Nutrient limitation of the macroalgaEnteromorpha intestinalis collected along a resource gradient in a highly eutrophic estuary

We conducted a laboratory experiment to quantify nutrient (nitrogen and phosphorus) limitation of macroalgae collected along a gradient in water column nutrient availability in Upper Newport Bay estuary, a relatively nutrient-rich system in southern California, United States. We collectedEnteromorpha intestinalis and water for use in the experiment from five sites ranging from the lower end of the estuary to the head. Initial algal tissue N and P concentrations and molar N∶P ratios—as well as water column NO₃ and total Kjeldahl nitrogen (TKN)—increased along a spatial gradient from the lower end toward the head. Water column soluble reactive phosphorus (SRP) varied among sites as well but did not follow a pattem of increasing from the seaward end toward the head. Algae from each site were assigned to one of four experimental treatments: control (C), nitrogen enrichment (+N), phosphorus enrichment (+P), and nitrogen and phosphorus enrichment (+N+P). Each week for 3 wk we replaced the water in each unit with the appropriate treatment water to mimic a poorly flushed estuary. After 3 wk, the degree of nutrient limitation ofE. intestinalis varied spatially with distance from the head of the estuary. Growth ofE. intestinalis collected from several sites increased with N enrichment alone and increased further when P was added in combination with N This indicated that N was limiting and that when N was sufficient, P became limiting. Sites from whichE. intestinalis exhibited nutrient limitation spanned the range of background water column NO₃ (12.9±0.4 to 55.2±2.1 μM) and SRP (0.8±0.0 to 2.9±0.2 μM) concentrations. Algae that were N limited had initial tissue N levels ranging from 1.18±0.03 to 2.81±0.08% dry weight and molar N∶P ratios ranging from 16.75±0.39 to 26.40±1.98.     

Nutrient inputs to the Choptank River estuary: Implications for watershed management

Degraded water quality due to water column availability of nitrogen and phosphorus to algal species has been identified as the primary cause of the decline of submersed aquatic vegetation in Chesapeake Bay and its subestuaries. Determining the relative impacts of various nutrient delivery pathways on estuarine water quality is critical for developing effective strategies for reducing anthropogenic nutrient inputs to estuarine waters. This study investigated temporal and spatial patterns of nutrient inputs along an 80-km transect in the Choptank River, a coastal plain tributary and subestuary of Chesapeake Bay, from 1986 through 1991. The study period encompassed a wide range in freshwater discharge conditions that resulted in major changes in estuarine water quality. Watershed nitrogen loads to the Choptank River estuary are dominated by diffuse-source inputs, and are highly correlated to freshwater discharge volume. in years of below-average freshwater discharge, reduced nitrogen availability results in improved water quality throughout most of the Choptank River. Diffuse-source inputs are highly enriched in nitrogen relative to phosphorus, but point-source inputs of phosphorus from sewage treatment plants in the upper estuary reduce this imbalance, particularly during summer periods of low freshwater discharge. Diffuse-source nitrogen inputs result primarily from the discharge of groundwater contaminated by nitrate. Contamination is attributable to agricultural practices in the drainage basin where agricultural land use predominates. Groundwater discharge provides base flow to perennial streams in the upper regions of the watershed and seeps directly into tidal waters. Diffuse-source phosphorus inputs are highly episodic, occurring primarily via overland flow during storm events. Major reductions in diffuse-source nitrogen inputs under current landuse conditions will require modification of agricultural practices in the drainage basin to reduce entry rates of nitrate into shallow groundwater. Rates of subsurface nitrate delivery to tidal waters are generally lower from poorly-drained versus well-drained regions of the watershed, suggesting greater potential reductions of diffuse-source nitrogen loads per unit effort in the well-drained region of the watershed. Reductions in diffuse-source phosphorus loads will require long-term management of phosphorus levels in upper soil horizons. *** DIRECT SUPPORT *** A01BY074 00021     

Distribution and abundance of ichthyoplankton in the Manicouagan River estuary,a tributary of the lower St. Lawrence estuary

The species composition and relative abundance of ichthyoplankton were investigated during summer 1986 at four stations along the salinity gradient in the Manicouagan River estuary, a tributary of the lower St. Lawrence estuary. Physical characteristics of water masses indicated the presence of a strong saline front (>10‰ per km) delineating the freshwater and marine section of the Manicouagan estuary. The estuary supports a depauperate ichthyoplankton community, including four species of pelagic fish eggs and eight species of fish larvae. Species richness increased with salinity. The ichthyoplankton fauna can be divided into two distinct groups: freshwater and marine. These two groups result initially from spawning preferences exhibited by the different species abundance of freshwater larvae was maximal at the head of the estuary and marine larvae were most abundant at the most saline station. The length frequency distribution suggests that marine larvae are not effectively retained within the estuary. The Manicouagan estuary cannot be considered as a major spawning site nor an important nursery zone for any fish found in this area.     

