeling work and to assess and predict the internal nutrient loading following climate change.

Interactive Effects of Seagrass and the Microphytobenthos on Sediment Suspension Within Shallow Coastal Bays Abstract The suspension and transport of sediments in coastal environments influences water column clarity, and also affects the growth of photosynthetic organisms. The presence of benthic vegetation, such as seagrass, can attenuate wave and tidal energy, thereby altering suspended sediment concentrations (SSC) and microphytobenthos (MPB) biomass that secrete biogenic compounds that can increase sediment cohesion. The dual role of seagrass and MPB in altering the seasonal critical bed shear stress, τc, necessary to suspend sediment was studied within a Zostera marina seagrass meadow and an adjacent unvegetated region within a shallow coastal bay in Virginia, USA. Hydrodynamics and MPB biomass were recorded seasonally to determine the critical bed shear stress and subsequent SSC response. Results show that seagrasses reduced mean currents and waves, thus lowering SSC within the meadow. In addition, seagrass created favorable conditions for MPB growth, with annual mean sediment carbohydrate concentrations, a proxy for MPB activity, to be double within the seagrass compared to the unvegetated site. Sediment carbohydrate concentrations within the seagrass bed were higher during winter than summer due to enhanced light penetration, which coincided with an increase in τc to 0.056 Pa compared to 0.024 Pa. τc was found to be 0.021 Pa at the unvegetated site, with bed shear exceeding this threshold > 85% of the time. These findings suggest both MPB and seagrass play an important and interactive role in regulating seasonal sediment resuspension, and constant reworking of the bed sediments in high shear regions prevented the establishment of MPB.

Influences of a River Dam on Delivery and Fate of Sediments and Particulate Nutrients to the Adjacent Estuary: Case Study of Conowingo Dam and Chesapeake Bay Abstract Dams impact the magnitude and nature of material transport through rivers to coastal waters, initially trapping much material in upstream reservoirs. As reservoirs fill, trapping decreases and bottom sediments can be scoured by high flows, increasing downstream delivery. This is the case for the Conowingo Dam, which historically has trapped much of the sediment and particulate nutrients carried by the Susquehanna River otherwise bound for Chesapeake Bay but has now reached dynamic equilibrium. While previous studies primarily focus on either delivery of river inputs or their fate in the Bay, this study synthesizes insights from field observations and modeling along the Reservoir-Bay continuum to evaluate potential impacts of infilling on Bay biogeochemistry. Results show most Susquehanna sediment and particulate nutrient loading occurs during high-flow events that occur only ~ 10% of the time. While loading during these events has increased since the late 1970s, consistent with a decreasing scour threshold for Reservoir sediments, loading during low-flow periods has declined. Loads entering the estuary are largely retained within the upper Bay but can be transported farther downstream during events. Reservoir sediments are highly refractory, and inputs of reservoir-like organic matter do not enhance modeled sediment-nutrient release in upper Bay sediments. These findings and an emerging literature highlight the Bay’s resilience to large sediment loads during events (e.g., Tropical Storm Lee in 2011), likely aided by ongoing restoration efforts and/or consistently low-moderate recent inflows (2012–2017). Thus, while events can have major short-term impacts, the long-term impact to Bay biogeochemistry is less severe.

Variability of the Thermohaline Field in a Large Tropical, Well-Mixed Estuary: the Influence of an Extreme Draught Event Abstract Todos os Santos Bay (BTS) is one of eleven large estuaries along the semi-arid Brazilian northeast coast. It is a positive, well-mixed estuary undergoing progressive fresh water limitations due to natural climatic processes and direct human intervention to the fluvial hydrology. This study investigates the variability of the thermohaline field along three water years (2013–2015) that encompassed close-to-normal hydric conditions and the most severe regional drought in the historical record. Moored temperature and conductivity sensors, monthly CTD profiling along the bay axis, and historical records of meteorology and river discharges showed that the seasonal oscillations of the thermohaline field are 2- to 3-fold larger than higher, tidal frequency oscillations, and that BTS becomes a seasonally negative, hypersaline estuary during a drought. Also, a density plug and inverse estuarine circulation can arise with a delay of the rainy season. Negative ecological impacts have been associated with such hydrographic conditions. Long-term climatic trends and future climatic projections indicate that the BTS and its catchment area may undergo more frequent and acute droughts, suggesting that the bay may often become a seasonally negative estuary.

