Fish assemblage structure in relation to environmental variation in a Texas Gulf coastal wetland

We described seasonal fish-assemblages in an estuarine marsh fringing Matagorda Bay, Gulf of Mexico. Habitat zones were identified by patterns of fish species abundance and indicator species optima along gradients in salinity, dissolved oxygen (DO), and depth in our samples. Indicators of the lower brackish zone (lower lake and tidal bayou closest to the bay) were gulf menhaden (Brevoortia patronus), bay anchovy (Anchoa mitchilli), silver perch (Bairdiella chrysoura), and spotted seatrout (Cynoscion nebulosus) at salinity >15‰, DO 7–10 mg l⁻¹, and depth <0.5 m. Indicators of the upper brackish zone (lake and fringing salt marsh) were pinfish (Lagodon rhomboides) and spot (Leiostomus xanthurus) at salinity 10–20‰, DO >10 mg l⁻¹, and depth <0.5 m. In the freshwater wetland zone (diked wetland, ephemeral pool, and perennial scour pool), indicators were sheepshed minnow (Cyprinod on variegatus), rainwater killifish (Lucania parva), mosquitofish (Gambusia affinis), and sailfin molly (Poecilia latipinna) at salinity <5‰, DO <5 mg l⁻¹, and depth ≥1 m. In the freshwater channelized zone (slough and irrigation canal), indicators were three sunfish species (Lepomis), white crappie (Pomoxis annularis), and gizzard shad (Dorosoma cepedianum) at salinity <5‰, DO <5 mg l⁻¹, and depth >1.5 m. In brackish zones, seasonal variation in species diversity among sites was positively correlated with temperature, but assemblage structure also was influenced by depth and DO. In the freshwater zones, seasonal variation in species diversity among sites was positively correlated with depth, DO, and salinity, but assemblage structure was weakly associated with temperature. Species diversity and assemblage structure were strongly affected by the connectivity between freshwater wetland and brackish zones. Uncommon species in diked wetlands, such as tarpon (Megalops atlanticus) and fat sleeper (Dormitator maculatus), indicated movement of fishes from the brackish zone as the water level rose during natural flooding and scheduled (July) releases from the diked wetland. From September to July, diversity in the freshwater wetland zone decreased as receding waters left small isolated pools, and fish movement became blocked by a water-control structure. Subsequently, diversity was reduced to a few species with opportunistic life histories and tolerance to anoxic conditions that developed as flooded vegetation decayed.     

Spatial and temporal variation in recent growth, overall growth, and mortality of juvenile weakfish (Cynoscion regalis) in Delaware Bay

We examined relative abundance of juvenile weakfish,Cynoscion regalis, collected during 1986 and 1987 and tested for spatial differences in growth and survival within Delaware Bay. Juvenile weakfish recruit to all areas of Delaware Bay, and two cohorts were present during each year of the study. Although catch per unit effort (CPUE) varied among areas within the bay, there was a general trend of higher CPUE at lower salinities; abundance quickly declined near the end of September in all areas of the bay. Estimated growth rates from otolith increment analysis of juvenile weakfish ranged from 0.69 mm d⁻¹ to 0.97 mm d⁻¹. Spatial and temporal patterns in recent growth rate followed a general pattern: highest in the middle bay, lowest in the upper bay, and intermediate in the lower bay. Mortality rates were usually lowest in the low salinity region of the middle and upper bay during both years. There was no difference in mortality between cohorts in the middle bay, while in the upper bay the later-spawned fish had lower mortality and in the lower bay the early-spawned fish had lower mortality. Analysis of spatial and temporal patterns in growth and mortality suggests that there is a seasonal trade-off between habitat usage and resource availability for juvenile weakfish. The function of oligohaline and mesohaline waters as optimal nursery areas (in terms of growth and survival) changes due to the seasonally dynamic physicochemical characteristics in Delaware Bay.     

