Low-inflow Estuaries Research Articles

Spatial and Temporal Changes in the Fish Fauna of a Low-Inflow Estuary following a Mass Mortality Event and Natural and Artificial Bar Breaches

Estuaries that become closed from the ocean through the formation of a sand bar are particularly susceptible to degradation and often experience poor water quality and fish kills. Sampling was conducted seasonally for two years in the deeper waters of the Vasse–Wonnerup (southwestern Australia) after a large fish mortality event to identify the fish species present and determine if richness, abundance, diversity and faunal composition differed among regions and over time. Species richness, abundance and diversity were far greater in the downstream regions than in the upstream regions, reflecting patterns in salinity. Catch rates of two marine estuarine-opportunist mugilids (Mugil cephalus and Aldrichetta forsteri) and an estuarine-resident sparid (Acanthopagrus butcheri), which were the species most affected by the mortality event, were relatively stable. It is suggested that before sampling commenced, mugilids had entered the estuary following an artificial sand bar breach, while the loss of 0+ (new recruits) A. butcheri in the mortality event and subsequent recruitment failure prevented stocks from increasing. Temporal shifts in fish composition were driven by less abundant species, which utilized the estuary more opportunistically. Increases in the occurrences and abundances of these species coincided with an open bar and salinities close to those of seawater. The data also show how the estuary responds to differing hydrodynamic phases and artificial breaches.

Transitions between different estuarine states in a low-inflow estuary of arid coasts (Mond Estuary, Persian Gulf) with a focus on freshwater lenses

This study shows that the Mond Estuary, Persian Gulf experiences notable time-dependent longitudinal salinity variations. Monthly-seasonal variability in salinity changes from a positive estuarine system in wintertime to inverse and salt-plug estuaries during summertime. A highly unsteady state (time scale of hours-days) occurs in the wintertime named the freshwater lens estuary (FLE), in which a low-salinity water mass (lens) is confined by both marine and riverine saltwater sources. Here, FLE dynamics is investigated using realistic and idealized numerical models and two 25-hour data sets collected immediately before a hypersaline runoff (weak FLE condition) and during another one (strong FLE condition). The Péclet Number (Pe) reveals a much stronger advective transports than diffusive one (Pe>5.7) throughout the estuary. An exception from this behavior (stronger mixing with Pe=0.8) is observed at the estuary downstream, where a significant amount of freshwater mixes with seawater during the weak FLE conditions. Also, the minimum hypersaline runoff of ∼400 m3/s estimated by the numerical model can dampen tidal waves at the head of the estuary. In an idealized basin, the model shows that FLE longitudinal flows form two cells of positive estuarine circulation separated by a maximum mixing zone (the potential energy anomaly <0.75 J/m3). In the Mond Estuary, a sea-level rise of 20 cm increases salinity in the FLE state and the positive estuary during winter. Conversely, it decreases salinity in the inverse and salt-plug estuaries in summer.

Isotopic compositions of organic and inorganic nitrogen reveal processing and source dynamics at septic influenced and undeveloped estuary sites

Historically, dissolved organic nitrogen (DON) has not been characterized in the nitrogen profiles of most estuaries despite its significant contribution to total nitrogen and projected increase in loading. The characterization of dissolved inorganic nitrogen (DIN) and DON processing from groundwater to surface water also remains unconstrained. This study attempts to fill in these knowledge gaps by quantifying the DON pool and potential sources in a semiarid, low inflow estuary (Baffin Bay, Texas) using stable isotope techniques. High NO3− and DON concentrations, and high δ15N-NH4+ (+55.0 ± 56.7 ‰), δ15N-NO3− (+23.9 ± 8.6 ‰) and δ15N-DON (+22.3 ± 6.5 ‰) were observed in groundwaters of a septic-influenced estuarine area, indicating coupled septic contamination and nitrification/denitrification. In contrast, groundwater of an undeveloped area provided evidence of inundation by bay water through high NH4+ concentrations and δ15N-NH4+ (+8.4 ± 3.0 ‰) resembling estuary porewater. NH4+ was the dominant nitrogen species in porewater of both areas and δ15N-NH4+ indicated production via organic nitrogen mineralization and dissimilatory nitrate reduction to ammonium. Surface water had similar nitrogen profiles (DON constituted ∼98 % of dissolved nitrogen pool) and potential source contributions, despite distinct nitrogen processing and profiles found in each water table. This was attributed to low nitrogen removal rates and prolonged mixing associated with long residence time. This study emphasizes the importance of DON in a low-inflow estuary and the isotopic approach to comprehensively examine both inorganic and organic N processing and sources serving as a guide to investigate N cycling in high DON estuaries globally.

