Couplings of watersheds and coastal waters: Sources and consequences of nutrient enrichment in Waquoit Bay,Massachusetts

Human activities on coastal watersheds provide the major sources of nutrients entering shallow coastal ecosystems. Nutrient loadings from watersheds are the most widespread factor that alters structure and function of receiving aquatic ecosystems. To investigate this coupling of land to marine systems, we are studying a series of subwatersheds of Waquoit Bay that differ in degree of urbanization and hence are exposed to widely different nutrient loading rates. The subwatersheds differ in the number of septic tanks and the relative acreage of forests. In the area of our study, groundwater is the major mechanism that transports nutrients to coastal waters. Although there is some attenuation of nutrient concentrations within the aquifer or at the sediment-water interface, in urbanized areas there are significant increases in the nutrient content of groundwater arriving at the shore’s edge. The groundwater seeps or flows through the sediment-water boundary, and sufficient groundwater-borne nutrients (nitrogen in particular) traverse the sediment-water boundary to cause significant changes in the aquatic ecosystem. These loading-dependent alterations include increased nutrients in water, greater primary production by phytoplankton, and increased macroaglal biomass and growth (mediated by a suite of physiological responses to abundance of nutrients). The increased macroalgal biomass dominates the bay ecosystem through second- or third-order effects such as alterations of nutrient status of water columns and increasing frequency of anoxic events. The increases in seaweeds have decreased the areas covered by eelgrass habitats. The change in habitat type, plus the increased frequency of anoxic events, change the composition of the benthic fauna. The data make evident the importance of bottom-up control in shallow coastal food webs. The coupling of land to sea by groundwater-borne nutrient transport is mediated by a complex series of steps; the cascade of processes make it unlikely to find a one-to-one relation between land use and conditions in the aquatic ecosystem. Study of the process and synthesis by appropriate models may provide a way to deal with the complexities of the coupling.     

Evolving Paradigms and Challenges in Estuarine and Coastal Eutrophication Dynamics in a Culturally and Climatically Stressed World

Coastal watersheds support more than one half of the world’s human population and are experiencing unprecedented urban, agricultural, and industrial expansion. The freshwater–marine continua draining these watersheds are impacted increasingly by nutrient inputs and resultant eutrophication, including symptomatic harmful algal blooms, hypoxia, finfish and shellfish kills, and loss of higher plant and animal habitat. In addressing nutrient input reductions to stem and reverse eutrophication, phosphorus (P) has received priority traditionally in upstream freshwater regions, while controlling nitrogen (N) inputs has been the focus of management strategies in estuarine and coastal waters. However, freshwater, brackish, and full-salinity components of this continuum are connected structurally and functionally. Intensification of human activities has caused imbalances in N and P loading, altering nutrient limitation characteristics and complicating successful eutrophication control along the continuum. Several recent examples indicate the need for dual N and P input constraints as the only nutrient management option effective for long-term eutrophication control. Climatic changes increase variability in freshwater discharge with more severe storms and intense droughts and interact closely with nutrient inputs to modulate the magnitude and relative proportions of N and P loading. The effects of these interactions on phytoplankton production and composition were examined in two neighboring North Carolina lagoonal estuaries, the New River and Neuse River Estuaries, which are experiencing concurrent eutrophication and climatically driven hydrologic variability. Efforts aimed at stemming estuarine and coastal eutrophication in these and other similarly impacted estuarine systems should focus on establishing N and P input thresholds that take into account effects of hydrologic variability, so that eutrophication and harmful algal blooms can be controlled over a range of current and predicted climate change scenarios.     

Context-Dependent Eelgrass–Macroalgae Interactions Along an Estuarine Gradient in the Pacific Northwest,USA

Land-based eutrophication is often associated with blooms of green macroalgae, resulting in negative impacts on seagrasses. The generality of this interaction has not been studied in upwelling-influenced estuaries where oceanic nutrients dominate seasonally. We conducted an observational and experimental study with Zostera marina L. and ulvoid macroalgae across an estuarine gradient in Coos Bay, Oregon. We found a gradient in mean summer macroalgal biomass from 56.1 g dw 0.25 m⁻² at the marine site to 0.3 g dw 0.25 m⁻² at the riverine site. Despite large macroalgal blooms at the marine site, eelgrass biomass exhibited no seasonal or interannual declines. Through experimental manipulations, we found that pulsed additions of macroalgae biomass (+4,000 mL) did not affect eelgrass in marine areas, but it had negative effects in riverine areas. In upwelling-influenced estuaries, the negative effects of macroalgal blooms are context dependent, affecting the management of seagrass habitats subject to nutrient inputs from both land and sea.     