Distribution and Trophic Importance of Anthropogenic Nitrogen in Narragansett Bay: An Assessment Using Stable Isotopes

Narragansett Bay has been heavily influenced by human activities for more than 200 years. In recent decades, it has been one of the more intensively fertilized estuaries in the USA, with most of the anthropogenic nutrient load originating from sewage treatment plants (STP). This will soon change as tertiary treatment upgrades reduce nitrogen (N) loads by about one third or more during the summer. Before these reductions take place, we sought to characterize the sewage N signature in primary (macroalgae) and secondary (the hard clam, Mercenaria mercenaria) producers in the bay using stable isotopes of N (δ¹⁵N) and carbon (δ¹³C). The δ¹⁵N signatures of the macroalgae show a clear gradient of approximately 4‰ from north to south, i.e., high to low point source loading. There is also evidence of a west to east gradient of heavy to light values of δ¹⁵N in the bay consistent with circulation patterns and residual flows. The Providence River Estuary, just north of Narragansett Bay proper, receives 85% of STP inputs to Narragansett Bay, and lower δ¹⁵N values in macroalgae there reflected preferential uptake of ¹⁴N in this heavily fertilized area. Differences in pH from N stimulated photosynthesis and related shifts in predominance of dissolved C species may control the observed δ¹³C signatures. Unlike the macroalgae, the clams were remarkably uniform in both δ¹⁵N (13.2 ± 0.54‰ SD) and δ¹³C (−16.76 ± 0.61‰ SD) throughout the bay, and the δ¹⁵N values were 2–5‰ heavier than in clams collected outside the bay. We suggest that this remarkable uniformity reflects a food source of anthropogenically heavy phytoplankton formed in the upper bay and supported by sewage derived N. We estimate that approximately half of the N in the clams throughout Narragansett Bay may be from anthropogenic sources.     

Spatial and temporal growth rate variation of Bay Anchovy (Anchoa mitchilli) larvae in the Mid Hudson River estuary

Estuaries are critical habitats for larvae and juveniles of many marine fishes, possibly because they promote high growth rates and survival rates. We investigated spatial and temporal changes in growth rate of larval bay anchovy (Anchoa mitchilli), in the middle Hudson River estuary where abundance of larvae is high. In two consecutive summer seasons, we sampled larvae at 4 sites evenly spaced over 45 km, at weekly intervals for up to a month. We examined otoliths to determine age in days and then used age-length regressions to estimate growth rate. In 1995, larval anchovy growth rates varied from 0.39 to 0.88 mm d⁻¹ (median=0.48 mm d⁻¹). In 1996, growth rates varied from 0.41 to 0.77 mm d⁻¹ (median=0.55 mm d⁻¹). In both years, we found significant spatial and temporal variation in growth rate. Larvae collected in the upper portion of Haverstraw Bay tended to grow more slowly than larvae collected in other sites. The dates on which the most rapidly growing larvae were collected varied from site to site. Neither temperature nor salinity variations explained growth rate differences. Growth rate variation, probably governed by patches of zooplankton, occurred on temporal scales of a week and spatial scales of 15 km.     

Microzooplankton grazing and nitrogen excretion across a surface estuarine-coastal interface

The role of the microzooplankton community in regulating phytoplankton biomass was examined across a gradient from a river-dominated estuary to an oceanic-influenced coastal zone. Three stations located along a salinity gradient from the central region of Mobile Bay to 10 km off the coast were sampled from May 1994 to August 1995. Microzooplankton herbivory rates on phytoplankton and microzooplankton excretion of nitrogen derived from phytoplankton were estimated using the dilution technique. Microzooplankton grazing rates (range of station means=0.57–1.10 d⁻¹) and phytoplankton growth rates (0.70–1.62 d⁻¹) both increased across the salinity gradient from the bay station to the offshore station. However, the percent of primary production grazed per day was highest at the bay station (mean=83%) and decreased to a low at the offshore station (mean=64%). Excretion of phytoplankton-derived nitrogen by the microzooplankton was greatest at the bay and bay mouth stations. Excreted nitrogen could potentially supply 39%, 29%, and 20% of phytoplankton nitrogen demand at the bay, bay mouth, and offshore stations, respectively. These results support the idea that herbivorous microzooplankton are important in mediating nitrogen flow to both lower and higher trophic levels. *** DIRECT SUPPORT *** A01BY085 00012     

