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

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

Methane distribution and cycling in Tomales Bay, California

Cycling of methane (CH₄) in Tomales Bay, a 28-km² temperature estuary in northern California with relatively low inputs of organic carbon, was studied over a 1-yr period. Water column CH₄ concentrations showed spatial and temporal variability (range=8–100 nM), and were supersaturated with respect to the atmosphere by a factor of 2–37. Rates of net water column CH₄ production-oxidation were determined by in situ experiments, and were not found to be significantly different from zero. Fluxes across the sediment-water interface, determined by direct measurement using benthic chambers, varied from ?0.1 μmol m^?2 d^?1 to +16 μmol m^?2 d^?1 (positive fluxes into water). Methane concentrations in the two perennial creeks feeding the bay varied annually (140–950 nM); these creeks were a significant CH₄ source to the bay during winter. In addition, mass-balance calculations indicate a significant additional inter CH₄ source, which is hypothesized to result from storm-related runoff from dairy farms adjacent to the bay. Systemwide CH₄ budgets of the 16-km² inner bay indicate benthic production (110 mol d^?1) and atmospheric evasion (110 mol d^?1) dominated during summer, while atmospheric evasion (160 mol d^?1) and runoff from dairy farms (90 mol d^?1) dominated during winter.     

Applications of aerial photography to population estimation in Nigeria

For effective planning purposes, reliable and up-to-date statistics on population is very important in any country. However, in Nigeria, population census is such an infrequent exercise that data on city population is not readily available. The inherent data delivery lag time period, between the physical collection and actual publication of the results of censuses contributes to the obsolescency of census data.This situation calls for the development of appropriate techniques of collecting up-to-date population data in Nigeria.The use of aerial photography for collecting certain types of census information has been suggested as a procedure that can successfully supplement existing techniques. Three models viz: (1) the Land Use Model (2) the House Counting Method and (3) the Land Area Method were developed and applied in the estimation of the 1973 population of Ilorin. It is being suggested here that these methods can be employed in making the much needed intercensal population estimation for planning purposes readily available. In addition, this approach is not as sensitive as census per se in Nigeria.     

Late Holocene δC and pollen records of paleosalinity from tidal marshes in the San Francisco Bay estuary, California

Records of stable carbon isotopes (δ¹³C) are presented from cores collected from four San Francisco Bay marshes and used as a proxy for changes in estuary salinity. The δ¹³C value of organic marsh sediments are a reflection of the relative proportion of C₃ vs. C₄ plants occupying the surface, and can thus be used as a proxy for vegetation change on the marsh surface. The four marshes included in this study are located along a natural salinity gradient that exists in the San Francisco Bay, and records of vegetation change at all four sites can be used to infer changes in overall estuary paleosalinity. The δ¹³C values complement pollen data from the same marsh sites producing a paleoclimate record for the late Holocene period in the San Francisco Bay estuary. The data indicate that there have been periods of higher-than-average salinity in the Bay estuary (reduced fresh water inflow), including 1600-1300 cal yr B.P., 1000-800 cal yr B.P., 300-200 cal yr B.P., and ca. A.D. 1950 to the present. Periods of lower-than-average salinity (increased fresh water inflow) occurred before 2000 cal yr B.P., from 1300 to 1200 cal yr B.P. and ca. 150 cal yr B.P. to A.D. 1950. A comparison of the timing of these events with records from the California coast, watershed, and beyond the larger drainage of the Bay reveals that the paleosalinity variations reflected regional precipitation.     

The population ecology of an exotic mussel,Musculista senhousia,in a Southern California Bay

Musculista senhousia is a small, infaunal mussel native to Asia. Over the last 75 yrs, humans have unintentionally introduced it to various parts of the world, including the Mediterranean, Australasia, and North America. Recently,M. senhousia has become abundant in Mission Bay, San Diego, California. Here, the mussel exhibits many traits typical of opportunistic species. It is small, only attaining a maximum length of about 32 mm, and is capable of rapid growth, reaching up to 25 mm after 1 yr. In San Diego, there was a major peak of recruitment during the late fall of 1990, and low levels of background recruitment throughout much of the year. Although the maximum life span ofM. senhousia is approximately 2 yr, mortality is high and most of the population are only annuals. OverallM. senhousia appears to display similar life-history characteristics both in this invaded system and within its native range.     

Distribution patterns of zooplankton in Tomales Bay,California

Spatial patterns of abundance of the zooplankton of Tomales Bay, California, were studied over one year from August 1987 to September 1988. Samples were taken on six transects up the long axis of the bay, and the species composition and abundance of common species were determined. Distribution patterns were similar to those observed in other estuaries and bays, with species from nearby neritic waters occurring in the outer bay and a few resident species in the inner bay. This pattern may be best explained by size-selective predation within the bay. Most alternative explanations can be ruled out for Tomales Bay, except for possible temperature effects on cool-temperate neritic species. The four common species ofAcartia in Tomales Bay were in two subgenera, each of which included a neritic species and a smaller inner-bay species. The occurrence of the smaller of each pair in the inner bay, which has been observed forAcartia and other species in other estuaries and bays, may also be a result of size-selective predation.     

