Non-monotonic Responses of Phytoplankton Biomass Accumulation to Hydrologic Variability: A Comparison of Two Coastal Plain North Carolina Estuaries

Freshwater inputs often play a more direct role in estuarine phytoplankton biomass (chlorophyll a) accumulation than nitrogen (N) inputs, since discharge simultaneously controls both phytoplankton residence time and N loading. Understanding this link is critical, given potential changes in climate and human activities that may affect discharge and watershed N supply. Chlorophyll a (chla) relationships with hydrologic variability were examined in 3-year time series from two neighboring, shallow (<5?m), microtidal estuaries (New and Neuse River estuaries, NC, USA) influenced by the same climatic conditions and events. Under conditions ranging from drought to floods, N concentration and salinity showed direct positive and negative responses, respectively, to discharge for both estuaries. The response of chla to discharge was more complex, but was elucidated through conversion of discharge to freshwater flushing time, an estimate of transport time scale. Non-linear fits of chla to flushing time revealed non-monotonic, unimodal relationships that reflected the changing balance between intrinsic growth and losses through time and along the axis of each estuary. Maximum biomass occurred at approximately 10-day flushing times for both systems. Residual analysis of the fitted data revealed positive relationships between chla and temperature, suggesting enhanced growth rates at higher temperatures. N loading and system-wide, volume-weighted chla were positively correlated, and biomass yields per N load were greater than other marine systems. When combined with information on loss processes, these results on the hydrologic control of phytoplankton biomass will help formulate mechanistic models necessary to predict ecosystem responses to future climate and anthropogenic changes.     

Particle suspensions and their regions of effect in the Neuse River Estuary: Implications for water quality monitoring

Monitoring of estuarine condition depends on water quality parameters that have significant and interpretable ecological effects and can be assessed either in situ or via rapid laboratory techniques. Two commonly measured parameters are water column turbidity (NTU) and total suspended solids (TSS). Under certain conditions, either of these measures could represent a proxy for runoff and provide rapid, in situ measures to improve protection of the public by decreasing the time required to detect and monitor associated effects (e.g., reduced water clarity and eutrophication). The Neuse River Estuary (NRE) has experienced a decline in water quality due to anthropogenic inputs, including stormwater containing nutrient and particle loads. Water samples were collected from the NRE during both dry weather and storm events over 16 mo across the entire estuarine gradient. Particle size distributions, ratio of particulate organic carbon to nitrogen, chlorophylla (chla), TSS, and NTU were measured in each of these samples, with the data separated into regions based on salinity and depth of sample collection. Particle analyses were directed at identifying suspensions dominated by phytoplankton, runoff particles, or resuspended sediments. Particle size distributions for suspensions in Region I (all samples with salinity less than 2) varied little during sampling, resulting in a robust NTU-TSS relationship. This relationship confirmed the usefulness of turbidity as a measure of runoff and resuspension of recently deposited runoff in the upper NRE. Phytoplankton cells represented a majority of the particles in Region II (surface samples with salinity greater than 2), based on the close relationship between chla and total particle volume in these samples. Suspensions of large, nearly uniform diameter particles, which are likely aggregates and resuspended sediment, were observed in Region III (bottom samples with salinity greater than 2). Using these techniques as part of routine monitoring, particle suspension measures in different hydrographic regions of an estuary provide evidence useful for identifying the source and water quality consequences of particle suspensions (e.g., microbial contamination and algal blooms).     

Changes in Phytoplankton Biomass in the Western Scheldt Estuary During the Period 1978–2006

We investigated whether climate change results in long-term changes in phytoplankton biomass and phenology in a turbid eutrophic coastal plain estuary. Changes in annual mean chlorophyll a (chla) concentrations were studied for the period 1978–2006 in the eutrophic and turbid macro-tidal Western Scheldt estuary. Three stations were investigated: WS1, at the mouth of the estuary; station WS6, halfway up the estuary; and station WS11, near the Dutch–Belgian border near the upstream end of the estuary. No significant long-term changes in yearly averaged chla concentrations were observed in WS1 and WS6, but in WS11 the phytoplankton biomass decreased considerably. This is most likely due to an increase in grazing pressure as a result of an improvement in the dissolved oxygen concentrations. Spectral analyses revealed a possible periodicity of 7 years in the mean chla which was related to periodicity in river discharge. We also observed strong phenological responses in the timing of the spring/summer bloom which were related to a well-documented increase in the temperature in the estuary. The fulcrum, the center of gravity or the day at which 50% of the cumulative chla was reached during the year, advanced by 1–2 days/year. A similar trend was observed for the month in which the maximum bloom was observed, with the exception of station WS1. All stations showed an earlier initiation of the bloom, whereas the day at which the phytoplankton bloom was terminated also moved forward in time excepted for WS11. As a result, the bloom length decreased at station WS1, remained the same at station WS6, and increased at WS11. This complicated pattern in bloom phenology demonstrates the complex nature of ecosystem functioning in estuaries.     