Benthic macrofauna communities reflect eutrophic condition in a low-inflow estuary

Nutrient enrichment drives potentially severe changes in low-inflow and temporarily closed estuaries where tidal mixing and freshwater flushing is limited, and bottom-water anoxia occurs frequently. Our research assesses the influence of nutrient enrichment by comparing macrobenthos community metrics in a persistently hypereutrophic South African estuary. This is then compared to published literature for an estuary with a similar catchment size and mean annual runoff, but which experiences limited anthropogenic nutrient input. Macroinvertebrate abundance and richness were higher in an estuary with limited anthropogenic nutrient loading, and this varied seasonally for both estuaries. Oxygen saturation had a significant influence on taxonomic level abundances, with lower oxygen levels in the polluted estuary reducing macroinvertebrate abundances. Macroinvertebrates tolerant of organic pollution (e.g., Oligochaeta) were also more abundant in the polluted estuary. This study highlights the effects of increased nutrient loading in low-inflow estuaries and confirms the supposition from several previous studies that macrozoobenthos could be used as a responsive tool from a bioindicator perspective to monitor eutrophication.

Recognizing Low-Inflow Estuaries as a Common Estuary Paradigm

It has become clear that estuaries with low rates of freshwater inflow are an important but overlooked sphere of estuarine science. Low-inflow estuaries (LIEs) represent a major class of estuary long downplayed because observations do not fit well in the dominant estuary paradigm, which was developed in perennially wet climates. Rather than being rare and unusual, it is now evident that LIEs are common globally and an alternate estuary paradigm within the idea of an estuary as the place where a river meets the sea. They are found mostly in areas with arid, semi-arid, or seasonally arid climates, but LIE phenomena are also found in estuaries along mountainous coasts with small watersheds and short-tailed hydrographs. Inflows can be defined as “low” relative to basin volume, tidal mixing, evaporative losses, or wave forcing at the mouth. The focus here is on common physical phenomena that emerge in low-inflow estuaries—how low river flow is expressed in estuaries. The most common is hypersalinity (and the associated potential for inverse conditions), which develops where there is a net negative water balance. However, in small microtidal estuaries, low inflow results in mouth closure even as a positive water balance may persist, accounting for extreme stratification. Attention is also given to the longitudinal density gradient and the occurrence of thermal estuaries and inverse estuaries. Finally, ocean-driven estuaries are highlighted where marine subsidies (nutrients, particulates) dominate watershed subsidies. While climate change is altering freshwater inflow to estuaries, locally driven changes are generally more important and this presents an opportunity to restore estuaries through restoring estuarine hydrology.

Variation in Larval Fish Assemblage Dynamics Associated with Harmful Algal Blooms in a Temperate Estuary, South Africa

