Sediment-water oxygen and nutrient exchanges along the longitudinal axis of Chesapeake Bay: Seasonal patterns,controlling factors and ecological significance

Sediment-water oxygen and nutrient (NH₄ ⁺, NO₃ ^?+NO₂ ^?, DON, PO₄ ^3?, and DSi) fluxes were measured in three distinct regions of Chesapeake Bay at monthly intervals during 1 yr and for portions of several additional years. Examination of these data revealed strong spatial and temporal patterns. Most fluxes were greatest in the central bay (station MB), moderate in the high salinity lower bay (station SB) and reduced in the oligohaline upper bay (station NB). Sediment oxygen consumption (SOC) rates generally increased with increasing temperature until bottom water concentrations of dissolved oxygen (DO) fell below 2.5 mg l^?1, apparently limiting SOC rates. Fluxes of NH₄ ⁺ were elevated at temperatures >15°C and, when coupled with low bottom water DO concentrations (<5 mg l^?1), very large releases (>500 μmol N m^?2 h^?1) were observed. Nitrate + nitrite (NO₃ ^?+NO₂ ^?) exchanges were directed into sediments in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were high (>18 μM N); sediment efflux of NO₃ ^?+NO₂ ^? occurred only in areas where bottom water NO₃ ^?+NO₂ ^? concentrations were relatively low (<11 μM N) and bottom waters well oxygenated. Phosphate fluxes were small except in areas of hypoxic and anoxic bottom waters; in those cases releases were high (50–150 μmol P m^?2 h^?1) but of short duration (2 mo). Dissolved silicate (DSi) fluxes were directed out of the sediments at all stations and appeared to be proportional to primary production in overlying waters. Dissolved organic nitrogen (DON) was released from the sediments at stations NB and SB and taken up by the sediments at station MB in summer months; DON fluxes were either small or noninterpretable during cooler months of the year. It appears that the amount and quality of organic matter reaching the sediments is of primary importance in determining the spatial variability and interannual differences in sediment nutrient fluxes along the axis of the bay. Surficial sediment chlorophyll-a, used as an indicator of labile sediment organic matter, was highly correlated with NH₄ ^?, PO₄ ^3?, and DSi fluxes but only after a temporal lag of about 1 mo was added between deposition events and sediment nutrient releases. Sediment O:N flux ratios indicated that substantial sediment nitrification-denitrification probably occurred at all sites during winter-spring but not summer-fall; N:P flux ratios were high in spring but much less than expected during summer, particularly at hypoxic and anoxic sites. Finally, a comparison of seasonal N and P demand by phytoplankton with sediment nutrient releases indicated that the sediments provide a substantial fraction of nutrients required by phytoplankton in summer, but not winter, especially in the mid bay region.     

Nutrient dynamics in the Pomeranian Bay (southern Baltic): Impact of the Oder River outflow

The Pomeranian Bay is a coastal region fed by the Oder River, one of the seven largest Baltic rivers, whose waters flow through a large and complex estuarine system before entering the bay. Nutrients (NO₃ ⁻, NO₂ ⁻, NH₄ ⁺, Ntot, PO₄ ³⁻, P_tot, DSi), chlorophylla concentrations, oxygen content, salinity, and temperature were measured in the Pomeranian Bay in nine seasonally distributed cruises during 1993–1997. Strong spatial and temporal patterns were observed and they were governed by: the seasonally variable riverine water-nutrient discharges, the seasonally variable uptake of nutrients and their cycling in the river estuary and the Bay, the character of water exchange between the Pomeranian Bay and the Szczecin Lagoon, and the water flow patterns in the Bay that are dominated by wind-driven circulation. Easterly winds resulted in water and nutrient transport along the German coastline, while westerly winds confined the nutrient rich riverine waters to the Polish coast and transported them eastward beyond the study area. Two water masses, coastal and open, characterized by different chemical and physical parameters and chla content were found in the Bay independently of the season. The role of the Oder estuary in nutrient transformation, as well as the role of temperature in transformation processes is stressed in the paper. The DIN:DIP:DSi ratio indicated that phosphorus most probably played a limiting role in phytoplankton production in the Bay in spring, while nitrogen did the same in summer. During the spring bloom, predominated by diatoms, the DSi:DIN ratio dropped to 0.1 in the coastal waters and to 0.6 in the open bay waters, pointing to silicon limitation of diatom growth, similar to what is being observed in other Baltic regions.     

