Estuarine Outflow Research Articles

Phytoplankton communities of the west coast of Florida – multiyear and seasonal responses to nutrient enrichment

Blooms of the harmful algae species Karenia brevis are frequent off the southwest coast of Florida despite having relatively slow growth rates. The regional frequency of these harmful algal blooms led to the examination of the dominant estuarine outflows for effects on both K. brevis and the phytoplankton community in general. There is comparatively little information on the growth rates of non-Karenia taxonomic groups other than diatoms. A seasonally based series (Fall, Winter, and Spring) of bioassay experiments were conducted to determine the nutrient response of the coastal phytoplankton community. Treatments included estuarine waters (Tampa Bay, Charlotte Harbor, and the Caloosahatchee River) applied in a 1:25 dilution added to coastal water to mimic the influence of estuarine water in a coastal environment. Other treatments were 5–15 μM additions of nitrogen (N), phosphorus (P), and silica (Si) species, amino acids, and N (urea) + P added to coastal water. Incubations were conducted under ambient conditions with shading for 48 h. Analyses of dissolved and particulate nutrients were coupled with HPLC analysis of characteristic photopigments and taxonomic assignments of biomass via CHEMTAX. The coastal phytoplankton community, dominated by diatoms, cyanophytes and prasinophytes, was significantly different both by bioassay and by season, indicating little seasonal fidelity in composition. Specific growth rates of chlorophyll a indicated no significant difference between any controls, any estuarine treatment, P, or Si treatments. Conditions were uniformly N-limited with the highest growth rates in diatom biomass. Despite differing initial communities, however, there were seasonally reproducible changes in community due to the persistent growth or decline of the various taxa, including haptophytes, cyanophytes, and cryptophytes. For the one bioassay in which K. brevis was present, the slow growth of K. brevis relative to diatoms in a mixed community was evident, indicating that identifying the seasonally based behavior of other taxa in response to nutrients is critical for the simulation of phytoplankton competition and the successful prediction of the region's harmful algal blooms.

Environmental drivers and ecological implications of a ctenophore swarm: a case study from the Bay of Bengal, northern Indian Ocean

Abstract Ctenophores are quick responders to coastal environmental changes and play a crucial role in marine food web dynamics. We report the environmental drivers of a ctenophore swarm (Pleurobrachia spp.) and associated ecological changes in estuarine and coastal waters of the Sundarbans mangrove–estuarine complex in the northern Bay of Bengal (BoB). Plankton and fish samples were collected from estuarine and coastal waters at the peak of estuarine outflow in the northeast monsoon (NEM). Sampling locations at the estuarine mouth witnessed ctenophores (Pleurobrachia pileus and Pleurobrachia globosa) in swarming proportions. Multivariate statistical analysis revealed significant differences in hydrographical and biological properties between the swarm and non-swarm areas. The sea surface salinity, ammonium, and mesozooplankton (MSP) density were positively correlated while microzooplankton (MCZ) density was negatively correlated with the ctenophore swarm. Fish caught from swarm locations, dominated by microbial food web consisted of species of less market value, while those from the non-swarm locations, dominated by conventional plankton food web consisted mainly of commercially important species. Thus, in the first-ever record from a commercially important fishing ground in the BoB, this study provides evidence of how a ‘dead end’ plankton predator affects the plankton food web organization and fisheries in tropical coastal waters.

Spatial and temporal dynamics of water isotopes in the riverine‐marine mixing zone along the German Baltic Sea coast