Benthic metabolism and nutrient cycling along an estuarine salinity gradient

Benthic metabolism and nutrient exchange across the sediment-water interface were examined over an annual cycle at four sites along a freshwater to marine transect in the Parker River-Plum Island Sound estuary in northeastern Massachusetts, U.S. Sediment organic carbon content was highest at the freshwater site (10.3%) and decreased along the salinity gradient to 0.2% in the sandy sediments at the marine end of the estuary. C:N ratios were highest in the mid estuary (23:1) and lowest near the sea (11:1). Chlorophyll a in the surface sediments was high along the entire length of the estuary (39–57 mg chlorophyll a m⁻²) but especially so in the sandy marine sediments (172 mg chlorophyll a m⁻²). Chlorophyll a to phaeophytin ratios suggested most chlorophyll is detrital, except at the sandy marine site. Porewater sulfide values varied seasonally and between sites, reflecting both changes in sulfate availability as overlying water salinity changed and sediment metabolism. Patterns of sediment redox potential followed those of sulfide. Porewater profiles of inorganic N and P reflected strong seasonal patterns in remineralization, accumulation, and release. Highest porewater NH₄ ⁺ values were found in upper and mid estuarine sediments, occasionally exceeding 1 mM N. Porewater nitrate was frequently absent, except in the sandy marine sediments where concentrations of 8 μM were often observed. Annual average respiration was lowest at the marine site (13 mmol O₂ m⁻² d⁻¹ and 21 mmol TCO₂ m⁻² d⁻¹) and highest in the mid estuary (130 mmol O₂ m⁻² d⁻¹ and 170 mmol TCO₂ m⁻² d⁻¹) where clam densities were also high. N₂O and CH₄ fluxes were low at all stations throughout the year: Over the course, of a year, sediments varied from being sources to sinks of dissolved organic C and N, with the overall spatial pattern related closely to sediment organic content. There was little correlation between PO₄ ³⁻ flux and metabolism, which we attribute to geochemical processes. At the two sites having the lowest salinities, PO₄ ³⁻ flux was directed into the sediments. On average, between 22% and 32% of total system metabolism was attributable to the benthos. The mid estuary site was an exception, as benthic metabolism accounted for 95% of the total, which is attributable to high densities of filter-feeding clams. Benthic remineralization supplied from less than 1% to over 190% of the N requirements and 0% to 21% of the P requirements of primary producers in this system. Estimates of denitrification calculated from stoichiometry of C and N fluxes ranged from 0% for the upper and mid estuary site to 35% for the freshwater site to 100% of sediment organic N remineralization at the marine site. We hypothesize that low values in the upper and mid estuary are attributable to enhanced NH₄ ⁺ fluxes during summer due to desorption of exchangeable ammonium from rising porewater salinity. NH₄ ⁺ desorption during summer may be a mechanism that maintains high rates of pelagic primary production at a time of low inorganic N inputs from the watershed.     

Stable isotope analyses (δ<Superscript>15</Superscript>N and δ<Superscript>13</Superscript>C) of the trophic relationships of<Emphasis Type="Italic">Callinectes sapidus</Emphasis> in two North Carolina estuaries

The present study focused on detecting variations in trophic relationships among blue crab (Callinectes sapidus) consumers according to water quality along two estuaries in North Carolina. Stable isotope (δ¹⁵N and δ¹³C) analyses of particulate organic matter and bivalve(Rangia cuneata andCorbicula fluminea) food sources were examined in combination with an Isosource mixing model. Results suggest that blue crab δ¹³C values increased significantly with increasing salinity from upper to lower sites along the Neuse River estuary (NRE; R2 = 0.87, p < 0.01) and Alligator River estuary (R² = 0.92, p < 0.01). There was a positive relationship between blue crab δ¹⁵N values and nitrate concentrations for the NRE (R² = 0.48, p = 0.12). This study found that blue crab δ¹³C values increased with salinity from upper to lower regions along both estuaries. Results suggest that blue crab production may have used alternative food sources that were isotopically (δ¹³C) depleted, especially in the upper NRE, and enriched sources in the mid to lower regions of both estuaries. Consumers sampled from the upper NRE may be influenced by higher nitrogen input from urban land use and municipal wastewater.     

Crystal size distribution in Jurassic Ferrar flows and sills (Victoria Land,Antarctica): evidence for processes of cooling,nucleation, and crystallisation