Inorganic and Organic Nitrogen Uptake by Phytoplankton and Bacteria in Hong Kong Waters

Measurements of uptake rates of inorganic (NO₃⁻ and NH₄⁺) and organic (urea, glycine, and glutamic acid) N, and indirect estimates of total N uptake by bacteria, were made in four contrasting environments in sub-tropical Hong Kong waters in summer of 2008. In addition, the effects of several days of rain on N uptake rates were studied in eastern waters. Although ambient NO₃⁻ was the dominant form of N in Hong Kong waters, the dominant N form taken up by phytoplankton was usually NH₄⁺ and organic N, including urea and amino acids, rather than NO₃⁻. Hence, because of the low NO₃⁻ uptake, there was a long turnover time for NO₃⁻ (100 days), and most of the NO₃⁻ was apparently transported offshore into deeper shelf waters. In eastern waters where NH₄⁺ was undetectable, NO₃⁻ uptake rates were positively correlated with phytoplankton cell size. In contrast, potential rates of glutamic acid uptake were negatively correlated with phytoplankton size. N uptake rates in the smaller size fraction (0.7–2.8 μm) were less affected by the rain event, and smaller phytoplankton appeared to outcompete larger cells after several days of rain. The surface (PN)-specific N uptake rates in the >8-μm fraction decreased from 0.02 to 0.0001 h⁻¹, while the smaller fraction only exhibited a one- to threefold decrease after the rainfall. In contrast, bacterial production and N uptake were not affected by the rain event, and bacteria N uptake accounted for 10–60% of the total N uptake by phytoplankton.     

The effect of low-rate sand filtration and modified feed management on effluent quality,pond water quality and production of intensive shrimp ponds

Pond water in intensive shrimp ponds is typically high in nutrients, solids, and BOD and may have an adverse environmental impact when discharged to natural waters. As part of ongoing research to develop intensive production technologies that minimize the potential for environmental impact, a study investigated the effect of lowrate, coarse-grain sand filtration on the quality of effluent being discharged from an intensive shrimp pond receiving 5% d^?1 water exchange. The effect of sand filtration on a recirculating no-exchange pond was also examined. For comparison, a third pond received no water exchange and no filtration. Sand filtration removed some particulate matter, but levels of dissolved and particulate organic and inorganic nutrients were changed little and were sometimes higher as the result of in situ decomposition. Low-rate sand filtration is not seen as a cost-effective method of increasing the carrying capacity of no-exchange shrimp ponds or drastically improving the effluent quality of ponds with water exchange. Compared to previous studies with decreased or no water exchange, the application of feed in these ponds was more stable with small portions fed at frequent intervals with a constant rate of 80 kg ha^?1 d^?1. These ponds, stocked with 40 m^?2 juvenilePenaeus vannamei, had excellent survival and normal growth, resulting in productions levels approaching 7,000 kg ha^?1 crop^?1 without water exchange. This indicates that intensive shrimp farming may be possible in static no-exchange systems, thereby minimizing the potential impact of effluent as long as feed inputs do not overwhelm the assimilative capacity of the pond ecosystem. *** DIRECT SUPPORT *** A01BY069 00011     

Environmental impacts of shrimp farming with special reference to the situation in the continental United States