AbstractRiver estuaries are characterized by mixing processes between freshwater discharge and marine water masses. Since the first are depleted in heavier stable isotopes compared with the marine realm, estuaries often show a linear correlation between salinity and water stable isotopes (δ18O and δ2H values). In this study, we evaluated spatial and seasonal isotope dynamics along three estuarine lagoon transects, located at the northern German Baltic Sea coast. The data show strong seasonality of isotope values, even at locations located furthest from the river mouths. They further reveal a positive and linear salinity‐isotope correlation in spring, but ‐in two of the three studied transects‐ hyperbolic and partially reverse correlations in summers. We conclude that additional hydrological processes partially overprint the two‐phase mixing correlation during summers: aside from the isotope seasonality of the riverine inflows, the shallow inner lagoons in the studied estuaries are influenced by evaporation processes. In contrast the estuarine outflow regions are under impact of significant salinity and isotope fluctuations of the Baltic Sea. Deciphering those processes is crucial for the understanding of water isotope and salinity dynamics. This is also of relevance in context of ecological studies, for example, when interpreting oxygen and hydrogen isotope data in aquatic organisms that depend on ambient estuarine waters.

Wind‐Driven Currents in a “Wide” Narrow Channel, With Application to Douglas Channel, BC

AbstractThis paper applies a structured grid, 3‐Dimensional Regional Ocean Modeling System to examine wind‐driven currents of an idealized stratified channel, representative of Douglas Channel, British Columbia, Canada, where the increased marine activities require an improved understanding of the physical oceanography. The surface along‐channel elevation slope resulting from the wind stress is strongly affected by the surface stratification and can serve as a proxy for gauging surface stratification in operational systems. In the case of strong surface stratification, due to rotational effects an apparently narrow (width ≪ length) channel can be dynamically wide with pronounced cross‐channel variation. The thickness of the surface wind‐driven layer is scaled using the thermal wind relation, which provides a scaling factor to estimate the thickness of the surface layer. This scaling factor is not restricted to the wind‐driven flow and could be expanded to the surface mean estuarine outflow layers.

Impact of Mesoscale Circulation on the Structure of River Plumes During Large Rainfall Events Inshore of the East Australian Current

Estuarine outflow can have a significant impact on physical and ecological systems in the coastal ocean. Along southeastern Australia, inshore of the East Australian Current, the shelf is narrow, the coastal circulation is advection dominated, and river estuarine outflow tends to be low, hence river plumes have largely been ignored. For these reasons, we lack an understanding of the spatial and temporal evolution of river plumes during large rainfall events (which are projected to increase in frequency and intensity), and the interaction of the mesoscale circulation with the estuarine outflow remains to be explored. Using a high-resolution (750 m) hydrodynamic model, we simulate idealized plumes from 4 estuaries during three different mesoscale circulation scenarios and investigate the spatial and temporal evolution of the estuarine outflow under two contrasting rainfall events (normal and large). We explore the plume from the largest of the 4 rivers, the Hawkesbury River, to understand the impact of the mesoscale circulation. During the first EAC mode, the plume spreads both northward and southeastward. The offshore spread of the plume is the largest in this scenario (~12.5 km east of the river mouth) in the wet event. In the second EAC mode, this plume dispersal is toward the north and east, driven by the proximity of a cyclonic eddy on the shelf, with an eastward extension of 11 km. In the third EAC mode, most of this river plume spreads southward with some to the north, again dictated by the position of the cyclonic eddy. The cross-shelf dispersal is a minimum of 9.5 km from the river mouth. It takes around 6 days for the freshwater spatial extent of the plume in the wet event to return to the base case. These results show the importance of mesoscale EAC circulation on the shelf circulation when considering river plumes dispersal. Knowledge of the ultimate fate of riverborne material, dilution and cumulative effects will enable better environmental management of this dynamic region for the local government.

Coastal gradients of small microplastics and associated pollutants influenced by estuarine sources

Small microplastics (SMPs) in the gulf of Cadiz was sampled at 5 m depth by pumping it through the ship's pipe system and filtered through a 45 μm mesh size net. Our study reveals that higher densities have been found (130 mg·m−3) compared to other regions worldwide and these densities decreased from the coastline to the outer stations, showing a general coastal gradient influenced by estuarine outflows. SMPs with a size range between 45 and 193 μm were predominant and most of them composed by polyethylene and polypropylene. The metals associated with the MPs were mainly Na (21.1%), K (11.3%), Fe (8.5%), Ca (2.1%), Cr (1.8%), Zr (13.3%) and Hf (0.7%). The high proportion of Zr compared to Fe, which is different from what can be found in the environment, suggests that this metal is intrinsic to the materials used in catalytic processes during plastic production.