 Jurassic Ferrar rocks in Victoria Land (Antarctica) occur predominantly as basaltic or andesitic flows and sills. Both show characteristic petrographical and chemical variations, which can be related to in-situ differentiation processes. Such characteristics have been investigated at one flow (“Colonnade flow”) and one sill (“Thumb Point sill”) in the Prince Albert Mountains (Central Victoria Land) based on a statistical grain size analysis and the application of the crystal size distribution theory. A third magma body (“HiTi-unit”), which in previous literature was described as a flow, does not show clear similarities to either the flow or sill. Sill and flow are in-situ differentiated with accumulation of silicic residual melt in the latest cooled parts. For the flow this part is located in the lower half and for the sill in the upper third of their thickness. Thumb Point sill additionally shows an accumulation zone of olivine in the lower third. The position of the residual melt accumulation zone is an indicator for an origin as flow or sill. The HiTi-unit, by contrast, exhibits, only moderate petrographical and chemical variations. Growth and nucleation rates have been determined using, with some modifications, the CSD theory introduced by Marsh (1988). Growth rates of plagioclase for the flow vary from 10⁻¹¹ to 10⁻¹⁰ cm/s and are comparable with literature values for basaltic lava lakes on Hawaii. Nucleation rates vary from 10⁻⁴ to 10⁻³ cm⁻³s⁻¹, which are generally smaller than for Hawaiian basalts. Growth and nucleation rates for the sill cover a large range from 10⁻¹³ to 10⁻¹¹ cm/s and 10⁻⁹ to 10⁻³ cm⁻³s⁻¹, respectively. Systematic variations of these parameters with vertical position were obtained for the sill with its extensive differentiation history. Nucleation and growth rates are dependant on the mode of cooling. Nevertheless, small but significant differences between the flow and sill exist. Growth and nucleation rates of HiTi-unit (10⁻¹² to 10⁻¹¹ cm/s and 10⁻⁶ to 10⁻⁵ cm⁻³s⁻¹) are intermediate between the flow and sill and thus do not allow a distinction of emplacement mode. Received: 6 December 1995 / Accepted: 3 May 1996     

Effects of menhaden predation of plankton populations in Narragansett Bay, Rhode Island

The Atlantic menhaden,Brevoortia tyrannus, is an abundant plankton-feeding fish that undertakes extensive seasonal migrations, moving from overwintering locations offshore south of Cape Hatteras to the mid-Atlantic Bight and New England inshore waters during spring and summer. A bioenergetic model, based on field and laboratory studies, shows that when large numbers of menhaden enter Narragansett Bay, Rhode Island, during spring and early summer, they significantly influence plankton populations through size-selective grazing and nutrient regeneration. A population biomass of 9.1×10⁶ kg of menhaden feeding for 12 h each day in the upper bay would result, in a substantial reduction of the instantaneous growth rate of the >20-μm phytoplankton. Instantaneous growth rates of zooplankton would be negative if the same population of menhaden was present, resulting in a reduction in the biomass of zooplankton. Given the ambient phytoplankton and zooplankton populations, menhaden could achieve the seasonal growth measured in Narragansett Bay during 1976 by feeding on average about 5 h d^?1. Daily nitrogen excretion rates of the 9.1×10⁶ kg menhaden population were 56.4% of the mean standing stock of ammonia-N in the upper bay. Because menhaden travel in schools their effects are likely to be intense but strongly localized, increasing spatial heterogeneity in the ecosystem. When the fish migrate southward in the fall they transfer between 3.3% and 6.2% of the nitrogen exported annually from the bay.     

Eutrophication processes and trophic interactions in a shallow estuary: Preliminary results based on stable isotope analysis (δ13C and δ15N)

Stable isotopes ratios (δ¹³C and δ¹⁵N) were measured in primary producers and consumers of two bays with contrasting eutrophic conditions in the Patos Lagoon estuary, southern Brazil: the Justino bay, a more pristine ecosystem, and the Mangueira bay, a heavily polluted region that receives the Rio Grande city sewage and effluencts of several industries. δ¹³C values of organisms collected in both subsystems were not different, but δ¹⁵N values had significant statistical differences, ca. 3.5‰ higher in the Mangueira bay. It is likely that primary producers and consumers in this subsystem are greatly influenced by higher nitrogen input due to domestic and industrial sewages. The stable isotope analysis also corroborated several trophic interactions previously established by gut content analysis, and due to its higher sensitivity, it was possible to better determine the contributions of different primary producers and detrital fractions to the consumers' diets. It was confirmed that plant detritus represents the main food source for most organisms. The stable isotope analysis also demonstrated that detritivorous benthic organisms in the same habitat have distinct diet compositions, with differential consumption of C3 and C4 plants. This technique showed that some consumers that eat detritus do not have in their stable isotopic signature any relationship with that of plants. It is likely that these consumers assimilate their carbon and nitrogen from other sources like microalgae or microorganisms that colonize decaying plants.     