Shrimp culture technology has resulted in development of a major shrimp farming industry worldwide. Without the shrimp farming industry, increasing demands for shrimp by consumers could not be met, resulting in increased pressure on wild shrimp resources. Unfortunately, there are realized and potential adverse environmental effects on estuarine ecosystems as a result of shrimp farming. The effects can be categorized as wetland destruction for construction of shrimp farms, hypernutrification of estuarine ecosystems by shrim pond effluent, “biological pollution” of native shrimp stocks through escapement of aquaculture stocks, water use and entrainment of estuarine biota, and impacts of shrimp farm chemicals on estuarine systems. While the shrimp farming industry in the United States is small, the United States is effectively addressing all the realized and potential environmental impacts through regulation and research at the federal and state levels. Areas of regulation and research include stringent prohibitions on wetland destruction, regulation of effluents and support of research to eliminate and/or reduce effluents, escapement prevention technology and development of high-health stocks, minimizing entrainment of estuarine biota through water conservation and screening technology, and regulation of chemical use in the shrimp farming industry and support of research on shrimp pathology and environmentally safe disease control. Work is still in progress and not all problems have been resolved to the complete satisfaction of shrimp farmers and estuarine conservationists. However, the situation in the United States should serve as a model of how to encourage sustainable economic development through commercial shrimp farming while abating adverse environmental impacts on estuarine systems. To further improve the situation, the development and adoption of “best management practices” for shrimp aquaculture are recommended.     

Long-Term Trends of Nutrients and Phytoplankton in Chesapeake Bay

Climate effects on hydrology impart high variability to water-quality properties, including nutrient loadings, concentrations, and phytoplankton biomass as chlorophyll-a (chl-a), in estuarine and coastal ecosystems. Resolving long-term trends of these properties requires that we distinguish climate effects from secular changes reflecting anthropogenic eutrophication. Here, we test the hypothesis that strong climatic contrasts leading to irregular dry and wet periods contribute significantly to interannual variability of mean annual values of water-quality properties using in situ data for Chesapeake Bay. Climate effects are quantified using annual freshwater discharge from the Susquehanna River together with a synoptic climatology for the Chesapeake Bay region based on predominant sea-level pressure patterns. Time series of water-quality properties are analyzed using historical (1945–1983) and recent (1984–2012) data for the bay adjusted for climate effects on hydrology. Contemporary monitoring by the Chesapeake Bay Program (CBP) provides data for a period since mid-1984 that is significantly impacted by anthropogenic eutrophication, while historical data back to 1945 serve as historical context for a period prior to severe impairments. The generalized additive model (GAM) and the generalized additive mixed model (GAMM) are developed for nutrient loadings and concentrations (total nitrogen—TN, nitrate?+?nitrate—NO₂?+?NO₃) at the Susquehanna River and water-quality properties in the bay proper, including dissolved nutrients (NO₂?+?NO₃, orthophosphate—PO₄), chl-a, diffuse light attenuation coefficient (K _D (PAR)), and chl-a/TN. Each statistical model consists of a sum of nonlinear functions to generate flow-adjusted time series and compute long-term trends accounting for climate effects on hydrology. We present results identifying successive periods of (1) eutrophication ca. 1945–1980 characterized by approximately doubled TN and NO₂?+?NO₃ loadings, leading to increased chl-a and associated ecosystem impairments, and (2) modest decreases of TN and NO₂?+?NO₃ loadings from 1981 to 2012, signaling a partial reversal of nutrient over-enrichment. Comparison of our findings with long-term trends of water-quality properties for a variety of estuarine and coastal ecosystems around the world reveals that trends for Chesapeake Bay are weaker than for other systems subject to strenuous management efforts, suggesting that more aggressive actions than those undertaken to date will be required to counter anthropogenic eutrophication of this valuable resource.     

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.     

Assessing the organic pollution of surface water of Medjerda watershed (NE Algeria)