The Optical Properties of Greater Florida Bay: Implications for Seagrass Abundance

Water column optical properties of Greater Florida Bay were investigated in the context of their impacts on seagrass distribution. Scattering played an important role in light attenuation throughout the shallow water system. The northwest region was characterized by an absence of seagrasses and the highest scattering by particles, mostly from resuspended carbonate sediments. Higher seagrass densities were observed in the open waters just north of the Florida Keys, where absorption coefficients were dominated by colored dissolved organic material and scattering was lower than in the northwest region. Patchy dense seagrass meadows were observed in the clear waters south of the Keys where scattering and absorption were low and contributed equally to light attenuation. In general, seagrasses were observed in areas where >7.5% of surface irradiance reached the plants and where optical properties were not dominated by scattering. Although the prevention of eutrophication and nuisance algal blooms may be necessary for preserving seagrass meadows in this system, our observations and model calculations indicate that nutrient control alone may be insufficient to permit seagrass recolonization if optical properties are dominated by particulate scattering from resuspended sediments.     

Modeling the effects of environmental variables on short-term spatial changes in phytoplankton biomass in a large shallow lake,Lake Taihu

Understanding the short-term response of phytoplankton biomass on environmental variables is needed for issuing early warnings of harmful algal blooms in aquatic ecosystems. Predicting harmful algal blooms are particularly challenging in large shallow lakes due to their complex mixing patterns. This study used a two-dimensional hydrodynamic–phytoplankton model to evaluate the effects of environmental variables on short-term changes in the horizontal distribution of phytoplankton biomass in a large shallow lake, Lake Taihu, China. Two simulations were performed using daily and hourly average wind condition and water temperature data collected in 2009. Other model inputs were identical for these two simulations. The response of phytoplankton to wind conditions, light intensity, water temperature, and total dissolved phosphorus and nitrogen concentrations were examined based on a sensitivity analysis using the hourly data. Hourly simulation achieved a more realistic distribution of phytoplankton biomass than the daily simulation. This finding implies that data with a higher temporal resolution are more useful for short-term prediction of phytoplankton biomass in this lake. Sensitivity analysis indicated that water temperature and light intensity dominate short-term changes in phytoplankton biomass in this lake. Wind conditions also affect phytoplankton biomass distribution by causing advective water movement.     

A review of land–sea coupling by groundwater discharge of nitrogen to New England estuaries: Mechanisms and effects

Hydrologists have long been concerned with the interface of groundwater flow into estuaries, but not until the end of the last century did other disciplines realize the major role played by groundwater transport of nutrients to estuaries. Mass balance and stable isotopic data suggest that land-derived NO₃, NH₄, and dissolved organic N do enter estuaries in amounts likely to affect the function of the receiving ecosystem. Because of increasing human occupancy of the coastal zone, the nutrient loads borne by groundwater have increased in recent decades, in spite of substantial interception of nutrients within the land and aquifer components of watersheds. Groundwater-borne nutrient loads have increased the N content of receiving estuaries, increased phytoplankton and macroalgal production and biomass, decreased the area of seagrasses, and created a cascade of associated ecological changes. This linkage between land use and eutrophication of estuaries occurs in spite of mechanisms, including uptake of land-derived N by riparian vegetation and fringing wetlands, “unloading” by rapid water removal, and direct N inputs to estuaries, that tend to uncouple the effects of land use on receiving estuaries. It can be expected that as human activity on coastal watersheds continues to increase, the role of groundwater-borne nutrients to the receiving estuary will also increase.     