Determining Spatial and Temporal Inputs of Freshwater,Including Submarine Groundwater Discharge,to a Subtropical Estuary Using Geochemical Tracers,Biscayne Bay,South Florida

Geochemical mixing models were used to decipher the dominant source of freshwater (rainfall, canal discharge, or groundwater discharge) to Biscayne Bay, an estuary in south Florida. Discrete samples of precipitation, canal water, groundwater, and bay surface water were collected monthly for 2 years and analyzed for salinity, stable isotopes of oxygen and hydrogen, and Sr²⁺/Ca²⁺ concentrations. These geochemical tracers were used in three separate mixing models and then combined to trace the magnitude and timing of the freshwater inputs to the estuary. Fresh groundwater had an isotopic signature (δ ¹⁸O = −2.66‰, δD −7.60‰) similar to rainfall (δ ¹⁸O = −2.86‰, δD = −4.78‰). Canal water had a heavy isotopic signature (δ ¹⁸O = −0.46‰, δD = −2.48‰) due to evaporation. This made it possible to use stable isotopes of oxygen and hydrogen to separate canal water from precipitation and groundwater as a source of freshwater into the bay. A second model using Sr²⁺/Ca²⁺ ratios was developed to discern fresh groundwater inputs from precipitation inputs. Groundwater had a Sr²⁺/Ca²⁺ ratio of 0.07, while precipitation had a dissimilar ratio of 0.89. When combined, these models showed a freshwater input ratio of canal/precipitation/groundwater of 37%:53%:10% in the wet season and 40%:55%:5% in the dry season with an error of ±25%. For a bay-wide water budget that includes saltwater and freshwater mixing, fresh groundwater accounts for 1–2% of the total fresh and saline water input.     

Patterns of mangrove forest structure and soil nutrient dynamics along the Shark River estuary, Florida

The basal area and productivity of managrove wetlands are described in relation to selected soil properties to understand the general pattern of optimum forest stature at the mouth of estuaries in the Everglades, such as the Shark River Slough, Florida (U.S.). The basal area of mangroves decreases from 40.4 m² ha⁻¹ and 39.7 m² ha⁻¹ at two stations 1.8 km and 4.1 km from the estuary mouth to 20.7 m² ha⁻¹ and 19.6 m² ha⁻¹ at two sites 9.9 km and 18.2 km from the mouth, respectively. The gradient in basal area at these four sites is mostly the result of approximately 34 yr of growth since Hurricane Donna. Wood productivity is higher in the lower estuary (10.7 Mg ha⁻¹ yr⁻¹ and 12.0 Mg ha⁻¹ yr⁻¹) than in the upper estuary (3.2 Mg ha⁻¹ yr⁻¹ and 4.2 Mg ha⁻¹ yr⁻¹). Porewater salinity among these four mangrove sites during seasonal sampling in 1994 and 1995 ranged from 1.6 g kg⁻¹ to 33.5 g kg⁻¹, while sulfide was generally<0.15 mM at all sites. These soil values indicate that abiotic stress cannot explain the decrease in forest structure along this estuarine gradient. Concentrations of nitrogen (N) and phosphorus (P) are more closely related to patterns of forest development, with higher soil fertility at the mouth of the estuary as indicated by higher concentrations of extractable ammonium, total soil P, and available P, along with higher ammonium production rates. The more fertile sites of the lower estuary are dominated by Laguncularia racemosa, whereas the less fertile sites in the intermediate and upper estuary are dominated by Rhizophora mangle. Relative N mineralization per unit of total N is higher in the lower estuary and is related positively to concentrations of available P, indicating the importance of turnover rates and nutrient interactions to soil fertility. Concentrations of Ca-bound P per volume soil in the lower estuary is 40-fold higher than in the upper estuary, and along with an increase in residual P in the upper estuary, indicate a shift from mineral to organic P along the estuarine gradient. Mineral inputs to the mouth of Shark River estuary from the Gulf of Mexico (rather than upland inputs) apparently control the patterns of mangrove structure and productivity.     