Eutrophication and concomitant harmful algal blooms (HABs) are on the rise globally and pose a threat to larval stages of fishes that rely on estuarine nursery habitats for growth and survival. The anthropogenically altered low inflow estuary (LIE), Sundays Estuary, South Africa, supports persistent and predictable HABs. This study investigated the effects of HABs on the environmental conditions and larval fish assemblage of this warm temperate nursery area. Sampling took place during the austral spring of 2016 and 2018 at three sites in the mesohaline zone where both larval fish abundance and HABs are known to reach maxima. Physico-chemical and phytoplankton data were collected during the day and night, while larval fishes were sampled after nightfall. Physico-chemical parameters and larval fish assemblages were compared by water column (surface and bottom) and sites within the mesohaline zone, years, and HAB intensity (hypereutrophic ≥ 2781 H. akashiwo cells•mL−1; standard ≥ 205 H. akashiwo cells•mL−1; none < 205 H. akashiwo cells•mL−1). A longer period of consecutive hypereutrophic bloom conditions was recorded during 2018 compared to 2016. Dissolved oxygen concentration was notably higher during hypereutrophic blooms (χ2 = 23.759, df = 2, P < 0.001) and reached a maximum of approximately 21 mg•L−1 during the day and 13 mg•L−1 at night. Density and similarity of estuarine resident larval assemblages were negatively correlated to supersaturated dissolved oxygen concentrations. Greater mean densities of estuarine resident larvae were recorded during hypereutrophic blooms compared to standard blooms and bloom absence and diversity was lower during 2018 when hypereutrophic bloom conditions were more persistent. These changes may have major implications for successful early development of fishes that rely on the Sundays Estuary and similar LIEs as a nursery.

Water Releases From Dams Improve Ecological Health and Societal Benefits in Downstream Estuaries

Abstract This review study investigated the response of low-inflow estuaries (LIEs) to dam releases as this type of estuary is particularly sensitive to freshwater inflow modification. LIEs occur in arid and semi-arid regions and are subject to periods of little to no freshwater inflow. Case studies were used to identify ecological health and societal benefits associated with flow releases. Successful releases have been made to keep the estuary mouth open, ensure mixing, facilitate a salinity gradient and provide recruitment pulses to the marine environment for fish and invertebrates. Baseflow inputs ensured ecosystem connectivity and maintained estuary water quality gradients. Flow pulse releases in certain seasons stimulated spawning migrations of fish between freshwater and estuarine habitats. Holistic and adaptive restoration approaches were successful in terms of providing ecosystem services and societal benefits such as improved fisheries and livelihoods. Ongoing engagement, inclusion of communities, support from river users, and cooperation between multiple agencies were also important. However, this management solution for LIEs is threatened by increasing abstraction of water, competing water uses, over allocation, and frequent droughts. Moving forward, freshwater releases from dams should be considered an important restoration action that can improve ecological health, estuary function, ecosystem services, and societal benefits. This should take place within a socio-ecological system framework using an adaptive management and monitoring approach. Other key considerations for planning and implementation of future dam releases to LIEs were recommended.

Flushing time variability in a short, low-inflow estuary

Flushing time, the time scale for exchange and mixing between embayed and oceanic waters in an estuary, plays an integral role in determining water quality and aquatic ecosystem health. Here, we investigated the spatiotemporal variability of flushing times throughout Morro Bay, a short, low-inflow estuary (LIE) on the California coast, using a calibrated and validated hydrodynamic model (Delft3D). Morro Bay has historically supported an extensive eelgrass (Zostera marina) habitat, which declined substantially from 139 to 5.4 ha during 2007–2017. Eelgrass decline motivated the current research into the role of changing bed roughness and oceanic drivers (i.e., tide and sea-level rise) on estuarine hydrodynamics and flushing times. We found that tidal variability exerts the strongest control on flushing times compared to other effects, i.e., bed roughness or sea-level rise. Additionally, we found that increasing sea level and decreasing bed roughness (associated with declining seagrass coverage) yielded higher rates of mixing (lower flushing times). We detected a strong correspondence between areas having shorter flushing times (e.g., near the estuary mouth) and areas occupied by resilient eelgrass populations in Morro Bay. Our findings further indicated that flushing times in short LIEs are particularly sensitive to several factors (e.g., bed roughness, sea level) that are susceptible to anthropogenic disturbance and future climate change.