Water quality in the Northern part of Mellegue-Medjerda watershed (East Algeria) has been adversely affected by important pollutants discharged into the Medjerda wadi without, in most cases, any treatment. Chemical and physical degradation are due to agricultural and industrial practices and domestic wastewaters. Over a three-month period, a study of the low-flow water quality characteristics throughout Medjerda wadi was undertaken. Longitudinal profiles of water quality were constructed using data from fourteen sites. All sewage, agricultural, and industrial inputs were included. Analyzed properties were nutrients (NO₃ ⁻, NO₂ ⁻, NH₄ ⁺, and PO₄ ³⁻), Biochemical oxygen demand after five days (BOD₅), chemical oxygen demand (COD), and dissolved oxygen (DO). Along Medjerda wadi, all values change because of conditions specific to each sampling station. Nitrate was the most important form of nitrogen-element load (94%). Its concentration reached 34.3 mg L⁻¹ at OM₄ point, downstream of domestic wastewater discharges. The spatial evolution of the organic pollution index (OPI) shows that the wastewater effluent constitutes the main source of pollution. Indeed, water quality goes from a moderate pollution state at some sampling stations not or slightly affected by wastewaters discharges to a very strong pollution state (OPI of about 1.75) downstream of the domestic effluents inputs of Souk-Ahras city.     

Phytoplankton community composition and size distribution in the Langat River estuary,Malaysia

Phytoplankton patchiness, as expressed by community composition and size distribution, during the rainy season in the Langat River estuary (Malaysia) is described. Four sites in the estuary were sampled on two different occasions. The sampling area covered a stretch of the river from upstream to downstream of aquaculture activities (shrimp farms). Water samples from a shrimp farm outlet were also analyzed for nutrient and phytoplankton content. Differences in community structure between stations were found by means of multivariate procedures. Genera composition and total biomass were related to environmental factors, revealing salinity, light, and nutrients as important explaining factors. Elevated phytoplankton biomass and total phosphorus concentration, as well as lower inorganic nitrogen: phosphorus ratios, were found downstream of the shrimp farming activities. The size distribution spectrum of the phytoplankton population downstream of the shrimp farms was significantly different from that at the other stations but not different than that found in the sampled effluent from the shrimp farms, where phytoplankton biomass was also high. Twenty-two of the 24 recorded genera from the shrimp farm outlet were also found downstream of the farming activities. A number of different environmental factors potentially alter conditions for phytoplankton in the lower reaches of the estuary as compared to the upper regions. A cause and effect relationship explaining the differences noted between the upper and lower reaches of the estuary cannot be established. This study suggests that nutrient enrichment from the shrimp farming activities is of a magnitude that may contribute to the phytoplankton community changes observed in the lower reaches of the estuary.     

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.     

The effects of semi-lunar spring and neap tidal change on nutrients cycling in the intertidal sediments of the Yangtze estuary

During a semi-lunar tidal cycle from full moon till new moon, tide is characterized by the periodic change in spring and neap tide. Under the in situ conditions of light and temperature, the influence of a semi-lunar spring and neap tidal cycle on nutrient cycling in intertidal flat of the Yangtze estuary was simulated in the laboratory in July 2002. Lab experiments showed that NH₄⁺ and PO₄^3– were always released into overlying waters, while NO₃^– was directed into sediments in the permanently waterlogged systems, suggesting that the long-term waterlogged sediment acts as a significant source for NH₄⁺ and PO₄^3–, and a sink for NO₃^– in water columns. In contrast, reflooding of intertidal sediments after long-term desiccation promoted the considerable effluxes of NH₄⁺, NO₃^– and PO₄^3– into overlying waters, reflecting that the long-term exposed sediment is an important source for nutrients in overlying waters. In addition, the semi-lunar tidal cycle led to the intricate depth distribution patterns of nutrients in intertidal sediments. During long-term exposure, NH₄⁺ in sediments was quickly transformed into NO₂^– and NO₃^–, while organic P pool might be converted to slightly adsorbed and iron-bound P. Therefore, it is considered that the semi-lunar tidal circulation has the significant influence on the biogeochemical cycle of nutrients in intertidal systems.     