The effects of eelgrass habitat loss on estuarine fish communities of southern New England

We studied the late June–August fish community in extant and former eelgrass (Zostera marina L.) habitats in 15 estuaries of Buzzards Bay, and in Waquoit Bay, Massachusetts, U.S. Our objective was to quantify the effects of eelgrass habitat loss on fish abundance, biomass, species composition and richness, life-history characteristics, and habitat use by examining the response of the fish community to eelgrass loss in Waquoit and Buttermilk Bays over an 11-yr period (1988–1999) and in 14 other embayments of Buzzards Bay during 1993, 1996, and 1998. Sampling sites were located in present-day or historical eelgrass beds and were classified according to eelgrass habitat complexity (zero complexity: no eelgrass; low complexity: <100 eelgrass shoots or <100 g wet weight m⁻²; high complexity: ≥100 shoots and ≥100 g wet weight m⁻²). Habitats that had lost eelgrass included a variety of substratum types, from bare mud bottom to dense accumulations of red, brown, and green macroalgae (up to 7,065 g wet weight m⁻²). Contemporaneous sampling of fish (by otter trawl) and vegetated habitat (by divers) was conducted at each site. Overall, fish abundance, biomass, species richness, dominance, and life history diversity decreased significantly along the gradient of decreasing eelgrass habitat complexity. Loss of eelgrass was accompanied by significant declines in these measures of fish community integrity. Ten of the 13 most common species collected from 1988–1996 in Waquoit and Buttermilk Bays showed maximum abundance and biomass in sites with high eelgrass habitat complexity. All but two common species declined in abundance and biomass with the complete loss of eelgrass.     

The cost of stress: Low dissolved oxygen and economic benefits of recreational striped bass (<Emphasis Type="Italic">Morone saxatilis</Emphasis>) fishing in the Patuxent River

A Poisson catch rate model for striped bass (Morone saxatilis) anglers in Chesapeake Bay was developed that incorporates the effect of bottom temperature and dissolved oxygen (DO). Angler catch rates are shown to be negatively affected by low DO. Predicted angler catch rates were then used in a random utility model of striped bass fishing location choice. Where anglers choose to fish is significantly related to expected catch rate and the travel cost and time from the anglers residence to the fishing location. Results from the random utility model were then used to simulate the economic welfare changes that result from changing DO levels in the Patuxent River. Since there are many substitute sites for fishing in the Patuxent River, the welfare effects are small. Increases in DO from current levels have a small effect on angler welfare, but if levels are allowed to deteriorate so they never exceed 5 mg l⁻¹, the welfare effects are much larger. Under this latter scenario, the net present value of angler losses exceeds 100,000, and are almost100,000, and are almost 300,000 if the fishing grounds are anoxic. Losses are considerably higher as the area impacted by low oxygen conditions increases.     

Long-Term and Seasonal Changes in Nutrients,Phytoplankton Biomass,and Dissolved Oxygen in Deep Bay,Hong Kong

Deep Bay is a semienclosed bay that receives sewage from Shenzhen, a fast-growing city in China. NH₄ is the main N component of the sewage (>50% of total N) in the inner bay, and a twofold increase in NH₄ and PO₄ concentrations is attributed to increased sewage loading over the 21-year period (1986–2006). During this time series, the maximum annual average NH₄ and PO₄ concentrations exceeded 500 and 39 μM, respectively. The inner bay (Stns DM1 and DM2) has a long residence time and very high nutrient loads and yet much lower phytoplankton biomass (chlorophyll (Chl) <10 μg L⁻¹ except for Jan, July, and Aug) and few severe long-term hypoxic events (dissolved oxygen (DO) generally >2 mg L⁻¹) than expected. Because it is shallow (~2 m), phytoplankton growth is likely limited by light due to mixing and suspended sediments, as well as by ammonium toxicity, and biomass accumulation is reduced by grazing, which may reduce the occurrence of hypoxia. Since nutrients were not limiting in the inner bay, the significant long-term increase in Chl a (0.52–0.57 μg L⁻¹ year⁻¹) was attributed to climatic effects in which the significant increase in rainfall (11 mm year⁻¹) decreased salinity, increased stratification, and improved water stability. The outer bay (DM3 to DM5) has a high flushing rate (0.2 day⁻¹), is deeper (3 to 5 m), and has summer stratification, yet there are few large algal blooms and hypoxic events since dilution by the Pearl River discharge in summer, and the invasion of coastal water in winter is likely greater than the phytoplankton growth rate. A significant long-term increase in NO₃ (0.45–0.94 μM year⁻¹) occurred in the outer bay, but no increasing trend was observed for SiO₄ or PO₄, and these long-term trends in NO₃, PO₄, and SiO₄ in the outer bay agreed with those long-term trends in the Pearl River discharge. Dissolved inorganic nitrogen (DIN) has approximately doubled from 35–62 to 68–107 μM in the outer bay during the last two decades, and consequently DIN to PO₄ molar ratios have also increased over twofold since there was no change in PO₄. The rapid increase in salinity and DO and the decrease in nutrients and suspended solids from the inner to the outer bay suggest that the sewage effluent from the inner bay is rapidly diluted and appears to have a limited effect on the phytoplankton of the adjacent waters beyond Deep Bay. Therefore, physical processes play a key role in reducing the risk of algal blooms and hypoxic events in Deep Bay.     