Assessing Nearshore Nekton Abundance, Substrate, and Environmental Conditions in the Northern Gulf of Mexico: Are There Differences Among Three Adjacent Coastal Areas and Have There Been Changes over Three Decades (1986–2015)? Abstract Fishery-independent data on fishes and crustaceans collected in spring and fall over three decades (1986–2015) from coastal areas of southeastern Louisiana, Mississippi, and Alabama were analyzed to determine if these areas differed in species composition and environmental conditions over this period. Multivariate community analyses revealed significant differences in species composition among the three areas in trawl collections for both spring (ANOSIM, R = 0.543, p < 0.001) and fall (R = 0.722, p < 0.001), while seine collections were not significantly different among the areas for either season (spring, R < − 0.06, p = 0.61; fall, R < 0.167, p = 0.14). The most important factor contributing to these differences was the presence of more shell substrate at the Louisiana sites (LINKTREE analysis, B% = 86, p < 0.05). Abundance data for common species were used to test for changes over the three decades. Blue crabs (Callinectes sapidus) and least puffers (Sphoeroides parvus) experienced decreases in four of eight area–season–gear scenarios. Four species of flatfishes also experienced multiple declines. Salinity increased at the Louisiana trawl sites over the period in both spring (+ 2.92; ANOVA, p < 0.001) and fall (+ 5.97; ANOVA, p = 0.001–0.002), while spring trawl sites became warmer in Mississippi (+ 2.15 °C; ANOVA, p = 0.001–0.002). Alabama trawl sites became warmer in both spring (+ 3.36 °C; ANOVA, p < 0.001) and fall (+ 1.91 °C; ANOVA, p < 0.001). With declines in species and changes in environmental conditions, this region faces multiple challenges in maintaining its estuarine fisheries.

Shoreline Retraction and the Opening of a New Inlet: Implications on Estuarine Processes Abstract The dynamics of estuarine systems is sensitive to changes in its forcing conditions, including the morphology of its inlets. Coastline retraction, which may be induced by climate change, can result in modifications of estuarine inlet morphology. Through the use of a validated numerical model, we evaluate the effects of the opening of a new inlet on a tide-dominated estuary (Caravelas estuary, Brazil). During the last decades, shoreline retraction and the breach of an internal drainage channel led to the formation of a new inlet that became the main estuarine channel. The morphological changes of the estuary resulted in changes to its estuarine processes, including the general increase in the influence of the tide on the system and changes to its asymmetry. Internal channels that interconnect adjacent estuaries present great changes caused by the morphological alterations, not only in the magnitude of the processes but also in the resulting net transport direction. The increase in the water flow caused by the opening of the channel leads to an increase in the amount of water and materials carried toward the estuary. The changes presented here for the Caravelas estuarine system and the possible implications for the functioning of such systems demonstrate the importance of evaluating morphological aspects in relation to their use and management.

Soil Structure and Its Relationship to Shallow Soil Subsidence in Coastal Wetlands Abstract Accelerating sea-level rise poses a threat to mangroves and salt marshes. Sediment accretion on the soil surface and belowground root production is proposed to increase soil elevation enabling these intertidal habitats to maintain their position relative to mean sea level. However, shallow soil subsidence is frequently observed, which may cancel out or even outweigh the positive effects of surface accretion and root production on soil elevation gains. The processes that lead to shallow soil subsidence have not often been investigated. Here, we used computed tomography (CT) imaging to determine the volume of coarse pores in the top 20 cm of 30-cm-deep soil cores collected from mangroves and salt marshes within Moreton Bay, eastern Australia. The four field sites have long-term observations of variation in soil surface elevation gain, surface accretion rate, and shallow subsidence rate using the rod surface elevation table-marker horizon (RSET-MH) method. Over our sites, we found that the rates of shallow soil subsidence were positively related to the volume of soil coarse pores and soil clay and silt content rather than to the volume of fine pores or total porosity or other soil properties. In turn, the volume of soil coarse pores was positively related to the volume of animal burrows and root mass. Fine-pore volume and total porosity of soils were negatively associated with soil bulk density, which was in turn negatively related to soil organic matter content. Our results indicate influences of differently sized soil pores in shallow soil subsidence and suggest that animal burrowing may play a role in shallow soil subsidence.

Recent Changes in Nitrogen Sources and Load Components to Estuaries of the Contiguous United States Abstract Regional Spatially Referenced Regressions on Watershed models were used to update 2002 delivered nitrogen (N) loads to estuaries of the contiguous US for 2011, supplemented by direct estuarine atmospheric deposition from the Community Multiscale Air Quality Model. Median 2011 watershed N yields were greatest for the Puget Trough, Virginian, and Oregon–Washington–Vancouver Coast marine ecoregions (MEs; 13.7, 11.0, and 9.9 kg N/ha watershed/year, respectively); intermediate for the Floridian, Southern California Bight, and Northern California MEs (4.4–6.3 kg N/ha watershed/year); and lowest for the Northern Gulf of Mexico, Carolinian, and Gulf of Maine MEs (2.4–3.2 kg N/ha watershed/year). Dominant sources varied across marine ecoregions, with direct atmospheric deposition as the dominant source only in the far northern Gulf of Maine ME. Delivered N loads from atmospheric deposition have significantly decreased (p < 0.05) for most estuaries on the Atlantic and Gulf coasts for 2002–2012. Estimated point source delivered N loads for 2002–2012 increased for most estuaries with upstream treatment plants, with estimated loads to only seven estuaries decreasing by more than 50%. Urban runoff increased for most estuaries in the Puget Trough and Carolinian MEs and either increased or had no significant trend for the remaining marine ecoregions. The magnitude of change in total N delivered loads is uncertain due to incomplete monitoring for most minor dischargers. In areas with increased population growth and decreases in agricultural land, decreasing agricultural fertilizer inputs have been insufficient to offset increases in urban runoff.