Microplastics transport in a low-inflow estuary at the entrance of the Gulf of California

Microplastics (MPs) are recognized as a global emergent pollution impact, which can affect all food chains. Estimating MPs transport pathways in coastal ecosystems is needed to assess their likely effects. Here, we studied MPs accumulation and transport pathways in the Estero de Urias lagoon system (low-inflow estuary) using field data and a 3D particle model. Field results showed that the MPs present similar abundances throughout the study area during the dry and rainy seasons. Model simulations indicated that i) morphology and tidal currents caused the MPs discharged in the lagoon to remain inside, and ii) wind-induced currents caused the MPs in the coastal area to be transported to the southwest. These transport processes may be responsible for homogenizing MPs concentrations through the studied area. In addition, model simulations suggested that EUL-dense waters can export MPs from the coastal area to the sea bottom.

Temporal Variability in Water Quality and Phytoplankton Biomass in a Low-Inflow Estuary (Baffin Bay, TX)

Temporal Variability in Water Quality and Phytoplankton Biomass in a Low-Inflow Estuary (Baffin Bay, TX)

Spatial distribution of seawater carbonate chemistry and hydrodynamic controls in a low-inflow estuary

Coastal and estuarine systems play an important role in the global carbon cycle and often have complex carbonate chemistry dynamics due to a multitude of biogeochemical and physical drivers. Compared to classic estuaries, mechanisms driving the distribution of carbonate parameters in low-inflow estuaries are understudied. The spatial distribution of carbonate chemistry and hydrodynamic parameters were characterized in Morro Bay, a short and seasonally hypersaline estuary on the Central California Coast, during the dry, low-inflow season to better understand in situ modifications. Sampling transects were completed in the main channel in June, August, and September of 2018, bracketing both a high and low tide on each date. Temperature, salinity, total alkalinity, and dissolved inorganic carbon all increased from the mouth to the back of the estuary, with larger values observed during the low tide. pH values decreased towards the back of the bay, and had little variation between high and low tide for June and August transects. Flushing times (estimated using a salt-budget model approach) also increased toward the back of the bay which led to hypersaline conditions. Salinity alone only explained 20–33% of observed changes in total alkalinity and 13–22% of observed changes in dissolved inorganic carbon throughout the bay. The remaining changes in total alkalinity and dissolved inorganic carbon were likely driven by biogeochemical modifications enhanced by extended flushing times, particularly in the back bay. Prior to this project, Morro Bay experienced a recent, rapid collapse of eelgrass, the major biogenic habitat. In the last four years eelgrass in Morro Bay appears to be on a recovery trajectory; therefore, this study provides a baseline whereby future studies can evaluate carbonate chemistry changes associated with potential eelgrass recovery and expansion. This study highlights the unique hydrodynamic exchange in seasonally low-inflow estuaries and its potentially large role in influencing local carbonate chemistry and ocean acidification.

Consequences of Mouth Closure and Hypoxia-Induced State Changes in Low-Inflow Estuaries: Benthic Community and Trait-Based Response

AbstractThe southern California coastline hosts low-inflow estuaries that have mouths that periodically close. Low-inflow estuaries can become hypoxic and are then often opened mechanically. The consequences of mouth closure and hypoxia (&lt; 2 mg L−1 O2) on macrobenthic densities, species richness, diversity, composition, and biological traits were evaluated for legacy data generated by the Pacific Estuarine Research Lab for Los Peñasquitos Lagoon (LPL) (1991–2006) and Tijuana Estuary (TJE) (1988–2004). LPL closed at least annually and TJE remained open during the study period. Effects were moderated by zone within the estuary (relative to the mouth) and season. Periodic closure in LPL was associated with raised macrofaunal density and diversity, especially at the mouth, and with suppressed seasonality. Closure favored soft-bodied (non-calcified) non-bioturbating, mobile, epifaunal taxa in LPL with planktotrophic development, large branchiae, and no vision. There were more spionid and capitellid polychaetes, Traskorchestia traskiana, Cerithideopsis californica, Tagelus californianus, and phoronids during closure. In contrast, hypoxia (&lt; 2 mg L−1) measured during faunal sampling was associated with lower densities in LPL and different taxonomic composition, but no difference in taxon richness or diversity. There were more corophiid amphipods, small snails, tubificid oligochaetes, Palaemon macrodactylus (shrimp), and Trichorixa reticulata (insects) under hypoxic conditions, and retention of taxa with very large or small bodies and with vision. TJE densities were nearly double those of LPL; taxon richness and diversity (H’) were also higher in TJE. TJE hosted more burrowing, large-bodied, highly calcified taxa with planktotrophic development and no vision than LPL. Differences in composition and traits between the two estuaries disappeared in the middle and upper reaches, where ocean flushing was more limited. Historical long-term monitoring data for benthos, such as the data set analyzed here, offer a valuable baseline for evaluating ecosystem response to changes induced by climate, infrastructure development, contamination, or restoration.

