Temporally-displaced Mississippi River spring flood pulse shows muted aquatic ecosystem response in estuarine waters: A climate change warning for coastal foodwebs.

Mississippi River water levels typically rise in the early spring after snow melt in the extensive watershed. However, in 2016, warm air temperatures coupled with high precipitation led to a historically early river flood pulse, resulting in the opening of a flood release valve (Bonnet Carré Spillway) in early January to protect the city of New Orleans, Louisiana. The goal of this research was to determine the ecosystem response of this wintertime nutrient flood pulse on the receiving estuarine system and compare it to historical opening responses, which are generally several months later. Nutrients, TSS, and Chl a were measured along a 30 km transect in the Lake Pontchartrain estuary, before, during, and after the river diversion event. In the past, NOx concentrations were quickly reduced to below detection in the estuary in <4 weeks post-event accompanied by a moderate phytoplankton bloom. However, due to seasonal limitations (cold water temperatures and light limitation) during the 2016 event, NOx remained elevated for >2 months post-closure and Chl a values were low, indicating limited assimilation of nutrients into phytoplankton biomass. Consequently, much of the bioavailable nitrogen was denitrified by sediments and dispersed to the coastal ocean over time, limiting the transfer of nutrients into the food web by means of a spring phytoplankton bloom. An increasing warming trend in temperate and polar river watersheds is leading to earlier spring flood pulses, altering the timing of coastal nutrient transport, decoupled from conditions supporting primary production, which could significantly affect coastal food webs.

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.

Domoic Acid and Pseudo-nitzschia spp. Connected to Coastal Upwelling along Coastal Inhambane Province, Mozambique: A New Area of Concern.

Harmful algal blooms (HABs) are increasing globally in frequency, persistence, and geographic extent, posing a threat to ecosystem and human health. To date, no occurrences of marine phycotoxins have been recorded in Mozambique, which may be due to absence of a monitoring program and general awareness of potential threats. This study is the first documentation of neurotoxin, domoic acid (DA), produced by the diatom Pseudo-nitzschia along the east coast of Africa. Coastal Inhambane Province is a biodiversity hotspot where year-round Rhincodon typus (whale shark) sightings are among the highest globally and support an emerging ecotourism industry. Links between primary productivity and biodiversity in this area have not previously been considered or reported. During a pilot study, from January 2017 to April 2018, DA was identified year-round, peaking during Austral winter. During an intense study between May and August 2018, our research focused on identifying environmental factors influencing coastal productivity and DA concentration. Phytoplankton assemblage was diatom-dominated, with high abundances of Pseudo-nitzschia spp. Data suggest the system was influenced by nutrient pulses resulting from coastal upwelling. Continued and comprehensive monitoring along southern Mozambique would provide critical information to assess ecosystem and human health threats from marine toxins under challenges posed by global change.

Marine phytoplankton responses to oil and dispersant exposures: Knowledge gained since the Deepwater Horizon oil spill.

The Deepwater Horizon oil spill of 2010 brought the ecology and health of the Gulf of Mexico to the forefront of the public's and scientific community's attention. Not only did we need a better understanding of how this oil spill impacted the Gulf of Mexico ecosystem, but we also needed to apply this knowledge to help assess impacts from perturbations in the region and guide future response actions. Phytoplankton represent the base of the food web in oceanic systems. As such, alterations of the phytoplankton community propagate to upper trophic levels. This review brings together new insights into the influence of oil and dispersant on phytoplankton. We bring together laboratory, mesocosm and field experiments, including insights into novel observations of harmful algal bloom (HAB) forming species and zooplankton as well as bacteria-phytoplankton interactions. We finish by addressing knowledge gaps and highlighting key topics for research in novel areas.

The polychaete, Paraprionospio pinnata, is a likely vector of domoic acid to the benthic food web in the northern Gulf of Mexico.