Phytoplankton Community Indicators of Short- and Long-term Ecological Change in the Anthropogenically and Climatically Impacted Neuse River Estuary, North Carolina, USA

Estuarine and coastal systems represent a challenge when it comes to determining the causes of ecological change because human and natural perturbations often interact. Phytoplankton biomass (chlorophyll a) and group-specific photopigment indicators were examined from 1994 to 2007 to assess community responses to nutrient and climatic perturbations in the Neuse River Estuary, NC. This system experienced nutrient enrichment and hydrologic variability, including droughts, and an increase in hurricanes. Freshwater input strongly interacted with supplies of the limiting nutrient nitrogen (N) and temperature to determine the location, magnitude, and composition of phytoplankton biomass. Multi-annual, seasonal, and episodic hydrologic perturbations, including changes in the frequency and intensity of tropical storms, hurricanes and droughts, caused significant shifts in phytoplankton community structure. Climatic oscillations can at times overwhelm anthropogenic nutrient inputs in terms of controlling algal bloom thresholds, duration, and spatial extent. Eutrophication models should incorporate climatically driven changes to better predict phytoplankton community responses to nutrient inputs and other anthropogenic perturbations.     

A particle conveyor belt process in the Columbia River estuary: Evidence from chlorophyll<Emphasis Type="Italic">a</Emphasis> and particulate organic carbon

Using both the photosynthetically active chlorophylla (chla) content of the organic carbon fraction of suspended particulate matter (chla/POC) and the percentage of photosynthetically, active chla in fluorometrically measured chla plus pheophytina (% chla), we determined that under specified hydrodynamic conditions, neap-spring tidal differentiation in particle dynamics could be observed in the Columbia River estuary. During summer time neap tides, when river discharge was moderate, bottom chla/POC remained relatively unchanged from riverine chla/POC over the full 0–30 psu salinity range, suggesting a benign trapping environment. During summertime spring tides, bottom chla/POC decreased at mid range salinities indicating resuspension of chla-poor POC during flood-ebb transitions. Bottom % chla during neap tides tended to average higher than that during spring tides, suggesting that neap particles were more recently hydrodynamically trapped than those on the spring tides. Such differentiation supported the possibility of operation of a particle conveyor belt process, a process in which low-amplitude neap tides favor selective particle trapping in estuarine turbidity maxima (ETM)., while high-amplitude spring tides favor particle resuspension from the ETM. Untrapped river-derived particles at the surface would continue through the estuary to the coastal ocean on the neap tide; during spring tide some particles eroded from the ETM would combine with unsettled riverine particles in transit toward the ocean. Because in tensified biogeochemical activity is associated with ETM, these neap-spring differences may be critical to maintenance and renewal of populations and processes in the estuary. Very high river discharge (15, 000 m³ s⁻¹) tended to overwhelm neap-spring differences, and significant oceanic input during very low river discharge (5,000 m³ s⁻¹) tended to do the same in the estuarine channel most exposed to ocean input. During heavy springtime phytoplankton blooms, development of a thick bottom fluff layer rich in chla also appeared to negate neapspring differentiation because spring tides apparently acted to resuspend the same rich bottom material that was laid down during neap tides. When photosynthetic assimilation numbers [μgC (μgchl,a)⁻¹h⁻¹] were measured across, the full salinity range, no neap-spring differences and no river discharge effects occurred, indicating that within our suite of measurements the compositional distinction of suspended particulate material was mainly a function of chla/POC, and to a lesser extent % chla. Even though these measurements suggest the existence of a conveyor belt process, proof of actual operation of this phenomenon requires scalar flux measurements of chla properties in and out of the ETM on both neap and spring tides.     