Copepod Assemblages at the Base of Mangrove Food Webs during a Severe Drought

The effect of severe drought on the functional groups that sustain the base of the mangrove food webs in semi-arid areas is largely unknown. We therefore analyzed the intra-annual variation in the assemblages and functional groups of copepods in a shallow, low-inflow estuary of the Brazilian semi-arid coast when the most severe drought ever occurred. The lowest density was found in April (upstream region) and the highest in August (downstream region). Three main functional groups were identified, sorted by spawning strategy, and further subdivided according to feeding strategy, trophic regime, and diel vertical migration behavior. The community was significantly influenced by the extreme drought period, presenting a temporal homogenization in terms of composition, and an expressive and unexpected increase in density in the dry period, possibly due to phytoplankton blooms resistant to hypersalinity and the occurrence of copepod species adapted to stressful conditions. The few stress-tolerant species sustaining the food webs with seasonal variations were observed simplifying the trophic variability. The results indicate that hypersalinity can induce changes in the zooplankton community, increasing copepod mortality risk and, so, promoting alteration in the trophic estuarine dynamic.

Managing the Seemingly Unmanageable: Water Quality and Phytoplankton Dynamics in a Heavily Urbanised Low-Inflow Estuary

Managing the Seemingly Unmanageable: Water Quality and Phytoplankton Dynamics in a Heavily Urbanised Low-Inflow Estuary

Phytoplankton biomass and community composition in three Texas estuaries differing in freshwater inflow regime

Because many estuaries worldwide are experiencing large-scale alterations in freshwater inflows due to climatic and human influences on watersheds, it is critical to understand ecosystem-level responses to freshwater inflow conditions and variability. This study compared environmental conditions and phytoplankton biomass/community composition among three Texas estuaries with differing freshwater inflow regimes to understand the impacts of freshwater inflow magnitude on phytoplankton communities. It was hypothesized that: 1) nutrient concentrations and phytoplankton biomass would be highest in San Antonio Bay (SA), the high inflow estuary and lower in Nueces-Corpus Christi Bay (NC) and Baffin Bay (BB) due to lower average inflows, and 2) the phytoplankton community would be dominated by large and/or fast-growing taxa in SA, with a greater fraction of small and/or slow-growing taxa in NC and BB. Highest inorganic nutrient concentrations were generally observed in SA, while high organic nutrient concentrations were found in BB. Chlorophyll a was relatively high in both SA and BB (mean 16.9–18.5 μg L−1) while phytoplankton biovolume was highest in BB. Despite distinct freshwater inflow, salinity and nutrient regimes, differences in phytoplankton community composition were less pronounced. Nano- or microplankton were the dominant size class of phytoplankton in each system, and diatoms were the dominant functional group, accounting for 27–49% of total biovolume on average. There were indications that the phytoplankton community was more diverse in SA, especially following inflow events, providing evidence that inflow may act as a disturbance that leads to greater phytoplankton diversity. Results from this study also showed that while freshwater inflow is important for nutrient delivery, low inflow estuaries such as BB are still susceptible to effects of eutrophication due to long residence times and nutrient retention/recycling. Overall, the differing responses of each of these ecosystems to freshwater inflow highlight the importance of system-specific management plans and consistent monitoring programs in coastal estuaries.