Influence of Hydrological Regime in Determining the Response of Macroalgal Blooms to Nutrient Loading in Two Irish Estuaries

Eutrophication and the development of persistent opportunistic macroalgal blooms are recognised as one of the main detrimental effects of increased anthropogenic pressures on estuarine and coastal systems. This study aimed to highlight the interplay between pressures and controlling physical factors on ecosystem functioning. The hypothesis that hydrological regime can control the growth of opportunistic macroalgae was tested with the study of two Irish estuaries, the Argideen and the Blackwater, with similar nutrient loading sources but divergent hydrological regimes. Seasonal monitoring data was initially examination, while the application of a pre-existing box model allowed a further analysis of the influence of residence time and nutrient load modifications on macroalgal growth. Seasonal oscillations in monitored river flow rates altered nutrient transfer from the catchments to the estuaries in both cases, as is shown through differences between winter and summer nutrient concentrations. In the Argideen, however, the relative contribution of phosphorus (P) from adjacent marine waters was high due to the shorter residence times and greater influx of marine water into the estuary. Modelling studies showed that in the Argideen Estuary, P load reduction would have potentially minimal impact on macroalgal growth due to the shorter residence time which increased the influx of P from marine sources. Nitrogen (N) load reduction of 60 % had a significant, albeit limited, impact on macroalgae and was insufficient in achieving the environmental objectives for this waterbody. For the more river-dominated Blackwater Estuary, modelled reductions in P resulted in a considerable decrease in biomass. Any further P decreases would accentuate the existing disparity in estuarine N:P ratios with possible repercussions for N transport to the coastal system. Hence, the hydrological complexity of estuarine systems demonstrated dictates that a portfolio of separate, but complimentary, management approaches may be required to address eutrophication in these estuaries.     

Impacts of temperature and nutrients on coastal lagoon plant communities

We investigated the independent and interactive effects of nutrient loading and summer water temperature on phytoplankton, drift macroalgae, and eelgrass (Zostera marina) in a coastal lagoon mesocosm experiment conducted from May through August 1999. Temperature treatments consisted of controls that approximated the 9-yr mean daily temperatures for Ninigret and Point Judith Lagoons in Rhode Island (United States) and treatments approximately 4°C above and 4°C below the controls. Nutrient treatments consisted of the addition of 6 mmol N m⁻²d⁻¹ and 0.5 mmol P m⁻² d⁻¹ to mesocosms 4°C above and 4°C below the 9-yr daily mean. Nutrient enrichment produced marked phytoplankton blooms in both cool and warm treatments during early summer. These were replaced after midsummer by dramatic growths of macroalgal mats ofEnteromorpha flexuosa and, to a lesser degree,Cladophora sericea. No phytoplankton blooms were observed in the cool unenriched treatments, but blooms did develop in the mean temperature and warm mesocosms during the second half of the summer that were similar in intensity, though of shorter duration, than those observed earlier in the enriched systems. Macroalgal blooms did not occur in the unenriched mesocosms. Sustained warm water temperatures markedly decreased eelgrass density and belowground production and increased the time interval between the initiation of new leaves, particuarly when the biomass of macroalgae was high. The negative effect of elevated water temperature on eelgrass was significantly increased under conditions of elevated inorganic nutrient input. By the end of summer, virtually all of the measures of eelgrass health declined in rank order from cool, to mean, to cool enriched, to warm, to warm enriched treatments. It is likely that the marked declines in eelgrass abundance observed during recent decades in the Northeast have resulted from an interaction of increasing nutrient enrichment combined with increasing summer water temperatures.     

Biotic effects of environmental catastrophes at the end of the Cretaceous and early Tertiary: Guembelitria and Heterohelix blooms