A somewhat disparate, yet temporally cohesive, set of phytoplankton abundance, microphytobenthos, including the diatom Pseudo-nitzschia, benthic infauna, and sediment toxin data were used to develop a theory for the transfer of domoic acid (DA) from the toxic diatom to the benthos in the highly productive waters of the northern Gulf of Mexico near the Mississippi River plume. Archived samples and new data were used to test the theory that DA is likely to be incorporated into benthic consumers. High spring abundances of potentially toxic Pseudo-nitzschia diatoms were simultaneously present in the surface waters, bottom waters and on the seafloor. Examination of the gut contents of a typical deposit-feeding and suspension-feeding polychaete, Paraprionospio pinnata, during similar periods of high Pseudo-nitzschia abundance in surface water indicated consumption of the diatoms. Demersal fishes, particularly Atlantic croaker, are known to consume these polychaetes, with a potential for transfer of DA to even higher trophic levels. These findings warrant a theory to be tested with further studies about the trophic linkage of a phytoplankton toxin into the benthic food web.

Divergent gene expression among phytoplankton taxa in response to upwelling.

Frequent blooms of phytoplankton occur in coastal upwelling zones creating hotspots of biological productivity in the ocean. As cold, nutrient-rich water is brought up to sunlit layers from depth, phytoplankton are also transported upwards to seed surface blooms that are often dominated by diatoms. The physiological response of phytoplankton to this process, commonly referred to as shift-up, is characterized by increases in nitrate assimilation and rapid growth rates. To examine the molecular underpinnings behind this phenomenon, metatranscriptomics was applied to a simulated upwelling experiment using natural phytoplankton communities from the California Upwelling Zone. An increase in diatom growth following 5 days of incubation was attributed to the genera Chaetoceros and Pseudo-nitzschia. Here, we show that certain bloom-forming diatoms exhibit a distinct transcriptional response that coordinates shift-up where diatoms exhibited the greatest transcriptional change following upwelling; however, comparison of co-expressed genes exposed overrepresentation of distinct sets within each of the dominant phytoplankton groups. The analysis revealed that diatoms frontload genes involved in nitrogen assimilation likely in order to outcompete other groups for available nitrogen during upwelling events. We speculate that the evolutionary success of diatoms may be due, in part, to this proactive response to frequently encountered changes in their environment.

The Effect of Atrazine on Louisiana Gulf Coast Estuarine Phytoplankton.

Pesticides may enter water bodies in areas with a high proportion of agricultural land use through surface runoff, groundwater discharge, and erosion and thus negatively impact nontarget aquatic organisms. The herbicide atrazine is used extensively throughout the Midwest and enters the Mississippi River through surface runoff and groundwater discharge. The purpose of this study was to determine the extent of atrazine contamination in Louisiana's estuaries from Mississippi River water under different flow and nutrient regimes (spring and summer) and its effect on the biomass and oxygen production of the local phytoplankton community. The results showed that atrazine was consistently present in these systems at low levels. Microcosm experiments exposed to an atrazine-dilution series under low and high nutrient conditions to determine the phytoplankton stress response showed that high atrazine levels greatly decreased phytoplankton biomass and oxygen production. Phytoplankton exposed to low and moderate atrazine levels under high nutrient conditions were able to recover after an extended acclimation period. Communities grown under high nutrient conditions grew more rapidly and produced greater levels of oxygen than the low nutrient treatment groups, thus indicating that atrazine exposure may induce a greater stress response in phytoplankton communities under low-nutrient conditions. The native community also experienced a shift from more sensitive species, such as chlorophytes, to potentially more resilient species such as diatoms. The phytoplankton response to atrazine exposure at various concentrations can be especially important to greater trophic levels because their growth and abundance can determine the potential productivity of the entire ecosystem.

Induction of reactive oxygen species in marine phytoplankton under crude oil exposure.