Effects of hurricane floyd inland flooding,September–October 1999, on tributaries to Pamlico Sound,North Carolina

Hurricane Floyd in September 1999 caused disastrous flooding from South Carolina to Massachusetts in the United States, with particularly severe and prolonged flooding in eastern North Carolina resulting in record flood-flow loadings of freshwater and contaminants to Pamlico Sound, North Carolina. The inland flooding, water quality, and loadings to Pamlico Sound were determined as part of a multi-agency response to the floods and in an effort to understand the effects of the floods on the greater Pamlico Sound Basin. All major river basins draining to Pamlico Sound experienced floods at the 500-yr recurrence level. The volume of flood waters entering Pamlico Sound during September–October 1999 was estimated to be equivalent to about 95% of the volume of Pamlico Sound, meaning that flood waters could have essentially displaced most of the water present in Pamlico Sound. Nitrogen and phosphorus loads to the Pamlico River estuary and Neuse River estuary, the two principal estuaries draining to Pamlico Sound, in a 36-d period during the flooding were between 50–90% of the long term average annual loads. Pesticide concentrations in flood waters were surprisingly high, given the amount of dilution produced by the floodwaters.     

Phytoplankton reference communities for Chesapeake Bay and its tidal tributaries

Phytoplankton reference communities for Chesapeake Bay were quantified from least-impaired water quality conditions using commonly measured parameters and indicators derived from measured parameters. A binning approach was developed to classify water quality. Least-impaired conditions had relatively high water column transparency and low concentrations of dissolved inorganic nitrogen and orthophosphate. Reference communities in all seasons and salinity zones are characterized by consistently low values of chlorophylla and pheophytin coupled with relative stable proportions of the phytoplankton taxonomic groups and low biomasses of key bloom-forming species. Chlorophyll cell content was lower and less variable and average cell size and seasonal picophytoplankton biomass tended to be greater in the mesohaline and polyhaline reference communities as compared to the impaired communities. Biomass concentrations of the nano-micro phytoplankton size fractions (2–200 μm) in 12 of the 16 season-specific and salinity-specific reference communities were the same or higher than those in impaired habitat conditions, suggesting that nutrient reductions will not decrease the quantity of edible phytoplankton food available to large consumers. High (bloom) and low (bust) biomass events within the impaired phytoplankton communities showed strikingly different chlorophyll cell content and turnover rates. Freshwater flow had little effect on phytoplankton responses to water quality condition in most of the estuary. Improved water column transparency, or clarity, through the reduction of suspended sediments will be particularly important in attaining the reference communities. Significant nitrogen load reductions are also required.     

Alternation of factors limiting phytoplankton production in the Cape Fear River Estuary

Phytoplankton nutrient limitation experiments were performed from 1994 to 1996 at three stations in the Cape Fear River Estuary, a riverine system originating in the North Carolina piedmont. Nutrient addition bioassays were conducted by spiking triplicate cubitainers with various nutrient combinations and determining algal response by analyzing chlorophyll a production and ¹⁴C uptake daily for 3 d. Ambient chlorophyll a, nutrient concentration, and associated physical data were collected throughout the estuary as well. At a turbid, nutrient-rich oligohaline station, significant responses to nutrient additions were rare, with light the likely principal factor limiting phytoplankton production. During summer at a mesohaline station, phytoplankton community displayed significant nitrogen (N) limitation, while both phosphorus (P) and N were occasionally limiting in spring with some N+P co-limitation. Light was apparently limiting during fall and winter when the water was turid and nutrient-rich, as well as during other months of heavy rainfall and runoff. A polyhaline station in the lower estuary had clearer water and displayed significant responses to nutrient additions during all enrichment experiments. At this site N limitation occurred in summer and fall, and P limitation (with strong N+P co-limitation) occurred in winter and spring. The data suggest there are two patterns controlling phytoplankton productivity in the Cape Fear system: 1) a longitudinal pattern of decreasing light limitation and increasing nutrient sensitivity along the salinity gradient, and 2) a seasonal alternation of N limitation, light limitation, and P limitation in the middle-to-lower estuary. Statistical analyses indicated upper watershed precipitation events led to increased flow, turbidity, light attenuation, and nutrient loading, and decreased chlorophyll a and nutrient limitation potential in the estuary. Periods of low rainfall and river flow led to reduced estuarine turbidity, higher chlorophyll a, lower ambient nutrients, and more pronounced nutrient limitation.     