Effects of zooplankton grazing on phytoplankton size-structure and biomass in the lower Hudson River estuary

The impact of mesozooplankton (>210 μm, mostly adult copepods and late-stage copepodites) and micrometazoa (64–210 μm, mostly copepod nauplii) on phytoplankton size structure and biomass in the lower Hudson River estuary was investigated using various¹⁴C-labeled algal species as tracers of grazing on natural phytoplankton. During spring and summer, zooplankton grazing pressure, defined as %=mg C ingested m^?2 h^?1/mg C produced m^?2 h^?1 (depth-integrated rates)×100, on total phytoplankton ranged between 0.04% and 1.9% for mesozooplankton and 0.1% and 6.6% for micrometazoa. The greatest grazing impact was measured in fall when 20.2% and 44.6%, respectively, of the total depth-integrated primary production from surface water phytoplankton was grazed. Mesozooplankton exhibited some size-selective grazing on phytoplankton, preferentially grazing the diatomThalassiosira pseudonana over the larger diatomDitylum brightwelli, but this was not found for micrometazoa. Neither zooplankton group grazed on the dinoflagellateAmphidinium sp. We conclude that metazoan zooplankton have a minimal role in controlling total phytoplankton biomass in the lower Hudson River estuary. Differences in the growth coefficients of various phytoplankton size-fractions—not grazing selectivity—may be the predominant factor explaining community size-structure.     

Nutrient cycling in the sub-tropical Brunswick estuary,Australia

A combination of mixing plots, one-dimensional salt balance modelling, nutrient loading budgets, and benthic flux measurements were used to assess nutrient cycling pathways in the enriched sub-tropical Brunswick estuary during different freshwater flows. A simple model accounting for freshwater residence times and nutrient availability was found to be a good predictor of phytoplankton biomass along the estuary, and suggested that biomass accumulation may become nutrient-limited during low flows and that recycling within the water column is important during blooms. Dissolved inorganic nitrogen (DIN) cycling budgets were constructed for the estuary during different freshwater flows accounting for all major inputs (catchment, sewage, and urban) to the estuary. Internal cycling due to phytoplankton uptake (based on measured biomass) and sediment-water fluxes (based on measured rates in each estuarine reach) was considered. Four different nutrient cycling states were identified during the study. In high flow, freshwater residence times are less than 1 d, internal cycling processes are bypassed and virtually all dissolved, and most particulate, nutrients are delivered to the continental shelf. During the growth phase of a phytoplankton bloom enhanced recycling occurs as residence times increase sufficiently to allow biomass accumulation. Remineralization of phytoplankton detritus during this phase can supply up to 50% of phytoplankton DIN demands. In post-bloom conditions, DIN uptake by phytoplankton decreases in the autumn wet season when biomass doubling times begin to exceed residence times. OM supply to the sediments diminishes and the benthos becomes nutrient-limited, resulting in DIN uptake by the sediments. As flows decrease further in the dry season, there is tight recycling and phytoplankton blooms, and uptake by the sediments can account for the entire DIN loading to the estuary resulting in complete removal of DIN from the water column. The ocean is a potentially important source of DIN to the estuary at this time. The results of the DIN cycling budgets compared favorably with mixing plots of DIN at each time. The results suggest that a combination of different approaches may be useful in developing a more comprehensive understanding of nutrient cycling behavior and the effects of nutrient enrichment in estuaries.     

Nutrient limitation of heterotrophic bacteria in Florida Bay

We examined heterotrophic bacterial nutrient limitation at four sites in Florida Bay, U. S. in summer 1994 and winter 1995. Bacterial growth and biomass production in this system were most limited by inorganic phosphorus (P) in the eastern and southern regions of the bay. Nutrient additions stimulated productivity and biomass accumulation mostly in summer. The magnitude of growth responses (thymidine incorporation) to nutrient additions was nearly an order of magnitude less in winter than summer. Biomass-normalized alkaline phosphatase activity in the northeast and south-central region was 5–20 times greater than in the northwest and north-central regions, suggesting that P is most limiting to planktonic growth in those areas. Chlorophyll levels were higher in the northwest and north-central regions and P-uptake into particles >1 μm, primarily phytoplankton, was also higher in these regions. Consistent with these observations, others have observed that P is advected into the bay primarily in the northwestern region. Abundant seagrasses in Florida Bay may promote heterotrophic bacterial production relative to phytoplankton production by releasing dissolved organic carbon that makes bacteria more competitive for limiting quantities of inorganic phosphate, especially in the eastern bay where turbidity is low, P is most limiting, and light levels reaching the benthic plants are high.     