Radioactive and stable isotopes reveal variations in nearshore submarine groundwater discharge composition and magnitude across low inflow northwestern Gulf of Mexico estuaries

To determine how submarine groundwater discharge (SGD) magnitudes and composition (fresh or saline/recirculated) vary in nearshore low inflow estuaries across ⁓125 km of a semiarid coastline, this study assessed three south Texas estuaries, using radon [222Rn], radium [226Ra and 224Ra], and water isotopes [δ18O and δD]. Mass balance models of time-series 222Rn, found to be representative of total SGD in this study, revealed much higher SGD inputs to the Nueces Estuary (average [x̅] Nueces, Corpus Christi and Oso Bays: 120, 83, and 44 cm·d−1, respectively), attributed to anthropogenically-disturbed substrates and potentially surfacing growth-faults. The lowest 222Rn-derived SGD occurred in the Upper Laguna Madre Estuary (x̅: Upper Laguna Madre and Baffin Bay: 21 and 18 cm·d−1, respectively), explained by the drier climate, lower anthropogenic disturbance, and neighboring groundwater cone of depression. Aransas Bay in the Mission Aransas Estuary received greater average annual precipitation but exhibited low total SGD rates (x̅: 23 cm·d−1). Seasonally, average 222Rn-derived SGD rates increased following Hurricane Harvey (43 cm·d−1 in spring to 64 cm·d−1 in summer). In the Nueces Estuary, the overall 222Rn-derived SGDs were substantially higher than SGDs from 224Ra and 226Ra. The closer agreement between 224Ra and 222Rn-derived SGD and larger 224Ra rates in the Upper Laguna Madre Estuary, Aransas Bay and Oso Bay indicate that saline/recirculated SGD contributions were significant. Values of δ18O and δD confirm these types of inputs, with effects of evaporation/salinization more pronounced where recirculation was predominant and the opposite where terrestrial/222Rn-derived SGD inputs dominate. 226Ra-derived SGDs were lower than the 224Ra due to different behavior of the two isotopes while released into water following transport through saline and fine-grained estuarine sediments or due to wind-driven disturbances.

Phytoplankton Dynamics in a Low-Inflow Estuary (Baffin Bay, TX) During Drought and High-Rainfall Conditions Associated with an El Niño Event

Low freshwater inflow estuaries are common worldwide, yet our understanding of phytoplankton dynamics in these systems is limited compared to river-influenced estuaries. Combining 3 years of monthly sampling data with analysis of a lower frequency, multidecadal dataset, we examined nutrient-phytoplankton dynamics in Baffin Bay-Upper Laguna Madre, TX (USA), a low-inflow lagoonal estuary. During a low rainfall, high salinity period (corresponding with non-El Nino conditions), phytoplankton community biovolume was high and consisted of a near monoculture of the harmful “brown tide” organism, Aureoumbra lagunensis. With the onset of El Nino conditions, rainfall increased and salinity decreased. Other phytoplankton groups became more prevalent, namely diatoms and the mixotrophic ciliate, Mesodinium sp., while prevalence of A. lagunensis declined. Although El Nino conditions corresponded with increased inorganic nitrogen concentrations, this did not lead to a near-term (weeks-months) increase in phytoplankton biovolume, indicating that the stimulatory effects of nutrient pulses may have been countered by the decreased residence times associated with increased inflow. Overall, results demonstrate that low freshwater inflows, as well as high residence times and salinities, can lead to increased phytoplankton biomass and decreased phytoplankton diversity, despite relatively low external nutrient loadings. With future expansion of arid/semiarid regions and/or increasing human freshwater demands, Baffin Bay and similar systems may experience lower inflows, more pronounced hypersalinity, and presumably less diverse phytoplankton communities, potentially dominated by harmful taxa as seen here.