Coupled modelling of the interaction between dissolved substances emitted by Minho and Lima estuarine outflows (Portugal)

The Minho and Lima estuaries comprise important and sensitive protected areas valuable for species and habitats conservation, constituting a reference for ecotoxicological and water quality studies. These estuaries, separated only by 20 km, are very productive systems with high metabolic rates that strongly influence the cycle of nutrients and the ecology of NW Iberian coast. This study aims to understand the propagation of Minho and Lima estuarine plumes in order to investigate the effects of buoyancy on the interaction between dissolved substances emitted by their outflows and its impacts on regional productivity. Using the Delft3D model, a deeper analysis of the intrusion and dynamics of outflows of these adjacent estuaries was performed, through the simulation of trajectories of passive tracers emitted in each estuary, considering river discharges and wind stress. The results show an interaction between these estuaries, both during the summer and the winter seasons. Both estuarine outflows tend to move southward in response to the predominant north wind in the summer season, while in winter the opposite is observed. The northward outflows tend to be transported very close to the coastline, while the southward and westward is diverted offshore. The estuarine outflows interaction promotes exchanges of physical-chemical properties and nutrients between these estuaries, generating ecological advantages for the smaller one. These results showed that the numerical model used accurately describes the dynamical processes in these estuarine systems, being essential for a proper assessment of the effects of forcing drivers and providing relevant information for coastal zone planning and management.

Dissolved and particulate carbon export from a tropical mangrove‐dominated riverine system

AbstractDespite being a major component in the mangrove carbon (blue carbon) budget, “outwelling” flux (or export to the sea) has gained little attention relative to other biogeochemical fluxes and reservoir carbon stock estimations. This study aims to estimate lateral exchange fluxes of dissolved and particulate organic carbon (DOC, POC) and dissolved inorganic carbon (DIC) from the watershed through a microtidal mangrove‐dominated estuary to the coastal sea in Panay Island, Philippines. Along the estuarine transect, consistent addition of DOC, DIC, and POC at higher salinities were attributed to mangrove organic matter input. Upstream groundwater input (carbonate weathering) and downstream mangrove organic matter decomposition (possibly sulfate reduction) were the main controls on DIC. DOC corresponded to relatively pure mangrove sources in creek water, while POC was a mixture of detrital and algal organic matter. The mangrove system acted as net exporter of carbon to the sea in both dry and wet seasons. From short‐term observations, outwelling fluxes of mangrove‐derived DOC, DIC, and POC contributed 27–53%, 8–31%, and 42%, respectively, to their estuarine outflow. Unlike other studies, such low percentage for DIC might result from other external nonmangrove input (e.g., watershed carbonate weathering). Overall estuarine carbon flux was dominated by DIC (90–95%) with only minor contribution from DOC. The approach utilized in this study to estimate lateral carbon flux specific to a small mangrove setting can be useful in delineating blue carbon budgets that avoid geographical and methodological biases.

Nearshore response to cold air outbreaks in the Gulf of Mexico

The nearshore response to synoptic meteorological patterns called Cold Air Outbreaks (CAOs) was studied with a cross-shelf mooring array located 7 km west of an estuarine outflow in the Northern Gulf of Mexico. The array consisted of 6 stations that spanned the nearshore measuring waves, currents, temperature, and salinity for a two-week period in December 2013. CAOs are prevalent during winter and are characterized by the passage of atmospheric cold fronts with offshore winds known to cool coastal waters. The prefrontal phases of CAOs coincided with downwelling favorable winds and westward flow that introduced a river plume into the nearshore. Offshore winds during the high pressure phase formed a two-layer circulation with offshore flow in the surface layer and onshore flow beneath, which was moderated by a river plume. The downwind advection of the river plume tended to enhance surface transport until the plume was flushed out of the nearshore, which formed adverse density gradients that reduced surface transport through vertical mixing. When the plume was not fully flushed out of the inner shelf, surface transport remained enhanced during the high pressure phase due to stratification that limited the formation of adverse density gradients. When the plume was initially shore detached at the onset of the high pressure phase, offshore flow was not enhanced by the downwind advection of the plume due to the existence of initial adverse density gradients. These results highlight the effects of nearshore river plumes in regulating inner shelf circulation during cold fronts.

