Phytoplankton biomass and composition in a well-flushed, sub-tropical estuary: The contrasting effects of hydrology, nutrient loads and allochthonous influences

Numerical study of the upwelling and downwelling effects of artificial reefs along tidal cycles in the Pearl River Estuary

Artificial reefs (ARs) are a preferred option for managers due to their distinctive hydrodynamic properties, which support a highly productive local ecosystem. However, the hydrodynamics characteristics of ARs in natural marine environments have not been conducted. Being the first to explore the spatiotemporal characteristic of flow fields around ARs along tidal cycles in marine environments, this study redefined the upwelling and downwelling of ARs, based on natural vertical velocities, and separated the upwelling into co-direction upwelling and re-direction upwelling, and the downwelling into co-direction downwelling and re-direction downwelling. This study simulated the flow field in the Wanshan ARs area of the Pearl River Estuary along the tidal cycles using the MIKE3-FM. Numerical simulations revealed that (1) co-direction upwelling and co-direction downwelling were the dominant components of the vertical flow field effects of ARs; (2) the areas sum of upwelling and downwelling were largest in the medium water column, with about 1.6 and 1.03 times as large as the bottom and surface water column, respectively, while the fluxes sum of the upwelling and downwelling were largest in bottom water column, with approximately 1.3 and 2.2 times larger than those in the middle and surface water columns; (3) the area and volume of the upwelling and downwelling gradually decreased along neap-spring tide, exhibited significantly negative correlations with current speeds; while the upwelling flux and downwelling flux gradually increased along neap-spring tide; exhibited a significantly positive correlation with current speed; (4) the effects of tide to upwelling and downwelling of AR are forced by the northward velocity of current speed, the net flux of upwelling and downwelling showed a significant positive correlation with the northward velocity of current speed (r = 0.94). These results could provide a reference for assessing the flow field effect of ARs and a guide for the configuration and management of ARs.

Seasonality of phytoplankton biomass and composition on the Cape Canaveral shelf of Florida: Role of shifts in climate and coastal watershed influences

Seasonal patterns of phytoplankton biomass and composition in the inner continental shelf off Cape Canaveral on the east coast of Florida were examined for a 6-year period (2013–2019). In situ water samples were collected and analyzed for chlorophyll a, phytoplankton biomass and composition, along with water quality parameters. Regional satellite data on chlorophyll a, and temperature was also obtained from NASA. Average chlorophyll a values over the study period ranged from 0.63 ± 0.03 μg L−1 in the summer to 2.55 ± 0.10 μg L−1 in the fall. Phytoplankton community composition also showed seasonal differences, with persistent dominance by picoplanktonic cyanobacteria in the summer, but mixed dominance by picocyanobacteria and dinoflagellates in the fall. Seasonal differences were attributed to a shift in predominant seasonal wind directions, which drive water along the coast from the north in the fall and winter, but from the south in the spring and summer, including eddies and upwelling from the Gulf Stream. Water masses moving along the Florida coast from the north are influenced by nutrient and phytoplankton-enriched inputs from estuaries along the north coast of Florida, explaining the higher phytoplankton biomass levels on the Cape Canaveral shelf in the fall and winter. Seasonal patterns observed in this study demonstrate the importance of allochthonous influences on phytoplankton biomass and composition, and highlight the potential sensitivity of phytoplankton communities to continuing cultural eutrophication and future climate changes, including the frequency and intensity of tropical storms, and alterations in discharges from land.