Oyster growth across a salinity gradient in a shallow, subtropical Gulf of Mexico estuary

Abstract An increase in oyster aquaculture as a sustainable method of shellfish production is one response to overharvest and degradation of natural oyster reefs over the past century. Successful aquaculture production requires determining the environmental conditions optimal for oyster growth. In this study, the salinity, temperature, chlorophyll a concentration and the growth of Crassostrea virginica were monitored at four locations within the Mission-Aransas Estuary, Texas (USA), a shallow subtropical estuary influenced by relatively low freshwater inflow. Mean growth of the oyster shell (0.205 mm d–1 and 0.203 g d–1) and soft tissues (3.447 mg d–1) was highest when salinity was low (mean = 15.5) and chlorophyll a concentration was high (8.4 μg l–1). Oyster growth also varied temporally with periods of spawning. In low-inflow estuaries such as the Mission-Aransas Estuary, oyster farms should be sited close to river mouths so that oysters can benefit from freshwater inflows and lower salinities.

Phytoplankton Community Structure in a Seasonal Low-Inflow Estuary Adjacent to Coastal Upwelling (Drakes Estero, CA, USA)

Low-inflow estuaries (LIEs) in Mediterranean climates located within regions of wind-driven coastal upwelling have seasonally minimal freshwater inflow, are supplied by nutrient-rich upwelled water, and often used for sustainable shellfish mariculture supported by a rich pelagic food web. They likely support endemic estuarine phytoplankton supplemented during the upwelling season with phytoplankton supplied from the ocean. There are few descriptions of LIE algal assemblages. This study characterizes the spatio-temporal phytoplankton community dynamics in a low-inflow estuary located along the northern California coast with high frequency sampling to assess seasonal influences on functional groups, blooms, and endemic populations. Phytoplankton biomass and community composition were measured in Drakes Estero monthly from June 2016 to November 2017. We predicted blooms with diatoms in summer and dinoflagellates in fall but had no prediction for non-bloom situations and what phytoplankton would occur year-round. Flagellates were ubiquitous throughout the Estero at all times. Two summer blooms occurred with diatoms characteristic of upwelling (Chaetoceros, Skeletonema, Asterionellopsis, Thalassiosira) and two blooms in fall with dinoflagellates dominated by Prorocentrum and high densities of the raphidophyte Heterosigma. Harmful algal bloom (HAB) taxa were observed, some at levels reported to have negative effects. Non-metric multi-dimensional scaling analyses showed the diatoms to be associated with upwelling, cooler temperatures and high nitrate, and the dinoflagellates and flagellates associated with warmer temperatures and reduced upwelling. Dinoflagellate and flagellate communities, especially HAB taxa, might be promoted with warmer temperatures or longer warm seasons accompanying climate change; an important consideration for managers of LIEs, especially those containing mariculture facilities.

Hydrodynamic Response of a Floating Aquaculture Farm in a Low Inflow Estuary

AbstractThe impact of a floating oyster aquaculture farm on the intratidal dynamics of a low inflow estuary was investigated using field observations and an idealized numerical model. Measurements of current velocities, temperature, electrical conductivity, and velocity shear were collected around a floating oyster farm during two semidiurnal tidal cycles in a curved portion of the estuary. During flood, farm‐induced friction enhanced the lateral straining of velocity shears, which induced vertical mixing near the surface and extended the farm's frictional footprint. The streamwise flow reduction near the farm limited the development of lateral circulation. During ebb, flows largely bypassed the farm, which resulted in a weaker streamwise flow reduction through the farm and allowed the lateral circulation to develop across the estuary. To capture the farm effects in numerical simulations, a bulk drag coefficient for the farm was calculated as 8.4 × 10−3 ± 9.1 × 10−4, while the drag for a single cage ranged from 0.58 to 0.92. An idealized simulation in the Regional Ocean Modeling System (ROMS) demonstrated that the limited development of lateral circulation during flood was due to farm‐imposed friction rather than the combined influence of channel‐shoal morphology and the natural channel bend. These results showed that even though a farm may encompass a small portion of the estuary, it can affect the momentum and mixing outside of the immediate farm area. Therefore, it is important to consider the hydrodynamic responses of farms in estimates of carrying capacity and siting decisions.