Salinization and arsenic contamination of surface water in southwest Bangladesh

To identify the causes of salinization and arsenic contamination of surface water on an embanked island (i.e., polder) in the tidal delta plain of SW Bangladesh we collected and analyzed water samples in the dry (May) and wet (October) seasons in 2012–2013. Samples were collected from rice paddies (wet season), saltwater ponds used for brine shrimp aquaculture (dry season), freshwater ponds and tidal channels (both wet and dry season), and rainwater collectors. Continuous measurements of salinity from March 2012 to February 2013 show that tidal channel water increases from ~0.15 ppt in the wet season up to ~20 ppt in the dry season. On the polder, surface water exceeds the World Health Organization drinking water guideline of 10 μg As/L in 78% of shrimp ponds and 27% of rice paddies, raising concerns that produced shrimp and rice could have unsafe levels of As. Drinking water sources also often have unsafe As levels, with 83% of tubewell and 43% of freshwater pond samples having >10 μg As/L. Water compositions and field observations are consistent with shrimp pond water being sourced from tidal channels during the dry season, rather than the locally saline groundwater from tubewells. Irrigation water for rice paddies is also obtained from the tidal channels, but during the wet season when surface waters are fresh. Salts become concentrated in irrigation water through evaporation, with average salinity increasing from 0.43 ppt in the tidal channel source to 0.91 ppt in the rice paddies. Our observations suggest that the practice of seasonally alternating rice and shrimp farming in a field has a negligible effect on rice paddy water salinity. Also, shrimp ponds do not significantly affect the salinity of adjacent surface water bodies or subjacent groundwater because impermeable shallow surface deposits of silt and clay mostly isolate surface water bodies from each other and from the shallow groundwater aquifer. Bivariate plots of conservative element concentrations show that all surface water types lie on mixing lines between dry season tidal channel water and rainwater, i.e., all are related by varying degrees of salinization. High As concentrations in dry season tidal channel water and shrimp ponds likely result from groundwater exfiltration and upstream irrigation in the dry season. Arsenic is transferred from tidal channels to rice paddies through irrigation. Including groundwater samples from the same area (Ayers et al. in Geochem Trans 17:1–22, 2016), principal components analysis and correlation analysis reveal that salinization explains most variation in surface water compositions, whereas progressive reduction of buried surface water by dissolved organic carbon is responsible for the nonconservative behavior of S, Fe, and As and changes in Eh and alkalinity of groundwater.     

Influences of microbial communities on groundwater component concentrations during managed artificial recharge

Managed aquifer recharge is one of the most popular methods for dealing with local water shortage issues, and the bacterial community could be a vital factor influencing groundwater quality during this process. In this study, analysis of variations in groundwater components during artificial recharge revealed three stages at a text site in China. During stage I, total iron and dissolved organic carbon levels are stable basically, dissolved oxygen and SO₄ ^2? levels have rising trends, NO₃ ^? curve varied not obviously. Variation curves show increases in dissolved oxygen, NO₃ ^?, SO₄ ^2? and stabilization in dissolved organic carbon and total iron at stage II. During stage III, dissolved oxygen and NO₃ ^? have rising trends, dissolved organic carbon, total iron, and SO₄ ^2? keep stable. At 25 and 70 days the Simpson and Shannon–Wiener indices show that microbial community richness and population diversity underwent a gradual dynamic change after recharge water arrived. Correlation analysis shows that the Simpson index was mainly affected by dissolved oxygen and NO₃ ^?. PCR-DGGE confirmed these findings. Overall, the results revealed that the main bacterial communities reduce total nitrogen, total phosphorous, and chemical oxygen demand, which corresponded to the calculated correlation index.     

Atmospheric deposition of nitrogen: Implications for nutrient over-enrichment of coastal waters