Phytoplankton Communities and Their Relationship with Environmental Factors in the Waters around Macau

An investigation of the waters around Macau collected 43 phytoplankton species belonging to 29 genera and 5 phyla, including 32 species from 22 genera of Bacillariophyta, 7 species from 3 genera of Pyrrophyta, 2 species from 2 genera of Cyanophyta, and 1 genus and 1 species from both Euglenophyta and Chromophyta. The dominant phytoplankton species in the study areas were Skeletonema costatum (Greville) Cleve, Aulacoseira granulata (Ehrenberg) Simonsen, Thalassiothrix frauenfeidii Grunow, and Thalassionema nitzschioides Grunow. The phytoplankton abundance in the waters around Macau was between 46,607.14 and 1,355,000 cells/m3, with the highest abundance noted in station S8. Diatoms were the main contributor to phytoplankton abundance in station S8, accounting for 96.2% of the total abundance. Station S4 exhibited the lowest phytoplankton abundance of 46,607.1 cells/m3, with diatoms and Chromophytaaccounting for 58.6% and 29.9% of the total phytoplankton abundance, respectively. Biodiversity analysis results showed that the phytoplankton richness index was 1.18−3.61, the uniformity index was 0.24−0.78, and the Shannon−Wiener index was 0.94−3.41. Correlation analysis revealed that ammonia nitrogen was significantly negatively correlated with the phytoplankton richness, uniformity, and Shannon−Wiener indices. Nitrite nitrogen, nitrate nitrogen, inorganic nitrogen, salinity, turbidity, and pH were positively correlated with the phytoplankton evenness index and Shannon−Wiener index. Cluster and non-metric multidimensional scaling analyses demonstrated that the phytoplankton community structure in the waters around Macau could be divided into three groups, with A. granulata, S. costatum, T. frauenfeidii, T. nitzschioides, Chaetoceros curvisetus Cleve, and Chaetoceros diadema (Ehrenberg) Gran being predominant in different grouping communities (contribution% > 10%). Biota-Environment Stepwise Analysis (BIOENV) showed a significant correlation between the phytoplankton community and nitrite nitrogen content in the waters around Macau (correlation: 0.5544, Mantel test: statistic 0.4196, p = 0.009), which was consistent with the results of the canonical correspondence analysis.

Phytoplankton dynamics in Louisiana estuaries: Building a baseline to understand current and future change

Louisiana estuaries are important habitats in the northern Gulf of Mexico, a region undergoing significant and sustained human- and climate-driven changes. This paper synthesizes data collected over multiple years from four Louisiana estuaries - Breton Sound, Terrebonne Bay, the Atchafalaya River Delta Estuary, and Vermilion Bay - to characterize trends in phytoplankton biomass, community composition, and the environmental factors influencing them. Results highlight similarities in timing and composition of maximum chlorophyll, with salinity variability often explaining biomass trends. Distinct drivers for biomass versus community structure were observed in all four estuarine systems. Systems shared a lack of significant correlation between river discharge and overall phytoplankton biomass, while discharge was important for understanding community composition. Temperature was a significant explanatory variable for both biomass and community composition in only one system. These results provide a regional view of phytoplankton dynamics in Louisiana estuaries critical to understanding and predicting the effects of ongoing change.

Polystyrene microplastic contamination versus microplankton abundances in two lagoons of the Florida Keys

A microscopic study of microplankton in two coastal lagoons in the Florida Keys coincidently, and unexpectedly, revealed the widespread presence of high concentrations of polystyrene microplastic particles. The polystyrene particles were first observed in the second year of a 2-year study of phytoplankton communities, with peak densities in the spring/summer of 2019 at all ten sampling sites in the two lagoons. Polystyrene particle densities reached levels up to 76,000 L⁻¹. The particles ranged in size from 33 to 190 µm, similar to the size range of microplanktonic algae (20–200 µm). Over the period of peak polystyrene densities, average particle densities were similar to average densities of microplanktonic algae cells. The latter observation highlights the potential significance of the microplastic particles for the ecology of the pristine waters of the Florida Keys, if they persist.