Nutrient limitation of phytoplankton growth in Georgia nearshore waters

Nutrient enrichment experiments were conducted to investigate the utilization of dissolved organic and inorganic nitrogen by marine phytoplankton in Georgia coastal waters. Natural populations of marine phytoplankton, enriched with different concentrations of ammonium chloride and other plant nutrients, were grown under controlled temperature and irradiance conditions until the populations reached “stationary phase.” Results showed that (1) phytoplankton are limited by DIN up to ca. 20μM, when another nutrient (phosphate or silicate) becomes limiting, (2) very little naturally-occurring DON is directly utilized for growth, (3) very little DON is indirectly made available for growth over time periods of days to ca. 1 week, and (4) trace metals and vitamins do not significantly limit phytoplankton growth.     

Nontidal circulation and mixing processes in the lower Potomac estuary

An analysis of the vertical structure of nontidal longitudinal currents and salinity in a reach of the lower Potomac River Estuary suggests that values for vertical eddy viscosity and eddy diffusivity scale with water depth H, tidal current amplitude U and bulk Richardson number according to conventional empirical formulas. However, the constant which relates the vertical eddy coefficients under conditions of neutral stability to UH is found to be an order of magnitude less than that expected for tidal conditions. Analyses also suggest that the degree of enhancement of longitudinal dispersion by the shear effect associated with the nontidal currents is a strong function of bulk Richardson number.     

Sediment transport and trapping in the Hudson River estuary

The Hudson River estuary has a pronounced turbidity maximum zone, in which rapid, short-term deposition of sediment occurs during and following the spring freshet. Water-column measurements of currents and suspended sediment were performed during the spring of 1999 to determine the rate and mechanisms of sediment transport and trapping in the estuary. The net convergence of sediment in the lower estuary was approximately 300,000 tons, consistent with an estimate based on sediment cores. The major input of sediment from the watershed occurred during the spring freshet, as expected. Unexpected, however, was that an even larger quantity of sediment was transported landward into the estuary during the 3-mo observation period. The landward movement was largely accomplished by tidal pumping (i.e., the correlation between concentration and velocity at tidal frequencies) during spring tides, when the concentrations were 5 to 10 times higher than during neap tides. The landward flux is not consistent with the long-term sediment budget, which requires a seaward flux at the mouth to account for the excess input from the watershed relative to net accumulation. The anomalous, landward transport in 1999 occurred in part because the freshet was relatively weak, and the freshet occurred during neapetides when sediment resuspension was minimal. An extreme freshet occurred during 1998, which may have provided a repository of sediment just seaward of the mouth that re-entered the estuary in 1999. The amplitude of the spring freshet and its timing with respect to the spring-neap cycle cause large interannual variations in estuarine sediment flux. These variations can result in the remobilization of previously deposited sediment, the mass of which may exceed the annual inputs from the watershed.     

Stratification and bottom-water hypoxia in the Pamlico River estuary

Relationships among bottom-water dissolved oxygen (DO), vertical stratification, and the factors responsible for stratification-destratification in this shallow, low tidal-energy estuary were studied using a 15-yr set of biweekly measurements, along with some recent continuous-monitoring data. Hypoxia develops only when there is both vertical water-column stratification and warm water temperature (>15°C). In July, 75% of the DO readings were <5 mg 1^?1, and one-third were <1 mg 1^?1. Severe hypoxia occurs more frequently in the upper half of the estuary than near the mouth. Both the time series data and correlation analysis results indicate that stratification events and DO levels are tightly coupled with variations in freshwater discharge and wind stress. Stratification can form or disappear in a matter of hours, and episodes lasting from one to several days seem to be common. Estimated summertime respiration rates in the water and sediments are sufficient to produce hypoxia if the water is mixed only every 6–12 d. There has been no trend toward lower bottom water DO in the Pamlico River Estuary over the past 15 yr. *** DIRECT SUPPORT *** A01BY059 00002     

Phosphorus distribution in sediments of the Delaware River estuary

In order to determine the primary factors related to the accumulation of phosphorus in estuarine sediments, a study of phosphorus fractions in sediments of the Delaware River Estuary was undertaken. A correlation matrix between the phosphorus fractions, determined by serial extraction, and 14 sediment variables was computed. Total phosphorus and total inorganic phosphorus in the sediment-phosphorus reservoir decreases with increasing salinity. This variation is correlated with decreasing iron oxyhydroxide content in the sediment. Neither clay content nor calcium carbonate content appear to be significantly correlated with variation in total inorganic phosphorus content in the fine-grained sediments of this estuary. Although calcium phosphate is concluded to be a major constituent of the sediment-phosphorus reservoir, there was no evidence found that it is authigenic in this environment.     