Atmospheric deposition of nitrogen (AD-N) is a significant source of nitrogen enrichment to nitrogen (N)-limited estuarine and coastal waters downwind of anthropogenic emissions. Along the eastern U.S. coast and eastern Gulf of Mexico, AD-N currently accounts for 10% to over 40% of new N loading to estuaries. Extension of the regional acid deposition model (RADM) to coastal shelf waters indicates that 11, 5.6, and 5.6 kg N ha⁻¹ may be deposited on the continental shelf areas of the northeastern U.S. coast, southeast U.S. coast, and eastern Gulf of Mexico, respectively. AD-N approximates or exceeds riverine N inputs in many coastal regions. From a spatial perspective, AD-N is a unique source of N enrichment to estuarine and coastal waters because, for a receiving water body, the airshed may exceed the watershed by 10–20 fold. AD-N may originate far outside of the currently managed watersheds. AD-N may increase in importance as a new N source by affecting waters downstream of the oligohaline and mesohaline estuarine nutrient filters where large amounts of terrestrially-supplied N are assimilated and denitrified. Regionally and globally, N deposition associated with urbanization (NO_x, peroxyacetyl nitrate, or PAN) and agricultural expansion (NH₄ ⁺ and possibly organic N) has increased in coastal airsheds. Recent growth and intensification of animal (poultry, swine, cattle) operations in the midwest and mid-Atlantic regions have led to increasing amounts of NH₄ ⁺ emission and deposition, according to a three decadal analysis of the National Acid Deposition Program network. In western Europe, where livestock operations have dominated agricultural production for the better part of this century, NH₄ ⁺ is the most abundant form of AD-N. AD-N deposition in the U.S. is still dominated by oxides of N (NO_x) emitted from fossil fuel combustion; annual NH₄ ⁺ deposition is increasing, and in some regions is approaching total NO₃ ⁻ deposition. In receiving estuarine and coastal waters, phytoplankton community structural and functional changes, associated water quality, and trophic and biogeochemical alterations (i.e, algal blooms, hypoxia, food web, and fisheries habitat disruption) are frequent consequences of N-driven eutrophication. Increases in and changing proportions of various new N sources regulate phytoplankton competitive interactions, dominance, and successional patterns. These quantitative and qualitative aspects of AD-N and other atmospheric nutrient sources (e.g., iron) may promote biotic changes now apparent in estuarine and coastal waters, including the proliferation of harmful algal blooms, with cascading impacts on water quality and fisheries.     

Nitrate sources and sinks in Elkhorn Slough,California: Results from long-term continuous in situ nitrate analyzers

Nitrate and water quality parameters (temperature, salinity, dissolved oxygen, turbidity, and depth) were measured continuously with in situ NO₃ analyzers and water quality sondes at two sites in Elkhorn Slough in Central California. The Main Channel site near the mouth of Elkhorn Slough was sampled from February to September 2001. Azevedo Pond, a shallow tidal pond bordering agricultural fields further inland, was sampled from December 1999 to July 2001. Nitrate concentrations were recorded hourly while salinity, temperature, depth, oxygen, and turbidity were recorded every 30 min. Nitrate concentrations at the Main Channel site ranged from 5 to 65 μM. The propagation of an internal wave carrying water from ≈100 m depth up the Monterey Submarine Canyon and into the lower section of Elkhorn Slough on every rising tide was a major source of nitrate, accounting for 80–90% of the nitrogen load during the dry summer period. Nitrate concentrations in Azevedo Pond ranged from 0–20 μM during the dry summer months. Nitrate in Azevedo Pond increased to over 450 μM during a heavy winter precipitation event, and interannual variability driven by differences in precipitation was observed. At both sites, tidal cycling was the dominant forcing, often changing nitrate concentrations by 5-fold or more within a few hours. Water volume flux estimates were combined with observed nitrate concentrations to obtain nitrate fluxes. Nitrate flux calculations indicated a loss of 4 mmol NO₃ m^?2 d^?1 for the entire Elkhorn Slough and 1 mmol NO₃ m^?2 d^?1 at Azevedo Pond. These results suggested that the waters of Elkhorn Slough were not a major source of nitrate to Monterey Bay but actually a nitrate sink during the dry season. The limited winter data at the Main Channel site suggest that nitrate was exported from Elkhorn Slough during the wet season. Export of ammonium or dissolved organic nitrogen, which we did not monitor, may balance some or all of the NO₃ flux.     