A simple empirical optical model for simulating light attenuation variability in a partially mixed estuary

Representation of the subsurface light field is a crucial component of pelagic ecosystem and water quality models. Modeling the light field in estuaries is a particularly complicated problem due to the significant influence of high concentrations of dissolved and particulate matter that are derived from both terrestrial and estuarine sources. The goal of this study was to develop a relatively simple but effective way to model light attenuation variability in a turbuid estuary (Chesapeake Bay, United States) in a coupled physical-biological model. We adopted a simple, nonspectral empirical approach. Surface water quality data (salinity was used as a proxy of chromophoric dissolved organic matter [CDOM]) and light measurements from the Chesapeake Bay Program were used to determine the absorption coefficients in a linear attenuation model using regression methods. This model predicts K_c (specific attenuation due to phytoplankton/chlorophylla [chla]), K_t (specific attenuation due to total suspended solids), and K_s (a function of specific attenuation coefficients of CDOM in relation to salinity). The Bay-wide fitted relation between the light attenuation coefficient and water quality concentrations gives generally good estimates of total light attenuation, K_d. The direct inclusion of salinity in the relationship has one disadvantage: it can yield negative values for K_d at high salinities. We developed two separate models for two different salinity regimes. This approach, in addition to solving the negative K_d problem, also accounts for some changes in specific light absorption by chla, seston (nonphytoplankton particulate matter), and CDOM that apparently occur in different salinity regimes in Chesapeake Bay. The resulting model predicts the statistical characteristics (i.e., the mean and variance) of K_d quite accurately in most regions of Chesapeake Bay. We also discuss in this paper the feasibility and caveats of using K_d converted from Secchi depth in the empirical method.     

A decade of change in the Skidaway River estuary II. Particulate organic carbon, nitrogen, and chlorophylla

A sampling program was initiated in 1986 in the Skidaway River estuary, a tidally dominated subtropical estuary in the southeastern USA. Hydrography, nutrients, particulate organic matter (POM), and microbial and plankton abundance and composition were measured at weekly intervals at high and low tide on the same day at a single site. Hydrographic and nutrient data during 1986–1996 were given in Verity (2002); particulate organic carbon (POC), nitrogen (PON) and chlorophylla (chla) are presented here; plankton data will be presented elsewhere. Chla was fractionated into <8 μm and >8 μm size classes. All classes of POM exhibited distinct seasonal patterns superimposed upon significant long-term increases during the study period. Total chla, <8 μm chla, and >8 μm chla increased 36%, 61%, and 18%, respectively, however the fraction of total biomass attributable to small phytoplankton (<8 μm) increased 25%. The annual amplitude between minimum and maximum stock sizes increased significantly, meaning that bloom events became larger. POC and PON also increased 16% over the decade and, as observed with patterns in chla, exhibited increases in annual amplitude. The C:N ratio was typically 6.4–6.6 (wt:wt) and did not change significantly, while the annual mean C:Chla ratio decreased 19% from 165 to 140. These characteristics indicated highly labile POM composed of significant amounts of detritus, but which became increasingly autotrophic with time. Averaged over the decade, temperature explained 45–50% of the variance in POM. Nutrients were even better predictors of POM, as 60–75% of the variance in chla, POC, and PON were explained by ambient concentrations of DIN, or PO₄. Combined with significant increases in NO₃, NH₄, PO₄, Si(OH)₄, and DON during 1986–1996, these data strongly suggest that anthropogenic activities contributed to increased loading of dissolved nutrients, which became incorporated into living and nonliving particulate organic matter.     

Estuarine Phytoplankton Responses to Hurricanes and Tropical Storms with Different Characteristics (Trajectory,Rainfall, Winds)