In this study we report similar biotic response patterns in planktic foraminiferal assemblages, whether in association with volcanism, impacts or climate change at the end of the Cretaceous and early Tertiary. During and after each type of catastrophe two groups dominate high stress assemblages: (1) the small Guembelitria species, which are interpreted as having thrived in eutrophic surface waters where other species rarely survived; and (2) the low oxygen tolerant small Heterohelix species, which thrived at times of an expanding oxygen minimum zone associated with high nutrients and a stratified water column. The ecosystem collapse appears to be primarily the result of high macro- and micronutrient influx (from impacts, volcanism and erosion) leading to eutrophication and phytoplankton blooms (i.e., primary producers) that result in toxic conditions for foraminifera. Once nutrients decrease due to consumption by phytoplankton, the first opportunistic foraminifera, the Guembelitria, appear and graze on phytoplankton, rapidly reproduce (heterochronic acceleration) and increase populations exponentially. With nutrient depletion Guembelitria populations rapidly decrease leading to ecologic niches for other generalists and ecosystem recovery. Small low O₂ tolerant heterohelicid populations mark this second stage, followed by small trochospiral and planispiral species. With further environmental recovery, increasing competition, niche development, and restoration of a well-stratified watermass, oligotrophic conditions are restored opening habitats for large, highly specialized species and a return to normal diverse assemblages. Such highly stressed ecological successions are observed in association with mantle plume volcanism in the Indian Ocean, Andean volcanism in Argentina and shallow inland seas in Egypt and Madagascar during the late Maastrichtian, the K-T impact, volcanism during the early Danian, and intense upwelling and climate extremes. We present a simple model to explain the ecological succession and recovery phases that follow major biotic perturbations.     

Multivariate Analysis of Water Quality and Plankton Assemblages in an Urban Estuary

Raritan Bay, located between the states of New York and New Jersey, has a long history of cultural eutrophication and associated harmful algal blooms (HABs). Despite striking chemical and biological alterations occurring in Raritan Bay, publications in the early 1960s were the last to report consecutive measurements of both water quality parameters and plankton species composition in this system. The objectives of this study were to characterize water quality trends and plankton composition in a eutrophic estuary, compare current environmental conditions to those documented in Raritan Bay 50 years ago (i.e., at the same six sampling sites), and to further clarify the relationship among nutrients, secondary consumers, and algal bloom generation in this system using ordination techniques. This study (monthly data collected from April 2010–October 2012) indicates that Raritan Bay continues to exhibit numerous symptoms of eutrophication, including high algal biomass, high turbidity, violations of the dissolved oxygen standard to protect fish health, and blooms of potentially harmful phytoplankton species. Altered spatial and temporal patterns for nitrate and soluble reactive phosphorus (SRP) over the past 50 years may suggest new, changing, or expanding sources of nutrients. A total of 14 HAB species have been identified, including Heterosigma akashiwo, which formed a bloom in the upper Raritan Bay during summer 2012 in association with hypoxic conditions. Multivariate analyses indicate that abundance of this species is positively associated with high temperature, salinity, nitrate, and SRP and negatively associated with spring river discharge rates and total zooplankton abundance in Raritan Bay.     

Variations in stable carbon isotope ratio of the copepod Acartia tonsa during the onset of the Texas brown tide

The Laguna Madre of South Texas is a shallow coastal lagoon whose dominant primary producers shifted from seagrasses to phytoplankton with the onset of the Texas brown tide, which persisted from 1990 through 1997. Acartia tonsa is the dominant component of the mesozooplankton and forms an important link in both the phytoplankton and detritus-based pelagic food webs. Stable carbon isotope ratios of A. tonsa, as well as the two major primary producers: phytoplankton (as particulate organic carbon) and seagrasses, were measured from March 1989 to October 1991. Zooplankton samples were collected at four locations in the Laguna Madre: two in shallow water (c. 1 m) over seagrass beds and two in slightly deeper water (c. 2–3 m) over a muddy bottom in a secondary bay without seagrasses. We found seasonal trends in the isotopic composition of A. tonsa collected within both habitats as well as distinct differences between the average {ie995-1} values of individuals collected in the two regions. Isotopic ratios of animals collected during the summer months were generally 4–8‰ enriched in ¹³C compared with those collected in the winter, at all stations. A. tonsa collected over seagrass beds were 2–5‰ more enriched in ¹³C than those collected over muddy bottoms. These observations suggest carbon derived from seagrasses can be an important source of nutrition for these copepods in summer, especially for copepods living over seagrass beds. The effects of the persistent brown tide decreased the contribution of seagrasses as a carbon source for A. tonsa during the summer of 1991. The pathway by which seagrass carbon enters the diet of A. tonsa is unclear, but the two pathways considered most likely are through copepods feeding on microzooplankton that have fed on bacteria nourished on seagrass carbon, or by copepods feeding directly on particles of seagrass detritus.     