Interstitial water – sediment geochemistry of N, P and Fe and its response to overlying waters of tropical estuaries: a case from the southwest coast of India

 The concentrations of N, P and Fe in surface sediments and interstitial and overlying (bottom and surface) waters of the Ashtamudi estuary located in the southwest coast of India are reported along with the various chemical species of N (NO₂–N, NO₃–N, NH₃–N and total N) and P (organic P, inorganic P and total P) in interstitial and overlying waters and discussed in terms of the physico-chemical environment of the system. The interstitial water exhibits higher salinity values compared to bottom and surface waters, indicating the coupled effects of salt-wedge phenomena and gravitational convection of more saline-denser marine water downward through surface sediments. N, P and Fe as well as their chemical forms are enriched in the interstitial water compared to bottom and surface waters. However, the dissolved oxygen (DO) shows an opposite trend. The marked enrichment of NH₃–N in the interstitial water and its marginal presence in bottom and surface waters, together with the substantial decrease in the DO concentrations of bottom water and consequent increase in the concentrations of NO₂–N and NO₃–N in interstitial and bottom waters, points to the nitrification process operating in the sediment-water interface of the Ashtamudi estuary. The enrichment of total N, P and Fe in the interstitial water compared to the overlying counterparts and the positive correlation of sediment N, P and Fe with mud contents as well as organic carbon indicate that these elements are liberated during the early diagenetic decomposition of organic matter trapped in estuarine muds. Received: 5 Oktober 1998 · Accepted: 9 February 1999     

Effect of fish and mesozooplankton manipulation on the phytoplankton community in the Patos Lagoon estuary,Southern Brazil

An enclosure experiment in the Patos Lagoon estuary, southern Brazil (32°S, 52°W) investigated the response of phytoplankton to the enhancement of mesozooplankton and fish abundance. Addition of nutrients (NO₃ ^? and PO₄ ^?) stimulated the growth of 3–20 μm diameter phytoplankton, especially the diatomCylindrotheca closterium, which, in turn, was heavily grazed by the dominant mesozooplankter, the copepodAcartia tonsa. Acartia did not consume small (2–3 μm) autotrophic flagellates and the cyanobacteriaAnabaena sp., despite their high cell number. Largest grazing ofC. closterium by the copepod occurred only after a decrease of ciliate abundance. The addition of mesozooplankton did not change the levels of primary production, but it significantly increased the phytoplankton assimilation number. Highest chlorophylla concentrations were measured in enclosures stocked with juveniles of the fishXenomelaniris brasiliensis. The large phytoplankton biomass and low mesozooplankton abundance found in this treatment indicates an, effective predator-mediated action on the phytoplankton community.     

Harmful algal blooms and eutrophication: Nutrient sources,composition, and consequences

Although algal blooms, including those considered toxic or harmful, can be natural phenomena, the nature of the global problem of harmful algal blooms (HABs) has expanded both in extent and its public perception over the last several decades. Of concern, especially for resource managers, is the potential relationship between HABs and the accelerated eutrophication of coastal waters from human activities. We address current insights into the relationships between HABs and eutrophication, focusing on sources of nutrients, known effects of nutrient loading and reduction, new understanding of pathways of nutrient acquisition among HAB species, and relationships between nutrients and toxic algae. Through specific, regional, and global examples of these various relationships, we offer both an assessment of the state of understanding, and the uncertainties that require future research efforts. The sources of nutrients potentially stimulating algal blooms include sewage, atmospheric deposition, groundwater flow, as well as agricultural and aquaculture runoff and discharge. On a global basis, strong correlations have been demonstrated between total phosphorus inputs and phytoplankton production in freshwaters, and between total nitrogen input and phytoplankton production in estuarine and marine waters. There are also numerous examples in geographic regions ranging from the largest and second largest U.S. mainland estuaries (Chesapeake Bay and the Albemarle-Pamlico Estuarine System), to the Inland Sea of Japan, the Black Sea, and Chinese coastal waters, where increases in nutrient loading have been linked with the development of large biomass blooms, leading to anoxia and even toxic or harmful impacts on fisheries resources, ecosystems, and human health or recreation. Many of these regions have witnessed reductions in phytoplankton biomass (as chlorophylla) or HAB incidence when nutrient controls were put in place. Shifts in species composition have often been attributed to changes in nutrient supply ratios, primarily N∶P or N∶Si. Recently this concept has been extended to include organic forms of nutrients, and an elevation in the ratio of dissolved organic carbon to dissolved organic nitrogen (DOC∶DON) has been observed during several recent blooms. The physiological strategies by which different groups of species acquire their nutrients have become better understood, and alternate modes of nutrition such as heterotrophy and mixotrophy are now recognized as common among HAB species. Despite our increased understanding of the pathways by which nutrients are delivered to ecosystems and the pathways by which they are assimilated differentially by different groups of species, the relationships between nutrient delivery and the development of blooms and their potential toxicity or harmfulness remain poorly understood. Many factors such as algal species presence/abundance, degree of flushing or water exchange, weather conditions, and presence and abundance of grazers contribute to the success of a given species at a given point in time. Similar nutrient loads do not have the same impact in different environments or in the same environment at different points in time. Eutrophication is one of several mechanisms by which harmful algae appear to be increasing in extent and duration in many locations. Although important, it is not the only explanation for blooms or toxic outbreaks. Nutrient enrichment has been strongly linked to stimulation of some harmful species, but for others it has not been an apparent contributing factor. The overall effect of nutrient over-enrichment on harmful algal species is clearly species specific.     