Nutrient biogeochemistry in an upwelling-influenced estuary of the Pacific northwest (Tillamook Bay,Oregon, USA)

Tillamook Bay, Oregon, is a drowned river estuary that receives freshwater input from 5 rivers and exchanges ocean water through a single channel. Similar to other western United States estuaries, the bay exhibits a strong seasonal change in river discharge in which there is a pronounced winter maximum and summer minimum in precipitation and runoff. The behavior of major inorganic nutrients (phosphorus, nitrogen, and silica) within the watershed is examined over seasonal cycles and under a range of river discharge conditions for October 1997–December 1999. Monthly and seasonal sampling stations include transects extending from the mouth of each river to the mouth of the estuary as well as 6–10 sites upstream along each of the 5 major rivers. Few studies have examined nutrient cycling in Pacific Northwest estuaries. This study evaluates the distributions of inorganic nutrients to understand the net processes occurring within this estuary. Based upon this approach, we hypothesize that nutrient behavior in the Tillamook Bay estuary can be explained by two dominant factors: freshwater flushing time and biological uptake and regeneration. Superimposed on these two processes is seasonal variability in nutrient concentrations of coastal waters via upwelling. Freshwater flushing time determines the amount of time for the uptake of nutrients by phytoplankton, for exchange with suspended particles, and for interaction with the sediments. Seasonal coastal upwelling controls the timing and extent of oceanic delivery of nutrients to the estuary. We suggest that benthic regeneration of nutrients is also an important process within the estuary occurring seasonally according to the flushing characteristics of the estuary. Silicic acid, nitrate, and NH₄ ⁺ supply to the bay appears to be dominated by riverine input. PO₄ ⁻³ supply is dominated by river input during periods of high river flow (winter months) with oceanic input via upwelling and tidal exchange important during other times (spring, summer, and fall months). Departures from conservative mixing indicate that internal estuarine sources of dissolved inorganic phosphorus and nitrogen are also significant over an annual cycle.     

Spatio-temporal scale assessment of an “upwelling shadow” in Northern Monterey Bay,California

Estuaries and Coasts - The time and space scales of an “upwelling shadow” in northern Monterey Bay, California, were assessed using hydrographic monitoring and satellite imagery....     

Minor elements in sediments of Great Bay estuary,New Hampshire

Concentrations of copper, zinc, chromium, lead, cadmium, and phosphorus were obtained from 81 samples of unconsolidated estuarine sediment from Great Bay, New Hampshire. Dispersal of aqueous chromium from localized industrial effluent is believed responsible for an increase in sediment chromium throughout the entire estuary. High phosphorus concentrations exist in sediment near the outfalls from several waste-water treatment plants. There is no evidence for any increase of copper, zinc, lead, or cadmium in this estuary, except for localized high concentrations close to industrial outfalls. Fine-grained sediments and organic carbon correlate highly with all the elements studied, except for chromium. This suggests that conventional agents of sedimentary adsorption are not adequate to explain the incorporation of chromium into sediment under the conditions of heavy industrial discharge which exist in this estuary. Sediment phosphorus correlates highly with minor elements, suggesting that it is an adsorption agent, similar to more typical sedimentary parameters such as organic matter and clay minerals. In such a capacity phosphorus may enhance the sedimentary uptake of other aqueous species, and account for higher chromium sediment concentrations. Comparative data from other sedimentary environments emphasize the environmental significance of these elements in Great Bay.     

Spatial and temporal distribution of methane in an extensive shallow estuary, South India

Sedimentary methane (CH₄) fluxes and oxidation rates were determined over the wet and dry seasons (four measurement campaigns) in Pulicat lake, an extensive shallow estuary in south India. Dissolved CH₄ concentrations were measured at 52 locations in December 2000. The annual mean net CH₄ flux from Pulicat lake sediments was 3.7 × 10⁹ g yr^-1 based on static chamber measurements. A further 1.7 × 10⁹g yr_-1 was estimated to be oxidized at the sediment-water interface. The mean dissolved concentration of CH₄ was 242nmol |^-1 (ranging between 94 and 501 nmol |^-1) and the spatial distribution could be explained by tidal dynamics and freshwater input. Sea-air exchange estimates using models, account only for ∼13% (0.5 × 10⁹ g yr-1) of the total CH₄ produced in sediments, whereas ebullition appeared to be the major route for loss to the atmosphere (∼ 63% of the net sediment flux). We estimated the total atmospheric source of CH₄ from Pulicat lake to be 0.5 to 4.0 × 10⁹g yr^-1.     