Diel oxygen dynamics and anoxic events in an eutrophic estuary of Waquoit Bay,Massachusetts

As a result of nutrient loading from septic systems, a thick canopy of macroalgae covers the bottom of Waquoit Bay, an embayment on Cape Cod, Massachusetts. Using automated conductivity-temperature-oxygen recorders and manual profiles, we measured diel water column O₂ changes during summer in the Childs River, the estuary of the bay with the highest housing density. At dawn in midsummer, bottom waters in the Childs River are chronically hypoxic due to high rates of benthic respiration. On sunny days benthic photosynthesis drives bottom water O₂ to 10–15 mg l^?1 by afternoon. The extent of the daily O₂ excursion is directly proportional to daily irradiance. Large diel O₂ excursions in bottom water are due to limited mixing of surface and bottom water. Density stratification exceeded two sigma-t units 85% of the time during midsummer in the Childs River. Because of stratification, hypoxia and even anoxia occur in this estuary. The first of several anoxic events was observed in Waquoit Bay in 1988, and we have attempted to evaluate factors that trigger anoxia. High rates of benthic respiration result in anoxia when replenishment of O₂ during the day is limited by insufficient light. Our analysis of meteorological records during two recent anoxic events shows that anoxia develops overnight in midsummer during periods of peak summertime temperatures after several days of cloudy, moderately calm weather. Similarly critical conditions existed most summers since 1975, yet anoxic events in the bay have not been reported historically. If climatic warming occurs, anoxic events in the bay may occur more frequently even if algal stocks remain unchanged. Eutrophication of Waquoit Bay is similar to many other embayments in populated coastal areas, and anoxic events may indicate a chronic growing problem in these important ecosystems. However, in shallow, stratified embayments, anoxia may be transient and easily missed without frequent monitoring.     

Extreme Eutrophication in Shallow Estuaries and Lagoons of California Is Driven by a Unique Combination of Local Watershed Modifications That Trump Variability Associated with Wet and Dry Seasons

Rapidly growing human populations have caused heavy modifications to the watersheds of many Mediterranean climate estuaries, subjecting them to excessive nutrient enrichment and harmful macroalgal blooms. Despite these impacts, comprehensive studies in these systems are rare and comparisons between systems are lacking. We surveyed five southern California estuaries that ranged in size from 93 to 1,000 ha and incorporated differing land usages and watershed sizes. We sampled environmental variables (sediment redox potential, organic content, total nitrogen and total phosphorus, water column nitrate, ammonium, and salinity) and macroalgal cover and biomass quarterly at three locations within each estuary over 15 months to compare spatial and wet vs. dry season patterns. Maximum mean water column nitrate concentration across all estuaries ranged from 47 to 1,700 μM, showing that all estuaries were highly enriched with nitrogen, at least at some times. Mean macroalgal biomass ranged from 0 to 1,500 g wet wt m^?2. However, neither nutrient concentrations nor algal biomass showed consistent seasonal patterns as maximum values occurred in different seasons in different estuaries. Three-dimensional principal components analysis followed by regression analyses confirmed that macroalgal abundance was not directly related to water or sediment N concentrations. Rather each of these southern California estuaries showed individual patterns in all measured variables, which were most likely induced by a suite of physical modifications unique to each system and its watershed.     

Hypoxia in a Coastal Embayment of the Chesapeake Bay: A Model Diagnostic Study of Oxygen Dynamics

Two distinct hypoxic patterns were revealed from high-frequency dissolved oxygen (DO) data collected from North Branch of Onancock Creek, a shallow coastal estuary of the Chesapeake Bay, from July to October 2004. Diurnal hypoxia developed associated with large DO swings during fair weather and hypoxia/anoxia developed for prolonged 2–5-day periods following rainfall events. A simplified diagnostic DO-algae model was used to investigate DO dynamics in the creek. The model results show that the modeling approach enables important features of the DO dynamics in the creek to be captured and analyzed. Large anthropogenic inputs of nutrients to the creek stimulated macroalgae blooms in the embayment. High DO production resulted in supersaturated DO in daytime, whereas DO was depleted at night as the high respiration overwhelmed the DO supply, leading to hypoxia. Unlike deep-water environments, in this shallow-water system, biological processes dominate DO variations. High macroalgae biomass interacting with low light and high temperature trigger the development of prolonged hypoxic/anoxic postrainfall events.     