Effect of municipal wastewaters on bottom sediment geochemistry and benthic foraminifera of two Red Sea coastal inlets, Jeddah, Saudi Arabia

Al-Arbaeen and Al-Shabab inlets are two Red Sea coastal inlets lying on the mid-coast of Jeddah City, Saudi Arabia. Forty-four surface sediment samples were collected from these inlets and surrounding areas during June 2010. Water depths and the overlying environmental parameters (temperature, salinity, pH and dissolved oxygen) of these samples were measured. Sediment samples were analyzed for variables, such as loss on ignition (LOI, organic matter), CaCO₃, heavy metals (Fe, Mn, Cu, Zn, Cd, Ni, Cr and Pb) and benthic foraminifera to assess any changes in the environment of the inlets and surrounding areas. Variables distribution patterns indicated that mud sediments dominated the inlets and enriched by LOI, heavy metals and Ammonia tepida–Quinqueloculina seminula assemblage, whereas coarse (sand–gravel) sediments dominated the substrates of surrounding areas and enriched by CaCO₃ and Coscinospira hemprichii–Peneroplis planatus–Varidentella neostriatula assemblage with low values of LOI and heavy metals. Highest concentrations of Cu, Zn, Cd, Ni, Cr and Pb were recorded inside the inlets, especially near the discharge points, and they were positively correlated with the LOI and mud fraction indicating their affinity to anthropogenic materials. However, highest concentrations of Fe and Mn were typically recorded in the whole study area. These metals were positively correlated with the LOI values of the surrounding area, but in the inlets they were negatively correlated with the LOI, indicating an existence of reducing conditions caused by limited dissolved oxygen conditions at bottom waters of the inlets. Changing the environment within the inlets, according to higher concentrations of heavy metals and LOI, is probably responsible for the existence of the low density and diversity of benthic foraminifera and the absence of (reefal) symbiont-bearing species.     

Phytoplankton population dynamics and productivity in a sewage-enriched tidal Creek in North Carolina

Calico Creek differs from neighboring estuaries in that it receives sewage effluent, and its waters therefore contain ample nutrients. High nutrient levels enable the phytoplankton population, which is probably light-limited, to reach densities of 10⁹ cells·1^?1 during the summer, 2 to 3 orders of magnitude higher than the surrounding, nutrient-limited populations. At cell densities greater than 10⁸ cells·1^?1 the number of dominant species is greatly reduced, and the level of diversity drops sharply. Calico Creek also differs by being very shallow; the population can be drastically affected by high runoff. The productivity of both the phytoplankton and the surroundingSpartina marsh is much higher than the neighboring unpolluted estuaries. Unlike the larger and more stable estuaries nearby, the dependence of the population on the effluent for nutrients, the possibility of toxic materials entering the creek with the effluent, and its small size make Calico Creek subject to sudden change.