珠江河口夏季缺氧现象的模拟

夏季底层水体缺氧现象是珠江河口存在的环境问题之一。使用三维水动力-生态耦合模型来分析珠江口缺氧现象的分布状况和产生原因。模拟结果表明:模型能很好的再现珠江口的缺氧敏感性区域和强度。珠江口存在的底层水体缺氧现象是水体强烈层化和生化耗氧过程共同作用的结果。缺氧现象的发展与减退受潮汐涨落的影响。珠江口盐度-潮汐混合锋面和层化作用控制低氧水团的范围和强度。     

Seasonal and spatial distribution of planktonic crustacea in the lower Saint John River,a multibasin estuary in New Brunswick,Canada

Five stations on the lower Saint John River, a complex multibasin estuary, were sampled semiquantitatively for zooplankton at biweekly intervals for one year, and qualitatively over a 4-year period. Planktonic Crustacea were dominated by the true estuarine copepods,Acartia tonsa andEurytemora affinis and the euryhaline marine copepodsOithona similis andPseudocalanus minutus. Atypical estuarine forms, confined to a lower fiord-like basin with salinity of 20‰, were the amphipod,Parathemisto abyssorum and the mysidErythrops erythrophthalma. River flows were highly variable from year to year. Certain basins function as lakes in some years and estuaries in other years, causing extreme zooplankton community fluctuations, and succession patterns dependent on salinity rather than season. On occasion freshwater zooplankters maintained viable populations at unusually high salinities (ca. 5‰). Vertical and horizontal distributions of zooplankters indicate that the estuary in fact comprises two systems: a true estuary in the upper reaches and the surface waters at the lower end, and a fiord in a subsidiary basin in the lower end.     

The distribution of shallow water juvenile fishes in an urban estuary: The effects of manmade structures in the lower Hudson River

The objective of this study was to determine what effect, if any, large pile-supported platforms (piers) have on the habitat distribution and abundance of juvenile fishes. Trapping techniques were used in 1993 and 1994 under piers, in pile fields, and in open-water habitat types in shallow areas (<5 m) in the lower Hudson River estuary (40°44′N, 70°01′W). Nearly 1500 fishes, mostly juveniles, representing 24 species were collected in 1865 trap-days from May through October in the 2-yr study. The presence of relatively large numbers of young-of-the-year (YOY) fish during both years lends support to the idea that shallow areas in the lower Hudson River estuary currently function as nursery habitats for a variety of fishes. Two seasonal assemblages were apparent, but their composition varied somewhat between years.Microgadus tomcod andPseudopleuronectes americanus YOY dominated an early summer assemblage (May–July) while large numbers of YOYMorone saxatilis were collected as part of a late summer assemblage (August–September). The effects of habitat type on fish assemblage structure were significant during both years. Fish abundance and species richness were typically low under piers; YOY fishes were rare andAnguilla rostrata accounted for a large proportion of the total catch. In contrast, YOY fishes dominated collections at pile field and open-water stations, where abundance and species richness were high. These results indicate that habitat quality under the platforms of large piers (>20,000 m²) is probably poor for YOY fishes when compared with nearby pile field and open-water habitat types.     

Hypoxia of bottom waters of the Razdolnaya River estuary

The hypoxia of the bottom waters in the Razdolnaya River estuary was observed for the first time in September 2014 during the survey. It is formed as in the seaward part: oxygen is absorbed as a result of destruction of excessive phytoplankton biomass that settles to the bottom and is synthesized on the upper horizon. The high value of primary production in the riverine part of the estuary was caused by the pycnocline formed. Thus, phytoplankton “blooms” above and undergoes destruction beneath the pycnocline. Oxygen is distributed symmetrically in both parts of the estuary with respect to a bar: similar oxygen concentrations, which are maximum on the surface and minimum at the bottom, 300 and <60 μm/L, respectively, are recorded. The anomalies of hydrochemical parameters that have been formed during this process are sharply different in the two parts of the estuary, which most vividly manifests itself in the N/P value and the partial pressure of carbon dioxide pCO₂. The causes of this unique situation are discussed.