Sediment and Lower Water Column Oxygen Consumption in the Seasonally Hypoxic Region of the Louisiana Continental Shelf

We report integrated measurements of sediment oxygen consumption (SOC) and bottom water plankton community respiration rates (WR) during eight cruises from 2003 to 2007 on the Louisiana continental shelf (LCS) where hypoxia develops annually. Averaged by cruise, SOC ranged from 3.9 to 25.8 mmol O₂ m⁻² day⁻¹, whereas WR ranged from 4.1 to 10.8 mmol O₂ m⁻³ day⁻¹. Total below-pycnocline respiration rates ranged from 46.4 to 104.5 mmol O₂ m⁻² day⁻¹. In general, below-pycnocline respiration showed low variability over a large geographic and temporal range, and exhibited no clear spatial or inter-annual patterns. SOC was strongly limited by dissolved oxygen (DO) in the overlying water; whereas, WR was insensitive to low DO, a relationship that may be useful for parameterizing future models. The component measures, WR and SOC, were similar to most prior measurements, both from the LCS and from other shallow estuarine and coastal environments. The contribution of SOC to total below-pycnocline respiration averaged 20 ± 4%, a finding that differs from several prior LCS studies, but one that was well supported from the broader estuarine and oceanic literature. The data reported here add substantially to those available for the LCS, thus helping to better understand oxygen dynamics on the LCS.     

Diel biogeochemical cycling in a hyperventilating shallow estuarine environment

A diel biogeochemical study was performed to assess the influence that periods of elevated biological activity have on the biogeochemical cycling of macronutrients and redox-sensitive elements in a natural estuarine environment. High-resolution data (15 min sampling) illustrates periodic extreme variations in dissolved oxygen (DO) in the shallow waters of Azevedo Pond, Elkhom Slough, California. During periods of low tidal flushing, DO values can range from highly oxic (>560 μM O₂: >250% saturation) during sunny days to suboxic conditions (<5 μM) at night. Nutrient cycling and redox-sensitive trace element biogeochemistry were evaluated in response to the extreme daily DO fluctuations. A diel sampling study was conducted over a 26-h period, where O₂ concentrations ranged from 346 μM to sustained non-detectable levels in the night hours. In concert with the DO fluctuations, diel phosphate cycling was on the order of 4 μM in response to tidal flushing events and biological assimilation and regeneration. The IO₃ ⁻/I⁻ redox couple quickly responded to suboxic conditions in the water column by a marked increase in I⁻ concentrations and corresponding depletion of IO₃ ⁻. The extreme fluctuations of the p∈ in the water column resulted in diel dissolved Mn²⁺ variations of nearly 5 μM, with observed dissolved Mn removal rates on the order of 1 μM h⁻¹. The elevated biogeochemical cycling of oxygen, nitrogen, phosphorus, iodine, manganese, and iron found in this shallow estuarine environment suggest that tidal restrictions and anthropogenic nutrient enrichments can amplify diel variations and potentially hinder the functional and ecological stability of these systems. These data suggest that accurate chemical monitoring of the health of an estuarine ecosystem must account for the diel variability inherent in these highly productive environments.     