Response of a Coastal Plume Formed by Tidally Modulated Estuarine Outflow to Light Upwelling-Favorable Wind

AbstractThis study presents observations of a buoyant plume off Winyah Bay, South Carolina, which was formed under conditions of high freshwater discharge and upwelling-favorable wind forcing. Analysis of observations demonstrates that the response of the anticyclonic bulge formed by tidally modulated estuarine outflow to the light upwelling-favorable wind is more complex than the previously studied far-field response. The latter can be described by a slab-like model with mixing concentrating at the offshore edge of a buoyant layer. The observed plume depth increased from ~3 m near the mouth to 6 m at the offshore edge, with plume depth changing in a steplike fashion rather than continuously. CTD profiles near these steps revealed overturning indicative of vigorous mixing. Estimates of the gradient Richardson number confirmed the likelihood of mixing/entrainment not only at the offshore edge of the plume but also in the proximity of the observed steps. We hypothesize that these steps represent tidal fronts that undergo geostrophic adjustment and are advected offshore by the superimposed Ekman drift. Scaling analysis suggests that mixing and entrainment at the observed interior fronts can be enhanced by superposition of geostrophic and wind-induced shear.

Variation in size at maturity by horse mackerel (Trachurus trachurus) within the central Mediterranean Sea: Implications for investigating drivers of local productivity and applications for resource assessments

Understanding drivers of fish maturity are essential to predict the productivity, stability, and resiliency of exploited populations. In terms of horse mackerel (Trachurus trachurus), in the eastern Atlantic Ocean and the Mediterranean Sea, size at maturity estimates date back to the 1940s and throughout its range. However, many older estimates may not reflect current maturity rates. This is because past methods were imprecise, or environmental or fishing effects may change maturity rates, and some stock areas have been overlooked. To address this data gap for the Central Mediterranean Sea, female horse mackerel size at maturity was estimated for two areas: the Strait of Sicily and the Tyrrhenian Sea. All ovaries were assigned a reproductive phase based on macroscopic methods, and the reliability of this method (94%) was validated with a microscopic method (i.e., gonad histology). Although the collected females exhibited similar condition, in terms of total body weight at a given length, as well as similar gonad-somatic indices, the size at median maturity in the Strait of Sicily was smaller (161 mm total length [TL]) than in the Tyrrhenian Sea (176 mm TL). Future sampling in a series of years with contrasting levels of productivity may help identify whether this is a broad latitudinal trend, or due to specific oceanographic drivers, such as estuarine outflow into the Tyrrhenian Sea or upwelling in the Strait of Sicily. Meanwhile, resource assessments should consider that these two areas represent two phenotypic stocks.

Subtidal variability of the Tagus river plume in winter 2013

The subtidal variability of surface plume response to local wind and river runoff is studied off central Portugal using a hindcast model for the Tagus Estuary (TE) plume. In addition to the model, the main patterns of variability of the TE plume were extracted in an automated mode using Self Organizing Maps (SOM). Three characteristic spatial patterns are extracted in a 3 × 2 SOM array: in synoptic time scales, northward and southward propagation of the plume and transitional periods. Results suggest that river runoff and wind induce strong variability in the coastal ocean, and that the TE plume has a rapid response to their changes. Under high river runoff there is a southern propagation of the plume, even during downwelling favorable winds, revealing bi-directional patterns due to the combined effect of estuarine outflow, wind direction and coastline in establishment the plume direction. The same is found during transitional periods, revealing that the TE plume has significant influence both north and south of the estuary mouth. SOM was applied for the first time to extract spatial and temporal features of the TE plume and prove to be a reliable tool to study large quantities of data in addition to the model results analysis.