Exposure of phytoplankton to the water-accommodated fraction of crude oil can elicit a number of stress responses, but the mechanisms that drive these responses are unclear. South Louisiana crude oil was selected to investigate its effects on population growth, chlorophyll a (Chl a) content, antioxidative defense, and lipid peroxidation, for the marine diatom, Ditylum brightwellii, and the dinoflagellate, Heterocapsa triquetra, in laboratory-based microcosm experiments. The transcript levels of several possible stress-responsive genes in D. brightwellii were also measured. The microalgae were exposed to crude oil for up to 96 h, and Chl a content, superoxide dismutase (SOD), the glutathione pool (GSH and GSSG), and lipid peroxidation content were analyzed. The cell growth of both phytoplankton species was inhibited with increasing crude oil concentrations. Crude oil exposure did not affect Chl a content significantly in cells. SOD activities showed similar responses in both species, being enhanced at 4- and 8-mg/L crude oil exposure. Only H. triquetra demonstrated enhanced activity in GSSG pool and lipid peroxidation at 8-mg/L crude oil exposure, suggesting that phytoplankton species have distinct physiological responses and tolerance levels to crude oil exposure. This study indicated the activation of reactive oxygen species (ROS) in phytoplankton under crude oil exposure; however, the progressive damage in cells is still unknown. Thus, ROS-related damage in nucleic acid, lipids, proteins, and DNA, due to crude oil exposure could be a worthwhile subject of study to better understand crude oil toxicity at the base of the food web.

Responses of sympatric Karenia brevis, Prorocentrum minimum, and Heterosigma akashiwo to the exposure of crude oil.

Impacts of the Deepwater Horizon oil spill on phytoplankton, particularly, the tolerability and changes to the toxin profiles of harmful toxic algal species remain unknown. The degree to which oil-affected sympatric Karenia brevis, Prorocentrum minimum, and Heterosigma akashiwo, all of which are ecologically important species in the Gulf of Mexico, was investigated. Comparison of their tolerability to that of non-toxic species showed that the toxin-production potential of harmful species does not provide a selective advantage. Investigated toxin profiles for K. brevis and P. minimum demonstrated an increase in toxin productivity at the lowest crude oil concentration (0.66 mg L(-1)) tested in this study. Higher crude oil concentrations led to significant growth inhibition and a decrease in toxin production. Findings from this study could assist in the assessment of shellfish bed closures due to high risk of increased toxin potential of these phytoplankton species, especially during times of stressed conditions.

Relative phytoplankton growth responses to physically and chemically dispersed South Louisiana sweet crude oil.

We conducted controlled laboratory exposure experiments to assess the toxic effects of water-accommodated fractions (WAFs) of South Louisiana sweet crude oil on five phytoplankton species isolated from the Gulf of Mexico. Experiments were conducted with individual and combinations of the five phytoplankton species to determine growth inhibitions to eight total petroleum hydrocarbon (TPH) equivalent concentrations ranging from 461 to 7,205 ppb. The composition and concentration of crude oil were altered by physical and chemical processes and used to help evaluate crude oil toxicity. The impact of crude oil exposure on phytoplankton growth varied with the concentration of crude oil, species of microalgae, and their community composition. At a concentration of TPH < 1,200 ppb, dinoflagellate species showed significantly better tolerance, while diatom species showed a higher tolerance to crude oil at higher concentrations of TPH. For both groups, the larger species were more tolerant to crude oil than smaller ones. The toxicity potential of crude oil seems to be strongly influenced by the concentration of polycyclic aromatic hydrocarbons (PAHs). The addition of the dispersant, Corexit® EC9500A, increased the amount of crude oil up to 50-fold in the water column, while the physical enhancement (vigorous mixing of water column) did not significantly increase the amount of TPH concentration in the water column. The species response to crude oil was also examined in the five-species community. Each phytoplankton species showed considerably less tolerance to crude oil in the five-species community compared to their individual responses. This study provides baseline information about individual phytoplankton responses to crude oil and dispersed crude oil for subsequent research efforts seeking to understand the impacts of oil on the phytoplankton in the bigger picture.

Distinct responses of Gulf of Mexico phytoplankton communities to crude oil and the dispersant corexit(®) Ec9500A under different nutrient regimes.