Stand structure and productivity of the introducedRhizophora mangle in Hawaii

Since its introduction in the early part of this century,Rhizophora mangle L. has spread extensively through most of the main islands of the Hawaiian Archipelago. We investigated the structural properties and estimated productivity of aR. mangle population at Nuupia Ponds Wildlife Management Area (NPWMA), on windward Oahu, where the mangroves were being controlled due to their propensity to overgrow archaeological sites and the habitat of endangered Hawaiian waterbirds. Mangroves within NPWMA were very dense (>24,000 trees ha⁻¹) and most were relatively small (only 3.3% of the trees were ≥10 cm DBH). Mean basal area, aboveground biomass, and number of seedlings were all high, at 37.2 m² ha⁻¹, 279 t (dry wt) ha⁻¹, and 121 m⁻², respectively. The seedling, density may be particularly unusual and appears to be due to extremely high rates of propagule, production coupled with low rates of propagule predation. Stand productivity was estimated by stem growth (allometry), litterfall, and a light attenuation approach to determining net canopy photosynthetic production. All three methods yielded estimates that are higher than previously, reported forR. mangle and comparable with estimates of highly productiveRhizophora spp.-dominated stands in Australia and Asia. The high density, biomass, and productivity of this stand relative to stands within the species' native range may be due to a combination of favorable site conditions, lack of competition from other woody plants, and very low rates of herbivory and propagule predation.     

Stable isotope indicators of movement and residency for brown shrimp (<Emphasis Type="Italic">Farfantepenaeus aztecus</Emphasis>) in coastal Louisiana marshscapes

Brown shrimp (Farfantepenaeus aztecus) are an important commercial aquatic species experiencing loss of inshore marsh nursery habitat in coastal Louisiana. To study inshore brown shrimp movements and identify aspects of essential habitat important for sustaining brown shrimp populations, we collected juvenile brown shrimp in April and May 2000, the time of annual maximum brown shrimp abundance, in a small 1-km² marsh area on the central Louisiana coast. Drop sampling showed average shrimp densities of 1.6–2.4 m⁻² in shallow marsh ponds and seining indicated lower densities of 0.5–0.9 m⁻² in nearby shallow channel and open bay sites. Smaller shrimp (< 50 mm) fed disproportionately on benthic diatoms and small harpacticoid copepods, while large shrimp fed more frequently on larger-bodied amphipods and tanaids. We developed novel chemical approaches to estimate patterns of shrimp residency and movement using carbon and nitrogen stable isotopic determinations. Resident shrimp had isotopic values similar to average foods and showed consistent isotopic spacings between fast and slow turnover tissues. Residency was highest (47–55%) in ponds and shallow channel habitats and much less in open bays and deep channels (4–27%). There was sparse evidence for dietary specialization among individull shrimp. The results support the view that small 10–20 mm postlarval and juvenile brown shrimp arriving in estuaries from offshore waters continue movement through sub-optimal habitats (deep channels and open bays), but exhibit much less movement once an optimal habitat (marsh ponds or shallow channel margins) is reached. This study also indicated that combining estimates of shrimp densities, residency, growth rate, and mortality allows evaluation of the importance of different habitat types for shrimp production. Shallow ponds that in many ways resemble fertile aquaculture ponds appear to be hot spots for brown shrimp production, and coastal preservation and restoration efforts should focus on these areas as important for sustaining shrimp fisheries.     

Linking juvenile fish and their habitats: An example from Narragansett Bay, Rhode Island

We used two methods and existing field survey data to link juvenile fish and their habitats. The first method used seine survey data collected monthly from July to October 1988–1996 at fixed stations in Narragansett Bay, Rhode Island. Thirteen fish species making up 1% or more of the catch were analyzed by principal components analysis for two time periods: July–August and September–October. The stations were then plotted by their principal component scores to identify station groupings and habitat types. The second method used environmental data collected in July and August 1996 at the established survey stations in a principal components analysis. The stations and 13 most abundant species were plotted by principal components scores resulting from the environmental data. For the environmental data, the first two principal components explained 59% of the variance. The first principal component described the amount of energy shaping the habitat and was positively correlated with salinity, dissolved oxygen, current flow, and slope, and negatively correlated with silt. The second component was positively correlated with depth and silt, and negatively correlated with dissolved oxygen. The environmental data grouped the stations according to their distance from the ocean and three habitat types emerged. The uppermost station was a silty barren having low salinities and dissolved oxygen. Three other stations grouped together as low energy, protected habitats with sandy substrates. Lower bay stations had higher salinities, higher dissolved oxygen, higher flow rates, greater slopes, and larger size substrates, mostly cobble and gravel. Results from the fish data grouped the stations similarly. Combining results from both datasets revealed the uppermost station had the highest catches, most species, and greatest number of winter flounder (Pseudopleuronectes americanus) juveniles. Plots of winter flounder catches with principal component scores from the environmental data indicated the winter flounder distribution in the bay has shrunk from baywide to mostly the upper estuary near their primary spawning grounds. Results illustrate the value of coupling historic fish survey data with environmental measurements for identifying previously undervalued habitats important to fish.