Lobe‐cleft instability in the buoyant gravity current generated by estuarine outflow

AbstractGravity currents represent a broad class of geophysical flows including turbidity currents, powder avalanches, pyroclastic flows, sea breeze fronts, haboobs, and river plumes. A defining feature in many gravity currents is the formation of three‐dimensional lobes and clefts along the front and researchers have sought to understand these ubiquitous geophysical structures for decades. The prevailing explanation is based largely on early laboratory and numerical model experiments at much smaller scales, which concluded that lobes and clefts are generated due to hydrostatic instability exclusively in currents propagating over a nonslip boundary. Recent studies suggest that frontal dynamics change as the flow scale increases, but no measurements have been made that sufficiently resolve the flow structure in full‐scale geophysical flows. Here we use thermal infrared and acoustic imaging of a river plume to reveal the three‐dimensional structure of lobes and clefts formed in a geophysical gravity current front. The observed lobes and clefts are generated at the front in the absence of a nonslip boundary, contradicting the prevailing explanation. The observed flow structure is consistent with an alternative formation mechanism, which predicts that the lobe scale is inherited from subsurface vortex structures.

Estuarine influence on biogeochemical properties of the Alabama shelf during the fall season

Estuarine-shelf exchange can drive strong gradients in physical and biogeochemical properties in the coastal zone and exert a significant influence on biological processes and patterns. Physical, biogeochemical, and plankton data from an across-shelf transect extending south of Mobile Bay, Alabama, in conjunction with regional time series data, were used to determine the relative importance of estuarine-shelf interactions on the physical-biological structuring of the shelf environment during fall conditions (i.e., well-mixed, low discharge). This period was also characterized by a relatively unique weather event associated with the remnants of Hurricane Patricia, which drove a meteorological flushing of estuarine water onto the shelf. Survey data indicated generally low N:P ratios across the shelf, with slightly elevated dissolved inorganic nitrogen in the Region of Freshwater Influence (ROFI) that extended approximately 30km offshore. The ROFI had higher values of chlorophyll-a, diatom-specific production, marine snow, and primary productivity, with notable contributions from the larger size cells (>5µm). Furthermore, stratification provided a niche opportunity for Trichodesmium sp. aggregates, a typically oligotrophic cyanobacteria, at the offshore edge of the ROFI. The lens of estuarine water may have limited the vertical extent to which this population was mixed, providing enhanced light availability relative to the well-mixed offshore conditions. Following the biogeochemical trend, the highest zooplankton abundances were also located within the estuarine outflow. While limited in spatial extent, the distinct geochemical and biological characteristics within the ROFI demonstrate the ecological impacts that estuarine-sourced waters can have during periods of generally low productivity in the Mississippi Bight.

Seasonal and interannual variability of the Douro turbid river plume, northwestern Iberian Peninsula

The Douro River represents the major freshwater input into the coastal ocean of the northwestern Iberian Peninsula. The seasonal and interannual variability of its turbid plume is investigated using ocean color composites from MODIS (Moderate Resolution Imaging Spectroradiometer) sensor aboard the Aqua and Terra satellites (2000–2014) and long-term records of river discharge, wind and precipitation rate. Regional climate indices, namely the Eastern Atlantic (EA) and North Atlantic Oscillation (NAO), were analyzed to identify the influence of atmospheric variability on the generation of anomalous turbid river plume patterns. The connection between the monthly time series of normalized water leaving radiance at 555nm (nLw(555)) and river discharge is high (r=0.81), which indicates a strong link between river outflow and turbidity levels in the river plume. The equivalent result is found between precipitation and nLw(555) time series, but the peak correlation was found with a 1-month lag, revealing the importance of the river dams on the outflow regulation (r=0.65). Lag correlations between nLw(555) and EA index show a peak at 1-month lag (r=0.51). The relation between NAO index and Douro river discharge is considerable (−0.50), for a time lag of 1-month as well. However, the correlation coefficient between nLw(555) and NAO index presents a maximum peak for a longer period (r=−0.42 at 3-month). Anomalous turbid plume patterns, not related with estuarine outflow, are found during autumn 2004. A coccolithophore bloom is proposed as a plausible explanation for these unexpected turbid patches.