Seasonal Dynamics of Dissolved Trace Metals in the Scheldt Estuary: Relationship with Redox Conditions and Phytoplankton Activity

Seasonal dynamics of dissolved trace metals (Cd, Co, Cu, Ni and Zn) and its relationship with redox conditions and phytoplankton activity has been studied in the Scheldt estuary, during nine surveys carried out between May 1995 and June 1996. Seasonal profiles of dissolved trace metals and general estuarine water quality variables are compared, to identify the geochemical and biological processes responsible for the observed trace metal distributions. In keeping with previous studies, the behavior of dissolved Cd, Cu, and Zn can be explained by the presence of anoxic headwaters and the restoration of dissolved oxygen within the estuary. In the river water, the concentration of dissolved Cu and Zn is generally low, except during winter when dissolved oxygen is present in the water column, although highly undersaturated. Mobilization of particle-bound Cd, Cu, and Zn occurs as dissolved oxygen increases with increasing salinity, possibly because of oxidation of metal sulfides in the suspended matter. The geochemistry of dissolved Co is also related to the redox conditions but in an opposite way. Dissolved Co is mobilized in the anoxic upper estuary, along with the reduction in Mn (hydro) oxides, and subsequently coprecipitated with Mn (hydro) oxides when dissolved oxygen is restored. Conservative behavior is observed for dissolved Ni within the estuary. In the middle estuary, Cd and Zn are readsorbed during phytoplankton blooms, as suggested by the low concentrations of these metals during the most productive periods in spring and early summer. The removal may be caused by direct biological uptake and/or increased adsorption to suspended matter because of the pH increase associated with algae blooms. In the lower estuary, chemical gradients are much weaker and dilution with seawater is the dominant process.     

松辽盆地嫩江组油页岩发育控制因素

松辽盆地嫩江组一段和二段发育的油页岩是极其优质烃源岩, 生物标志化合物参数指示油页岩母质生源主要来自于水生生物其繁盛受温湿气候和湖泊富营养化控制.岩石学、元素地球化学、生物标志化合物等参数表明, 高盐和缺氧条件下的稳定水体分层是嫩一段油页岩沉积时期有机质保存的主控因素, 而嫩二段油页岩沉积时期水体盐度分层明显减弱, 此时藻类个体增大和孤立藻数量增多条件下的高生物生产力有利于有机质富集.为揭示嫩江组不同沉积环境下油页岩发育的控制因素, 建立了嫩一段油页岩沉积时期保存条件控制和嫩二段油页岩沉积时期生物生产力控制的两种有机质富集模式, 这对含氧淡水大型陆相湖泊的有机质富集成因探讨具有重要借鉴意义.      

Eutrophication and management initiatives for the control of nutrient inputs to Rhode Island coastal lagoons

An assessment of developing eutrophic conditions in small temperate lagoons along the coast of Rhode Island suggests that in such shallow, macrophyte based systems the response to nutrient enrichment differs from that described for plankton based systems. The nitrogen loadings per unit area of the salt ponds are 240–770 mmol N per m² per year. Instead of the high nutrient concentrations, increased phytoplankton biomass and turbidity, leading to eventual loss of benthic macrophytes described for such systems as the Chesapeake, Patuxent and Appalachicola Bay, nutrient enrichment of the Rhode Island lagoons has led to increased growth of marine macroalgae. The increased macroalgal growth appears to alter the benthic habitat and a shift from a grazing to detrital food chain appears to be impacting important shellfisheries. As more extensive areas of organic sediments develop, geochemical cycling changes, resulting in higher rates of nitrogen remineralization and accelerated eutrophication. The major sources of nitrogen inputs to the salt ponds have been identified and a series of management initiatives have been designed to limit inputs from present and potential development within the watersheds of the lagoons.     