An assessment of trends in the frequency and duration of Karenia brevis red tide blooms on the South Texas coast (western Gulf of Mexico)

Limited data coverage on harmful algal blooms (HABs) in some regions makes assessment of long-term trends difficult, and also impedes understanding of bloom ecology. Here, observations reported in a local newspaper were combined with cell count and environmental data from resource management agencies to assess trends in Karenia brevis "red tide" frequency and duration in the Nueces Estuary (Texas) and adjacent coastal waters, and to determine relationships with environmental factors. Based on these analyses, the Coastal Bend region of the Texas coast has experienced a significant increase in the frequency of red tide blooms since the mid-1990s. Salinity was positively correlated with red tide occurrence in the Nueces Estuary, and a documented long-term increase in salinity of the Nueces Estuary may be a major factor in the long-term increase in bloom frequency. This suggests that freshwater inflow management efforts in Texas should consider impacts on red tide habitat suitability (i.e., salinity regime) in downstream estuaries. Natural climate variability such as the El Niño-Southern Oscillation, which is strongly related to rainfall and salinity in Central and South Texas, was also an influential predictor of red tide presence/absence. Though no significant change in the duration of blooms was detected, there was a negative correlation between duration and temperature. Specifically, summer-like temperatures were not favorable to K. brevis bloom development. The relationships found here between red tide frequency/duration and environmental drivers present a new avenue of research that will aid in refining monitoring and forecasting efforts for red tides on the Texas coast and elsewhere. Findings also highlight the importance of factors (i.e., salinity, temperature) that are likely to be altered in the future due to both population growth in coastal watersheds and anthropogenic climate change.

Documenting the duration and chlorophyll pigments of an allochthonous Karenia brevis bloom in the Loxahatchee River Estuary (LRE), Florida

In Fall 2017 a large bloom of the toxic dinoflagellate Karenia brevis developed in the Gulf of Mexico. After persisting for months, in Fall 2018 wind and water circulation patterns drove K. brevis towards the east coast of Florida. On September 29, 2018 Palm Beach County, FL beaches were closed due to respiratory and gastrointestinal issues associated with brevotoxins, and effects of brevotoxins were reported from within estuarine segments of the Loxahatchee River Estuary (LRE). This was the first apparent report of a K. brevis bloom impacting inshore portions of the LRE prompting us to question the longevity of K. brevis within a relatively shallow, well-flushed coastal-estuarine system. Within 3 days (October 1, 2018) of the first reported effects of toxins, K. brevis reached over one million cells/L and chlorophyll-a concentrations peaked at 13 µg L^-1. Within 11 days (October 9, 2018) both K. brevis and chlorophyll pigment concentrations significantly (p-perm ≤ 0.05) dropped to an average of ≤ 30,000 cells L^-1 and < 4 µg L^-1 chlorophyll-a, indicating that the bloom had diminished. Using distance-based linear modeling (DistLM) K. brevis abundance alone explained 66% of the variation in a multivariate measure of chlorophylls (driven by carotenoids and chlorophyll-c pigment concentrations), supporting a K. brevis dominated bloom. Following the K. brevis bloom, additional HAB species K. mikimotoi and Pseudo-nitzschia spp singularly explained 6% of the variations in the multivariate measure of chlorophylls. The low explanatory power of individual HAB species, including K. brevis (≤ 0%), signifies the recovery of the phytoplankton population, where non-HAB species likely contributed to the variability in the multivariate measure of chlorophylls and overall chlorophyll-a concentrations (average of 2 µg L^-1 during non-bloom conditions). Finally, we evaluated ambient and historical water quality data to assess how these parameters changed before, during, and after the 2018 K. brevis bloom. Temperature, salinity, and nutrients in the LRE were comparable to reports of other K. brevis bloom events along the west coast of Florida. Reduced ammonia-nitrogen (NH₃-N) concentrations and increased tidal amplitude coincided with the end of the bloom in 2018. More work is needed to understand the specific mechanisms constraining K. brevis blooms in tidal estuaries. We suggest that future research focus on water residence times along with nutrient availability in controlling allochthonous HABs in lotic and tidally flushed estuaries. Also, we anticipate this work may stimulate additional efforts to characterize HABs using in situ observations coupled with multivariate measures of chlorophylls, though we recognize much work remains to fully define the value of this approach.