A box model for representing estuarine physical processes in Earth system models

Appropriately treating riverine freshwater discharge into the oceans in Earth system models is a challenging problem. Commonly, the river runoff is discharged into the ocean models with zero salinity and arbitrarily distributed either horizontally or vertically over several grid cells. Those approaches entirely neglect estuarine physical processes that modify river inputs before they reach the open ocean. In order to realistically represent riverine freshwater inputs in Earth system models, a physically based Estuary Box Model (EBM) is developed to parameterize the mixing processes in estuaries. The EBM represents the estuary exchange circulation with a two-layer box structure. It takes as input the river volume flux from the land surface model and the subsurface salinity at the estuary mouth from the ocean model. It delivers the estuarine outflow salinity and net volume flux into and out of the estuary to the ocean model. An offline test of the EBM forced with observed conditions for the Columbia River system shows good agreement with observations of outflow salinity and high-resolution simulations of the exchange flow volume flux. To illustrate the practicality of use of the EBM in an Earth system model, the EBM is implemented for all coastal grid cells with river runoff in the Community Earth System Model (CESM). Compared to the standard version of CESM, which treats runoff as an augmentation to precipitation, the EBM increases sea surface salinity and reduces stratification near river mouths. The EBM also leads to significant regional and remote changes in CESM ocean surface salinities.

Different approaches to model the nearshore circulation in the south shore of O'ahu, Hawaii

Abstract. The dynamical interaction between currents, bathymetry, waves, and estuarine outflow has significant impacts on the surf zone. We investigate the impacts of two strategies to include the effect of surface gravity waves on an ocean circulation model of the south shore of O'ahu, Hawaii. This area provides an ideal laboratory for the development of nearshore circulation modeling systems for reef-protected coastlines. We use two numerical models for circulation and waves: Regional Ocean Modeling System (ROMS) and Simulating Waves Nearshore (SWAN) model, respectively. The circulation model is nested within larger-scale models that capture the tidal, regional, and wind-forced circulation of the Hawaiian archipelago. Two strategies are explored for circulation modeling: forcing by the output of the wave model and online, two-way coupling of the circulation and wave models. In addition, the circulation model alone provides the reference for the circulation without the effect of the waves. These strategies are applied to two experiments: (1) typical trade-wind conditions that are frequent during summer months, and (2) the arrival of a large winter swell that wraps around the island. The results show the importance of considering the effect of the waves on the circulation and, particularly, the circulation–wave coupled processes. Both approaches show a similar nearshore circulation pattern, with the presence of an offshore current in the middle beaches of Waikiki. Although the pattern of the offshore circulation remains the same, the coupled waves and circulation produce larger significant wave heights ( ≈ 10 %) and the formation of strong alongshore and cross-shore currents ( ≈ 1 m s−1).

Air–sea fluxes of CO<sub>2</sub> and CH<sub>4</sub> from the Penlee Point Atmospheric Observatory on the south-west coast of the UK