We examined the short-term (<1 month post-storm) impact of storms [Tropical Storm (TS) Helene in 2000, Hurricane (H) Isabel in 2003, H Alex, Tropical Depression (TD) Bonnie and TS Charley in 2004] varying in their trajectory, wind and rainfall characteristics, on water column structure, nutrients, and phytoplankton biomass in North Carolina’s Neuse R. Estuary (NRE). Data are presented from two sampling programs, ModMon (biweekly) and FerryMon (measurements made every 3 min daily). Helene’s winds mixed the previously stratified water column, delivering sediment-bound nutrients to the euphotic zone, and localized freshwater input from Helene was also evident. Mean chlorophyll a concentrations in the mesohaline portion of the NRE, where N was strongly limiting before the storm (molar DIN:DIP < 1), more than doubled after the storm. Unlike with Helene, the water column was well mixed before passage of Isabel, and nutrient concentrations were high. As a result, minimal impact on phytoplankton biomass was detected despite Isabel’s high winds and significant freshwater input. In fact, conditions became less favorable for phytoplankton growth after the storm. Alex was fast moving and relatively small, but its winds were sufficient to mix the water column. Although data from ModMon suggest that chlorophyll a was only slightly higher after passage of Alex, FerryMon detected an ephemeral bloom that was missed by ModMon. Overall, these results suggest that relatively small tropical storms and hurricanes can lead to significant increases in phytoplankton biomass. However, the phytoplankton response depends on both the characteristics of a particular storm and the physical–chemical conditions of the water column before storm passage. Finally, the ephemeral bloom that developed as a result of Alex, the strong response of phytoplankton in the mesohaline portion of the estuary to nutrient inputs, and their patchiness on several other occasions suggests that storms may create “hot spots” for trophic transfer and biogeochemical dynamics in estuaries. Adaptive sampling is necessary to capture these features and to fully understand the impact of perturbations such as storms on estuarine ecosystem functioning.     

Formation and seasonal evolution of Atlantic menhaden juvenile nurseries in coastal estuaries

A hypothesis on the formation and seasonal evolution of Atlantic menhaden (Brevoortia tyrannus) juvenile nurseries in coastal estuaries is described. A series of cruises were undertaken to capture postmetamorphic juvenile menhaden and to characterize several biological and physical parameters along estuarine gradients. The two study systems, the Neuse and Pamlico rivers in North Carolina, contain important menhaden nursery grounds. Juvenile menhaden abundance was found to be associated with gradients of phytoplankton biomass as evidenced by chlorophylla levels in the upper water column. Fish abundances were only secondarily associated with salinity gradients as salinity was a factor that moderated primary production in the estuary. The persistence of spatial and temporal trends in the distribution of phytoplankton in the Neuse and Pamlico estuaries was reviewed. The review suggested that postmetamorphic juvenile menhaden modify their distribution patterns to match those created by phytoplankton biomass, which in turn makes them most abundant in the phytoplankton maxima of estuaries. Because the location of these maxima varies with the mixing and nutrient dynamics of different estuaries, so will the location of the nursery.     

Phytoplankton, nutrients, and transparency in Danish coastal waters

We present a comparative analysis of 1400 data series of water chemistry (particularly nitrogen and phosphorus concentrations), phytoplankton biomass as chlorophylla (chla) concentrations, concentrations of suspended matter and Secchi depth transparency collected from the mid-1980s to the mid-1990s from 162 stations in 27 Danish fjords and coastal waters. The results demonstrate that Danish coastal waters were heavily eutrophied and had high particle concentrations and turbid waters. Median values were 5.1 μg chla 1⁻¹, 10.0 mg DW 1⁻¹ of suspended particles, and Secchi depth of 3.6 m. Chlorophyll concentration was strongly linked to the total-nitrogen concentration. The strength of this relationship increased from spring to summer as the concentration of total nitrogen declined. During summer, total nitrogen concentrations accounted for about 60% of the variability in chlorophyll concentrations among the different coastal systems. The relationship between chlorophyll and total phosphorus was more consistant over the year and correlations were much weaker than encountered for total nitrogen. Secchi depth could be predicted with good precision from measurements of chlorophyll and suspended matter. In a multiple stepwise regression model with In-transformed values the two variables accounted for most of the variability in water transparency for the different seasons and the period March–October as a whole (c. 80%). We were able to demonstrate a significant relationship between total nitrogen and Secchi depth, with important implications for management purposes.     