Nutrient pulses, plankton blooms, and seasonal hypoxia in western Long Island Sound

Development of seasonal hypoxia was studied weekly in the western narrows of Long Island Sound (WLIS) during the summers of 1992 and 1993 by measuring hydrographic properties, biological oxygen demand (BOD), biomass, production, and mortality of phytoplankton and bacterioplankton in the water column. Dissolved oxygen in bottom waters was low and variable during stratified periods (19–51% saturation), oscillating in and out of hypoxic conditions (defined as <3 mg O₂ l⁻¹ or 94 μM O₂). Hypoxia was more prevalent in 1993 than in 1992, corresponding to greater water column stratification in 1993. Microbial BOD in bottom waters appeared to be fueled by delivery of autochthonous carbon from phytoplankton blooms rather than allochthonous carbon input. Phytoplankton production responded to elevated NH₄ ⁺ concentrations, especially when the mixed layer was shallow. NH₄ ⁺ concentrations generally varied as a function of the preceding week's rainfall (r²=0.765). Bacterial production did not covary with phytoplankton production, yet was closely correlated with particulate organic carbon, which was chlorophyll-rich. Results indicate that the timing and severity of hypoxia development are strongly coupled to allochthonous input of NH₄ ⁺ after heavy precipitation. Observations illustrate for the first time that bottom waters in this system oscillate in and out of hypoxia on an almost weekly basis rather than sustain them over the entire stratified period. The frequency of these oscillations depends upon variations in nutrients, planktonic production and export, and bottom water ventilation.     

Drivers of Change in Shallow Coastal Photic Systems: An Introduction to a Special Issue

Coastal ecosystems are characterized by relatively deep, plankton-based estuaries and much shallower systems where light reaches the bottom. These latter systems, including lagoons, bar-built estuaries, the fringing regions of deeper systems, and other systems of only a few meters deep, are characterized by a variety of benthic primary producers that augment and, in many cases, dominate the production supplied by phytoplankton. These “shallow coastal photic systems” are subject to a wide variety of both natural and anthropogenic drivers and possess numerous natural “filters” that modulate their response to these drivers; in many cases, the responses are much different from those in deeper estuaries. Natural drivers include meteorological forcing, freshwater inflow, episodic events such as storms, wet/dry periods, and background loading of optically active constituents. Anthropogenic drivers include accelerated inputs of nutrients and sediments, chemical contaminants, physical alteration and hydrodynamic manipulation, climate change, the presence of intensive aquaculture, fishery harvests, and introduction of exotic species. The response of these systems is modulated by a number of factors, notably bathymetry, physical flushing, fetch, sediment type, background light attenuation, and the presence of benthic autotrophs, suspension feeding bivalves, and fringing tidal wetlands. Finally, responses to stressors in these systems, particularly anthropogenic nutrient enrichment, consist of blooms of phytoplankton, macroalgae, and epiphytic algae, including harmful algal blooms, subsequent declines in submerged aquatic vegetation and loss of critical habitat, development of hypoxia/anoxia particularly on short time scales (i.e., “diel-cycling”), fish kills, and loss of secondary production. This special issue of Estuaries and Coasts serves to integrate current understanding of the structure and function of shallow coastal photic systems, illustrate the many drivers that cause change in these systems, and synthesize their varied responses.     

Ecological Impacts of Macroalgal Blooms on Salt Marsh Communities

Despite excessive growth of macroalgae in estuarine systems, little research has been done to examine the impacts of increased algal biomass that drifts into nearby salt marshes and accumulates on intertidal flats. The accumulation of macroalgal mats and subsequent decomposition-related releases of limiting nutrients may potentially alter marsh communities and impact multiple trophic levels. We conducted a 2-year in situ study, as well as laboratory mesocosm experiments, to determine the fate of these nutrients and any bottom-up impacts from the blooms on the dominant salt marsh plant (Spartina alterniflora) and herbivores. Mesocosm results showed that macroalgal decomposition had a positive impact on sediment nitrogen concentrations, as well as S. alterniflora growth rates. In contrast, our in situ results suggested that S. alterniflora growth was hindered by the presence of macroalgal mats. From our results, we suggest that macroalgal accumulation and subsequent release of nitrogen during decomposition may be beneficial in nitrogen limited areas. However, as marshes are becoming increasingly eutrophic, releasing lower marsh plants from nitrogen limitation, this accumulation of macroalgal biomass may hinder S. alterniflora growth through smothering and breakage of culms. As macroalgal blooms are predicted to intensify with rising temperatures and increased eutrophication, the ecological impacts associated with these changes need to be continuously monitored in order to preserve these fragile ecosystems.