Landscape context and nutrients modify the effects of coastal urbanisation

Estuaries are focal points for coastal cities worldwide, their habitats frequently transformed into engineered shorelines abutting waters with elevated nutrients in an urbanised landscape. Here we test for relationships between shoreline armouring and nutrients on the diversity and trophic composition of fish assemblages across 22 estuaries in eastern Australia. Urbanisation was associated with fish diversity and abundance, but there were differences in the effects of shoreline armouring and nutrient level on the trophic composition of fish assemblages. Fish diversity and the abundance of most trophic groups, particularly omnivores, zoobenthivores and detritivores, was greatest in highly urban estuaries. We show that estuarine fish assemblages are associated with urbanisation in more nuanced ways than simple habitat transformation would suggest, but this depends on the broader environmental context. Our findings have wider implications for estuarine conservation and restoration, emphasizing that ecological benefits of habitat measures may depend on both landscape attributes and water quality in urban settings.

Light, but Not Nutrients, Drives Seasonal Congruence of Taxonomic and Functional Diversity of Phytoplankton in a Eutrophic Highland Lake in China

Information on temporal dynamics of phytoplankton communities and their responses to environmental factors can provide insights into mechanisms driving succession of phytoplankton communities that is useful in programs to manage and or remediate undesirable assemblages. Populations of phytoplankton can be controlled by bottom-up factors such as nutrients and temperature or top-down such as predation by zooplankton. Traditionally, taxonomic diversity based on morphologies has been the measure used for analysis of responses to environmental factors. Recently, according to functional groupings, including functional groups (FG), morpho-FG (MFG), and morphology-based FG (MBFG), functional diversity has been used to represent functional aspects of phytoplankton communities. However, to what extent these taxonomic and functional groupings are congruent at seasonal time-scales and the main environmental factors, which drive succession, have remained less studied. Here, we analyzed absolute and relative proportions of a phytoplankton community during a 3-year period in Lake Erhai, a eutrophic highland lake in China. Alpha diversity and beta diversity, as measured by Shannon-Wiener and Bray-Curtis indices of taxonomic grouping and three functional groupings (FG, MFG, and MBFG) were applied to investigate environmental factors determining diversity. Significant, positive relationships were observed between taxonomic diversity and functional diversity that were strongly linked through seasons. In order to exclude the influence of dominant species' tolerance to extreme environments, the dominant species were excluded one by one, and the results showed that residual communities still exhibited similar patterns of succession. This synchronous temporal pattern was not principally driven by the dominant genera (Microcystis, Psephonema, and Mougeotia). Instead, the entire phytoplankton community assemblages were important in the pattern. Most diversity indices of taxonomic and functional groupings were significantly correlated with solar irradiance, but not nutrient concentrations. Because the lake is eutrophic and there were already sufficient nutrients available, additional nutrients had little effect on seasonal taxonomic and functional diversity of phytoplankton in Lake Erhai.

Hurricanes, El Niño and harmful algal blooms in two sub-tropical Florida estuaries: Direct and indirect impacts

Future increases in the intensity of hurricanes and El Niño periods predicted by climate change models have focused attention on their role in stimulating harmful algal blooms (HABs). A series of hurricanes that recently impacted Florida (USA) provided a unique opportunity to explore the relationships between hurricanes, El Niño and HABs in two Florida estuaries subject to repeated intense ecosystem disruptive HABs, the Indian River Lagoon and the St. Lucie Estuary. The roles that hurricanes and El Niño play in contributing to HAB events are examined in the context of key structural and functional features of each estuary and their watersheds, including morphology, water residence time and hydrology, such as the influence of Lake Okeechobee discharges into the St. Lucie Estuary. The most direct impact was the increase in rainfall associated with hurricanes and El Niño, resulting in enhanced nutrient loads which drive HABs in the Indian River Lagoon and Lake Okeechobee. Major HABs in Lake Okeechobee also present an indirect threat of freshwater HAB blooms in the St. Lucie Estuary via mandated discharges from the lake into the estuary during high rainfall periods. Conversely, during the absence of HABs in Lake Okeechobee, short water residence times produced by discharges into the St. Lucie Estuary can result in lower bloom intensities.