Mechanisms of expansion for an introduced species of cordgrass, Spartina densiflora, in Humboldt Bay, California

The dominant plant in Humboldt Bay salt marshes in Spartina densiflora, a species of cordgrass apparently introduced from South America. At several salt marshes and restoration sites around Humboldt Bay, distribution of this plant has increased significantly. We investigated the relative contributions of vegetative tiller production and seed germination to the establishment and expansion of S. densiflora. Lateral spread of plants surrounded by competitors were compared to areas without competing plant species. Plants growing in areas without competitors had significantly higher rates of vegetative expansion (p<0.0001). Viable seed production, germination rates, seedling survivorship, and growth of adult plants were measured in six salinity treatments. Approximately 1,977±80 viable seeds are produced per plant (0.25–0.5 m²). The number of germinating seeds was inversely related to increases in salinity. Salinity treatments between 19‰ and 35‰ produced significantly lower germination rates than salinities of 0–18‰ (p<0.0001). Seedling survivorship was 50% at ≤4‰ and 8–14% at ≥11‰. Lateral expansion of adult, greenhouse-grown plants occurred in all salinity treatments, with modest decreases in the highest salinity treatments (p<0.05). Our findings indicate that S. densiflora expands primarily by vegetative expansion, and lateral tillers are produced by throughout the year. Spartina densiflora produces prolific amounts of seed, but recruitment in mature salt marshes may be limited by competitors and higher salinities. At restoration sites, planting of native species such as Salicornia virginica, Distichlis spicata, or Jaumea carnosa may prevent monospecific stands of S. densiflora from developing.     

Using winter flounder growth rates to assess habitat quality across an anthropogenic gradient in Narragansett Bay, Rhode Island

We used growth rates of juvenile winter flounderPseudopleuronectes americanus to assess anthropogenic influence on habitat quality at three sites in Narragansett Bay, Rhode Island. The upper bay site, Gaspee Point, had the highest population density and concentration of total nitrogen; human inputs decreased down bay. Growth rates of individually marked fish were measured in three 15-d experiments from June 8 to July 6, 1998 in 1-m² cages placed at upper, middle, and lower bay sites. Water temperature, salinity, dissolved oxygen (DO), and benthic food were also measured. Stable isotopes of nitrogen and carbon were measured in experimental fish as possible indicators of nutrient enrichment and to identify organic carbon sources. Growth rates were 0.22–0.60 mm d⁻¹, with the highest average at the mid-bay site. Growth was initially fastest at Gaspee Point, but dropped off as DO concentrations fell. Step-wise multiple regression indicated that location (upper, middle, or lower bay) explained most of the variability in fish growth (40%). Coefficients of other significant variables indicated that fish grew faster at lower salinities, smaller sizes, and with decreased time that DO was below 2.3 mg l⁻¹. Benthic prey varied among sites and there was significantly less food and fewer species at Gaspee Point.Polydora cornuta was a favored food at all sites and was found in over half the stomachs. Values of δ¹⁵N in fish and sediments did not reflect differences in total nitrogen concentrations recorded near the sites. We suggest that anthropogenic influences, such as nutrients and sewage, affected habitat quality by reducing DO, which lowered fish growth rates.     

Seasonal and spatial variability in macrobenthos communities in Jamaica Bay,New York—an urban estuary

Macrobenthos were sampled at 27 sites on a seasonal basis from October 1981 through November 1982. Cluster analyses and principal components analyses indicated that although spatial and species groupings were weak, an underlying sediment-correlated structure persisted for all seasons. Weak station groupings resulted from great seasonal and local variability in abundance of several dominant taxa. The most widely distributed species varied the least in density over time. Species richness (number of species) and dominance were uncorrelated. Dominance was correlated with sediment organic content [% total organic carbon (TOC)] and % Mud. Species richness increased with increasing % TOC, reaching a maximum in the range 0.7 to 1.0% TOC (primarily sand stations). At higher organic levels (muddy, sand stations), species richness declined. Muddy sand stations were dominated byAmpelisca abdita. In this community, species richness was inversely correlated both with percent TOC and heavy metals concentrations. In both sand and mud communities, species richness was positively correlated with density (mean number individuals of all species per sample).     