Abstract. We present air–sea fluxes of carbon dioxide (CO2), methane (CH4), momentum, and sensible heat measured by the eddy covariance method from the recently established Penlee Point Atmospheric Observatory (PPAO) on the south-west coast of the United Kingdom. Measurements from the south-westerly direction (open water sector) were made at three different sampling heights (approximately 15, 18, and 27 m above mean sea level, a.m.s.l.), each from a different period during 2014–2015. At sampling heights ≥ 18 m a.m.s.l., measured fluxes of momentum and sensible heat demonstrate reasonable ( ≤ ±20 % in the mean) agreement with transfer rates over the open ocean. This confirms the suitability of PPAO for air–sea exchange measurements in shelf regions. Covariance air–sea CO2 fluxes demonstrate high temporal variability. Air-to-sea transport of CO2 declined from spring to summer in both years, coinciding with the breakdown of the spring phytoplankton bloom. We report, to the best of our knowledge, the first successful eddy covariance measurements of CH4 emissions from a marine environment. Higher sea-to-air CH4 fluxes were observed during rising tides (20 ± 3; 38 ± 3; 29 ± 6 µmole m−2 d−1 at 15, 18, 27 m a.m.s.l.) than during falling tides (14 ± 2; 22 ± 2; 21 ± 5 µmole m−2 d−1), consistent with an elevated CH4 source from an estuarine outflow driven by local tidal circulation. These fluxes are a few times higher than the predicted CH4 emissions over the open ocean and are significantly lower than estimates from other aquatic CH4 hotspots (e.g. polar regions, freshwater). Finally, we found the detection limit of the air–sea CH4 flux by eddy covariance to be 20 µmole m−2 d−1 over hourly timescales (4 µmole m−2 d−1 over 24 h).

Observations of the frontal region of a buoyant river plume using an autonomous underwater vehicle

To characterize the transitional region from the near-field to far-field of a river plume entering coastal waters, we conducted four surveys using an autonomous underwater vehicle (AUV) to target the outflow of the New River Inlet, North Carolina, during maximum ebb tide. The utilization of a mobile sensor to synoptically observe current velocity data in tandem with natural river plume tracers (e.g., colored dissolved organic matter, salinity) was essential in understanding the mechanisms driving the observed circulation and mixing patterns within these waters. We find that this region is regularly impacted by two primary processes: (1) the interaction of an old dredged channel plume with the main discharge and (2) the recirculation of the discharge plume by an eddy that persistently forms between the old channel and main discharge location. Wind-driven processes in the nearshore can enhance the interaction of these two plumes resulting in unstable regions where mixing of the merged plume with the receiving waters is accelerated. We also conduct comparisons between AUV velocity observations from two surveys and their corresponding velocity outputs from a parallelized quasi-3-D model. We conclude that the ability to observe the estuarine outflow transitional region at near-synoptic temporal scales and resolutions discussed in this paper is key in providing the mechanisms driving local circulation which is essential for proper parameterization of high-resolution numerical coastal models.

Nitrogen, phosphorus and silica on the West Florida Shelf: Patterns and relationships with Karenia spp. occurrence

A large scale investigation was conducted in major estuaries and the oligotrophic coastal waters of the West Florida Shelf during the fall of 2007 through 2010 to identify statistically significant and persistent associations of the nearly annual blooms of the harmful algae, Karenia brevis, with dissolved and particulate nutrients, and with other taxonomic groups of the coastal phytoplankton community. Conflicting interannual patterns of Karenia densities and riverine flows indicated that links between Karenia blooms and terrestrial contribution of nutrients were neither direct nor certain. There was no evidence in these data that Karenia blooms were enhanced by estuarine outflow. There was no characteristic nutrient ensemble with which Karenia presence was associated, which implied a wide range of nutrient strategies for the organism. Drawdowns of urea and DIN were not a universal feature of the blooms sampled. The only persistent relationship with individual parameters were a correlation of Karenia cell density with DON and an inverse correlation with DIN:Si(OH)4. Temporal and spatial patterns of DON and δ15N provided evidence of regional processes controlling nitrogen supply. Correlation of δ15N values with cyanophyte percentage of biomass demonstrated the significance of N2-fixation to the region. Karenia was correlated with haptophyte and cyanophyte biomass and inversely correlated with diatoms. The phytoplankton community was not significantly different with Karenia present and indicated that blooms replaced differing components of the phytoplankton community at different times. Principal component analysis of phytoplankton community composition identified a modest association of Karenia with nutrient-poor conditions. The results emphasized that strong interannual variability existed in nutrient regimes, phytoplankton community, and nutrient-mediated Karenia responses.