Responses of estuarine phytoplankton communities to nitrogen form and mixing using microcosm bioassays

We examined the effects of different forms of nitrogen and mixed versus static conditions on the structure and function of natural Neuse River estuary phytoplankton communities incubated in 66-liter microcosms in March, May, August, and November 1999. Significant differences were found between effects of mixed versus static treatments in three of four experiments, but no differences were observed between effects of different forms of nitrogen. Mixed incubations resulted in higher contributions of diatoms to total community biomass (measured as chlorophylla) than in static tanks in May. Significantly higher rates of carbon fixation were also observed, likely due to increased suspension of diatoms in surface (illuminated) layers of the tanks. In August, we found significantly higher abundances of cyanobacteria, total community biomass, and rates of carbon fixation in static tanks than in tanks that were mixed. In November, static incubations showed significantly higher abundances of cryptophytes resulting in higher total community biomass and rates of carbon fixation in static tanks than in mixed tanks. Nitrogen additions significantly increased total community biomass relative to controls in May and August, indicating that the communities were nitrogen-limited at these times. We conclude that while nitrogen additions may result in increases in phytoplankton biomass when nitrogen is limiting, phytoplankton community structure in the Neuse River Estuary may be determined more by the hydrodynamics of the system (mixing versus stratification) than by the form of nitrogen available for growth.     

Nutrient limitation in the lower Housatonic River estuary

One of the most serious threats to freshwater and marine ecosystems is high rates of anthropogenic nutrient loading, particularly nitrogen (N) and phophorus (P). One of the major freshwater sources of nutrients to Long Island Sound (LIS) is the Housatonic River (HR). Current management plans that call for reducing N inputs without reducing P inputs may change the N: P ratio in the water column and the pattern of algal nutrient limitation and species composition in the tidal portion of the river. To assess the current pattern of algal nutrient limitation in the HR estuary, nutrient bioassays were conducted in spring, summer, and fall at 5 sites throughout the tidal portion and adjacent LIS. Diatoms were a dominant taxon at all sites throughout the sampling period. Other seasonally important taxa include cyanobacteria, cryptophytes, and euglenoids. Phytoplankton in LIS were always strongly N limited and were co-limited by P in spring. During low flow (summer), phytoplankton in the lower HR estuary were N limited. Phytoplankton in the middle reaches showed no evidence of N or P limitation and were likely limited by other factors. In spring, phytoplankton in the upper HR estuary were P limited. Periods of N or P limitation were better correlated with periods of lower concentrations of nitrate or phosphate than with differences in N: P ratio. These results suggest that decreases in N concentration could increase the prevalence of N limitation throughout the estuary that in turn may reduce phytoplankton biomass and alter species composition of the phytoplankton.     

The Influence of Coastal Nutrients on Phytoplankton Productivity in a Shallow Low Inflow Estuary,Drakes Estero,California (USA)

Seasonal wind-driven upwelling along the U.S. West Coast supplies large concentrations of nitrogen to surface waters that drives high primary production. However, the influence of coastal upwelled nutrients on phytoplankton productivity in adjacent small estuaries and bays is poorly understood. This study was conducted in Drakes Estero, California, a low inflow estuary located in the Point Reyes National Seashore and the site of an oyster mariculture facility that produces 40 % of the oysters harvested in California. Measurements of nutrients, chlorophyll a, phytoplankton functional groups, and phytoplankton carbon and nitrogen uptake were made between May 2010 and June 2011. A sea-to-land gradient in nutrient concentrations was observed with elevated nitrate at the coast and higher ammonium at the landward region. Larger phytoplankton cells (>5 μm diameter) were dominant within the outer and middle Estero where phytoplankton primary productivity was fueled by nitrate and f-ratios were >0.5; the greatest primary production rates were in the middle Estero. Primary production was lowest within the inner Estero, where smaller phytoplankton cells (<5 μm) were dominant, and nitrogen uptake was dominated by ammonium. Phytoplankton blooms occurred at the outer and middle Estero and were dominated by diatoms during the spring and dry-upwelling seasons but dinoflagellates during the fall. Small flagellated algae (>2 μm) were dominant at the inner Estero where no blooms occurred. These results indicate that coastal nitrate and phytoplankton are imported into Drakes Estero and lead to periods of high new production that can support the oyster mariculture; a likely scenario also for other small estuaries and bays.     

Environmental Forcing of Phytoplankton in a Mediterranean Estuary (Guadiana Estuary,South-western Iberia): A Decadal Study of Anthropogenic and Climatic Influences

Phytoplankton seasonal and interannual variability in the Guadiana upper estuary was analyzed during 1996–2005, a period that encompassed a climatic controlled reduction in river flow that was superimposed on the construction of a dam. Phytoplankton seasonal patterns revealed an alternation between a persistent light limitation and episodic nutrient limitation. Phytoplankton succession, with early spring diatom blooms and summer–early fall cyanobacterial blooms, was apparently driven by changes in nutrients, water temperature, and turbulence, clearly demonstrating the role of river flow and climate variability. Light intensity in the mixed layer was a prevalent driver of phytoplankton interannual variability, and the increased turbidity caused by the Alqueva dam construction was linked to pronounced decreases in chlorophyll a concentration, particularly at the start and end of the phytoplankton growing period. Decreases in annual maximum and average abundances of diatoms, green algae, and cyanobacteria were also detected. Furthermore, chlorophyll a decreases after dam filling and a decrease in turbidity may point to a shift from light limitation towards a more nutrient-limited mode in the near future.     