Knockdown of Dinoflagellate Cellulose Synthase CesA1 Resulted in Malformed Intracellular Cellulosic Thecal Plates and Severely Impeded Cyst-to-Swarmer Transition

Cellulose synthesis (CS) is conducted by membrane-bound cellulose synthase complexes (CSCs), containing cellulose synthases (CesA), that are either arranged in hexagonal structures in higher plants or in linear arrays in most microbial organisms, including dinoflagellates. Dinoflagellates are a major phytoplankton group having linear-type CSCs and internal cellulosic thecal plates (CTPs) in large cortical vesicles. Immunological study suggested CesA1p were cortically localized to the periphery of CTPs. During cyst-to-swarmer transition (T_C–S), synchronized peaks of CesA1 transcription, CesA1p expression, CS and CTP formation occurred in respective order, over 12–16 h, strategically allowing the study of CS regulation and CTP biogenesis. CesA1-knockdown resulted in 40% reduction in CesA1p level and time required for swarmer cells reappearance. CTPs were severely malformed with reduced cellulose content. As CTPs are deposited in internal organelle, the present study demonstrated dinoflagellate CesA1 ortholog was adapted for non-surface deposition; this is different to paradigm of other CesAps which require plasmamembrane for cellulose fiber deposition. This pioneer gene-knockdown study demonstrated the requirement of a gene for dinoflagellate cell wall remodeling and proper T_C–S, which are prominent in dinoflagellate life-cycles.

Modeling the exposure time in a tidal system: the impacts of external domain, tidal range, and inflows

Exposure time is an important characteristic for hydrodynamics that has simultaneous impacts on the biochemical processes in tidal systems. To eliminate man-made errors, decrease computational effort, and increase simulation efficiency, exposure time was evaluated under different hydrodynamic conditions for a bay to investigate the impact of the external domain on the accuracy of the computational results for exposure time. The exposure time was explicitly defined and computed using a hydrodynamic model and tracer experiments for a set of ten external domain sizes (EDS), five external domain lengths (EDL), and three special hydrodynamic conditions. The results indicated that the external domain had a significant influence on the exposure time, and the intensity of this influence was related to hydrodynamic conditions. The sensitivity of the exposure time to the external domain increased with increasing tidal range, while freshwater inflows decreased this sensitivity. However, the variation trends for exposure time with different EDS and EDL were independent of the hydrodynamic conditions. Considering the computational efficiency (maximum), the calculated error (minimum) of the exposure time, and the impact of the boundary conditions (minimum), the recommended EDS and EDL range from 9 to 13 times the initial domain size and 1.30 to 1.45 times the length in the bay, respectively. The research regarding exposure time and external domains not only helps to eliminate the errors caused by man-made factors and reduce the computational effort but also provides a reference for understanding the interrelationship between coastal waters, reciprocating flow, and the water environment.

Ocean acidification: One potential driver of phosphorus eutrophication

Harmful algal blooms which may be limited by phosphorus outbreak increases currently and ocean acidification worsens presently, which implies that ocean acidification might lead to phosphorus eutrophication. To verify the hypothesis, oxic sediments were exposed to seawater with different pH 30days. If pH was 8.1 and 7.7, the total phosphorus (TP) content in sediments was 1.52±0.50 and 1.29±0.40mg/g. The inorganic phosphorus (IP) content in sediments exposed to seawater with pH8.1 and 7.7 was 1.39±0.10 and 1.06±0.20mg/g, respectively. The exchangeable phosphorus (Ex-P) content in sediments was 4.40±0.45 and 2.82±0.15μg/g, if seawater pH was 8.1 and 7.7. Ex-P and IP contents in oxic sediments were reduced by ocean acidification significantly (p<5%). The reduced phosphorus in sediments diffused into water, which implied that ocean acidification was one potential facilitator of phosphorus eutrophication in oxic conditions.