This study examines the potential effects of exposure to South Louisiana sweet crude oil (LSC), Corexit(®) EC9500A, and dispersed oil on enclosed phytoplankton communities under different nutrient regimes. Three distinct microcosm experiments were conducted for 10 days to assess changes to the structure of natural communities from the Gulf of Mexico as quantified by temporal changes in the biomasses of different phytoplankton groups. Concentration of NO3, Si and PO4 were 0.83, 0.99 and 0.09 µM for the unenriched treatments and 14.07, 13.01 and 0.94 µM for the enriched treatments, respectively. Overall, the contaminants LSC and Corexit(®) EC9500A led to a decrease in the number of sensitive species and an increase in more resistant species. Phytoplankton communities showed more sensitivity to LSC under nutrient-limited conditions. The addition of nutrients to initially nutrient-limited treatments lessened the inhibitory effect of LSC in the short term. Centric diatoms benefited most from this enrichment, but pennate diatoms demonstrated considerably greater tolerance to crude oil at low crude oil concentrations in nutrient-enriched treatments. Dinoflagellates showed relatively higher tolerance in nutrient-limited treatments and high crude oil concentrations. Corexit(®) EC9500A inputs significantly increased the toxicity of crude oil. Corexit(®) EC9500A alone had a highly inhibitory effect at 63 ppm on phytoplankton communities. This study highlights the fact that different nutrient regimes play a major role in determining the shifts of the phytoplankton community in response to exposure to different concentrations of crude oil and dispersant. Determination of the functional equivalence of shifted phytoplankton groups could complement our research and allow for more pertinent extrapolation to real world conditions.

Can crude oil toxicity on phytoplankton be predicted based on toxicity data on benzo(a)pyrene and naphthalene?

Polycyclic aromatic hydrocarbons (PAHs), which are major components of crude oil, are responsible in large part for the toxicity of crude oil to phytoplankton. This study addressed the following question. Can reliable predictions of the aquatic toxicity of crude oil, a multi-component mixture, be described from toxicity data on individual PAH compounds? Naphthalene, the most abundant PAH compound, and benzo(a)pyrene, a highly toxic PAH compound, were selected as model compounds to quantify toxicity of crude oil on two phytoplankton species, Ditylum brightwellii and Heterocapsa triquetra, by analyzing the effects of different concentrations of these PAHs on growth rate. EC50 values suggested that the diatom D. brightwellii was more vulnerable to both toxicants than the dinoflagellate H. triquetra. However, a previous study, which investigated the impact of crude oil on the same two species, had opposite results. The differences in response from these phytoplankton species to naphthalene and benzo(a)pyrene toxicity compared to their response to crude oil suggest that they may not be solely used as surrogates to assess crude oil toxicity on phytoplankton.

Estuarine ecosystem response to three large-scale Mississippi River flood diversion events.

Large inflows of nitrogen (N)-rich freshwater to estuaries can lead to expressions of eutrophication including harmful algal blooms of cyanobacteria (CyanoHABs). Lake Pontchartrain is a large, oligohaline estuary that occasionally receives episodic diversions of N-rich Mississippi River water via the Bonnet Carré Spillway to alleviate flood threats to New Orleans, LA. The extreme flood stage of the Lower Mississippi River in May 2011 prompted the tenth opening of the spillway since 1937. The 2011 opening occurred later in the season than the previous two lower discharge events (1997 and 2008) and was characterized by dissolved inorganic N loads 1.7 and 2.6 times greater than the 1997 and 2008 events, respectively. Rapid depletion of riverine nitrate (21 days) occurred post-spillway closure in 2011 with no associated CyanoHAB and was followed by an internal pulse of phosphorus (P) from sediments to restore N-limitation. Our analysis of recent spillway openings indicates that there is not a simple stimulus-response relationship between N loading and CyanoHAB formation. We investigate the systemic causal relationships that determine ecosystem response to these nutrient-rich freshwater inflows and highlight several important parameters including: external N loading, timing, magnitude, plume hydrodynamics, nutrient molar ratios, internal P loading, weather, and northern tributary discharge. Our results suggest that the turbulent, fluctuating environment and nutrient composition during diversions does not favor CyanoHAB formation and that the immense size and timing of the 2011 diversion may have resulted in near complete post-diversion CyanoHAB suppression by hydraulic flushing.

Toxic diatoms and domoic acid in natural and iron enriched waters of the oceanic Pacific.