Human development is linked to multiple water body impairments along the California coast

To elucidate relationships between land cover and water quality along the central California coast, we collected monthly samples from 14 coastal waterway outlets representing various degrees of human development. Sites were distributed between three salinity categories, freshwater, estuarine, and marine, to better understand land cover-water quality relationships across a range of coastal aquatic ecosystems. Samples were analyzed for fecal indicator bacteria (FIB), dissolved nutrients, stable nitrogen isotopes in particulate organic matter, and chlorophylla (chla). Sediment samples from 11 sites were analyzed for the concentration of the anthropogenic organic contaminant perfluorooctane sulfonate and its precursors (ΣPFOS). While the data indicated impairment by nutrient, microbial, and organic contaminants at both agricultural and urban sites, the percentage of agricultural land cover was the most robust indicator of impairment, showing significant correlations (p<0.05) to FIB, nutrient, chla, and ΣPFOS levels. FIB densities were strongly influenced by salinity and were highest at sites dominated by agriculture and urbanization. Nutrient levels and chla correlated to both agricultural and urban land use metrics as well. Positive correlations among FIB, nutrients, chla, and ΣPFOS suggest a synergy between microbial, nutrient, and organic pollution. The results emphasize the importance of land management in protecting coastal water bodies and human health, and identify nutrient, microbial, and organic pollution as prevalent problems in coastal California water bodies.     

Fine-scale responses of phytoplankton to freshwater influx in a tropical monsoonal estuary following the onset of southwest monsoon

In May of 2007, a study was initiated by the National Institute of Oceanography (NIO), Goa, India, to investigate the influence of monsoonal rainfall on hydrographic conditions in the Mandovi River of India. The study was undertaken at a location ∼2 km upstream of the mouth of this estuary. During the premonsoon (PreM) in May, when circulation in the estuary was dominated by tidal activity, phytoplankton communities in the high saline (35–37 psu) waters at the study site were largely made up of the coastal neritic species Fragilaria oceanica, Ditylum brightwellii and Trichodesmium erythraeum. During the later part of the intermonsoon (InterM) phase, an abrupt decline in salinity led to a surge in phytoplankton biomass (Chlorophyll a ∼14 mg m^− 3), of a population that was dominated by Thalassiosira eccentricus. As the southwest monsoon (SWM) progressed and the estuary freshened salinity and Chlorophyll a (Chl a) concentrations decreased during the MoN, Skeletonema costatum established itself as the dominant form. Despite the low biomass (Chl a <2 mg m^− 3), the phytoplankton community of the MoN was the most diverse of the entire study. During the postmonsoon (PostM), the increase in salinity was marked by a surge in dinoflagellate populations comprising of Ceratium furca, Akashiwo sanguinea, and Pyrophacus horologium.     

Variations in the onset of bottom-water intrusions over the entrance sill of a fjord

Puget Sound is a fjord-like estuary and bottom-water intrusions are major circulation features which play a dominant role in the replacement of water below still depth. New observations on the inside and outside of the entrance sill show that, while intrusions occur during neap tides as previously thought, the onset of the intrusions is a results off fluctuations in the horizontal density gradient caused by salinity variations across the sill. Salinity changes outside the sill in the Strait of Juan de Fuca estuary appear to be the result of storms on the Pacific Coast causing reversals of surface flow and variations in deep flow more than 135 km from the coast. Previous observations have shown deep salinity variations midway along the straight, but these are the first to show this effect can penetrate the full length of the Strait causing near-bottom salinity variations of sufficient magnitude to influence flow into Puget Sound. This influence probably occurs from the onset of storms in autumn through subsidence in spring, although occasional large storms occur in summer. Although Puget Sound is more characteristic of a fjord, the simple model calculations here suggest similar processes may occur in lower-layer flow at the mouth of coastal plain estuaries.