Near-surface waters ranging from the Pacific subarctic (58°N) to the Southern Ocean (66°S) contain the neurotoxin domoic acid (DA), associated with the diatom Pseudo-nitzschia. Of the 35 stations sampled, including ones from historic iron fertilization experiments (SOFeX, IronEx II), we found Pseudo-nitzschia at 34 stations and DA measurable at 14 of the 26 stations analyzed for DA. Toxin ranged from 0.3 fg·cell(-1) to 2 pg·cell(-1), comparable with levels found in similar-sized cells from coastal waters. In the western subarctic, descent of intact Pseudo-nitzschia likely delivered significant amounts of toxin (up to 4 µg of DA·m(-2)·d(-1)) to underlying mesopelagic waters (150-500 m). By reexamining phytoplankton samples from SOFeX and IronEx II, we found substantial amounts of DA associated with Pseudo-nitzschia. Indeed, at SOFeX in the Antarctic Pacific, DA reached 220 ng·L(-1), levels at which animal mortalities have occurred on continental shelves. Iron ocean fertilization also occurs naturally and may have promoted blooms of these ubiquitous algae over previous glacial cycles during deposition of iron-rich aerosols. Thus, the neurotoxin DA occurs both in coastal and oceanic waters, and its concentration, associated with changes in Pseudo-nitzschia abundance, likely varies naturally with climate cycles, as well as with artificial iron fertilization. Given that iron fertilization in iron-depleted regions of the sea has been proposed to enhance phytoplankton growth and, thereby, both reduce atmospheric CO(2) and moderate ocean acidification in surface waters, consideration of the potentially serious ecosystem impacts associated with DA is prudent.

The role of domoic acid in abortion and premature parturition of California sea lions (Zalophus californianus) on San Miguel Island, California.

Domoic acid is a glutaminergic neurotoxin produced by marine algae such as Pseudo-nitzschia australis. California sea lions (Zalophus californianus) ingest the toxin when foraging on planktivorous fish. Adult females comprise 60% of stranded animals admitted for rehabilitation due to acute domoic acid toxicosis and commonly suffer from reproductive failure, including abortions and premature live births. Domoic acid has been shown to cross the placenta exposing the fetus to the toxin. To determine whether domoic acid was playing a role in reproductive failure in sea lion rookeries, 67 aborted and live-born premature pups were sampled on San Miguel Island in 2005 and 2006 to investigate the causes for reproductive failure. Analyses included domoic acid, contaminant and infectious disease testing, and histologic examination. Pseudo-nitzschia spp. were present both in the environment and in sea lion feces, and domoic acid was detected in the sea lion feces and in 17% of pup samples tested. Histopathologic findings included systemic and localized inflammation and bacterial infections of amniotic origin, placental abruption, and brain edema. The primary lesion in five animals with measurable domoic acid concentrations was brain edema, a common finding and, in some cases, the only lesion observed in aborted premature pups born to domoic acid-intoxicated females in rehabilitation. Blubber organochlorine concentrations were lower than those measured previously in premature sea lion pups collected in the 1970s. While the etiology of abortion and premature parturition was varied in this study, these results suggest that domoic acid contributes to reproductive failure on California sea lion rookeries.

From sanddabs to blue whales: the pervasiveness of domoic acid.

Domoic acid (DA) is a potent food web transferred algal toxin that has caused dramatic mortality events involving sea birds and sea lions. Although no confirmed DA toxicity events have been reported in whales, here we present data demonstrating that humpback and blue whales are exposed to the toxin and consume DA contaminated prey. Whale fecal samples were found to contain DA at levels ranging from 10 to 207microg DA g(-1) feces via HPLC-UV methods. SEM analysis of whale feces containing DA, collected from krill-feeding whales, revealed the presence of diatom frustules identified as Pseudo-nitzschia australis, a known DA producer. Humpback whales were observed feeding on anchovies and sardines that contained DA at levels ranging from 75 to 444microg DA g(-1) viscera. DA contamination of whale feces and fish occurred only during blooms of toxic Pseudo-nitzschia. Additionally, several novel fish species collected during a toxic diatom bloom were tested for DA. Fish as diverse as benthic sanddabs and pelagic albacore were found to contain the neurotoxin, suggesting that DA permeates benthic as well as pelagic communities.