The importance of autotroph distribution to mussel growth in a well-mixed,temperate estuary

In this study, we explored the extent to which secondary production in a well-mixed estuary reflects local differences in biotic and physical characteristics of habitats, or larger-scale, estuary-wide characteristics governed by a freshwater-marine gradient. We addressed the following questions: To what extent do organic components of seston within habitats in an estuary reflect distributions of local autotrophs and to what extent do estuarine consumers such as sessile filter-feeders, respond to small-scale, local differences in habitat characteristics in a wellmixed estuary? We contrasted habitat quality and consumer growth at four sites within Padilla Bay estuary, Washington, representing the major autotrophic sources of organic carbon in Pacific Northwest estuaries (i.e., phytoplankton, eelgrass (Zostera marina), epibenthic and macro-algal species, and marsh macrophytes.) The natural abundances of stable carbon isotopes {ie898-1} were used to resolve origins of organic carbon in diets of blue mussels (Mytilus edulis), a representative suspension feeder. To assess consumer responses to habitat, quality, we combined measures of sestonic food quantity and quality and physical parameters with in situ determination of mussel growth. We used measures of food quality {ie898-2} and consumer response (growth of transplanted mussels) to integrate the effects of high variability in estuarine physical and biological characteristics on primary and secondary production. Using ANOVA, we detected significant differences in the concentrations of sestonic food, seston composition as indicated by {ie898-3}, and mussel {ie898-4} values and growth rates among the four representative habitats. That significant differences in {ie898-5} values of mussel tissue corresponded to the significant differences in {ie898-6} values of local autotrophs and seston among habitats suggests that mussels in Padilla Bay rely primarily on local sources of carbon for food. Mussel growth throughout, the estuary was significantly correlated with both sestonic {ie898-7} and salinity. We conclude that differences in local seston composition and mussel growth rates reflect in part the heterogeneous, distribution of benthic primary producer habitats in Padilla Bay, despite its well-mixed nature. In addition, local differences in salinity levels, as opposed to the bay-wide freshwater-marine, gradient, explained a significant proportion of the variance in mussel growth within the bay. Our results counter the prediction that seston quality and consumer production are comparable throughout well-mixed estuaries, and suggest that the paradigm of physically and chemically determined gradients in estuarine secondary production needs to be broadened to include local biotic factors as well.     

Measurement of cordgrass,Spartina alterniflora,production in a macrotidal estuary,Bay of Fundy

A one-year field study was conducted of the growth, mortality, and loss dynamics of aSpartina alterniflora low marsh in the Minas Basin, a macrotidal estuary at the head of the Bay of Fundy. Data were used to examine the suitability of four methods for estimating annual net aerial primary production (NAPP) of a marsh subject to energetic tidal flooding. Shoots start to grow in April and reach maximum height (about 0.5 m) and weight in October. Maximum shoot density (900–1,600 m^?2) occurs around June and drops thereafter due to the export of entire shoots. The average shoot produces about seven leaves and at least 2–3 are lost during the growing season. All remaining vegetation dies before the end of November. Methods based on harvesting vegetation underestimated NAPP, especially at lower elevations where export is greater due to more frequent and prolonged tidal flooding. The highest NAPP values, on the order of 500–600 g m^?2 y^?1, were obtained using methods based on the population dynamics of individual shoots. These methods are recommended for energetic tidal environments because they include the production of vegetation exported during the growing season.     

Iron, sulfur, and carbon diagenesis in sediments of Tomales Bay, California

Analysis of 3-m sediment cores revealed that profiles of carbon (C), sulfur (S), and iron (Fe) varied with relative distance from marine and terrestrial sediment sources in Tomales Bay California. Despite relatively high sedimentation rates throughout the bay (historically 3–30 mm yr⁻¹), sulfate reduction of deposited organic matter led to free-sulfide accumulation in sediments only at the location farthest from terrestrial runoff, the source of reactive iron. Acid-volatile sulfide concentrations in all sediments (<10 μmol g⁻¹) were low relative to concentrations of chromiumreducible sulfide (up to 400 μmol g⁻¹ farthest from the reactive iron source). A calculated index of iron availability, used to describe sediment resistance to build-up of free sulfide, was lowest at this location. Recent, upward shifts in reactive Fe concentration and in the relative contribution of terrestrial orgnic carbon (measured as a shift in δ¹³C of bulk sediment organic matter) in all cores indicated that erosion and transport of sediments from the watershed surrounding Tomales Bay increased after European settlement in the 1850s.     

A method to assess the freshwater inflow requirements of estuaries and application to the Mtata estuary, South Africa

The National Water Act (Act 36 of 1998) in South Africa recognizes basic human water requirements as well as the need to sustain the country's freshwater and estuarine ecosystems in a healthy condition for present as well as future generations. In this Act, provision is made for a water reserve to be estimated prior to the authorization of water use (e.g., for agriculture, large volume residential and industrial uses) through licensing. This reserve is the water required to satisfy basic human needs (i.e., 25 1 person^?1 d^?1) and to protect aquatic ecosystems to ensure present and future sustainable use of the resource. This led the Departments of Water Affairs and Forestry and estuarine scientists throughout South Africa to develop a method to determine the freshwater inflow requirements of estuaries. The method includes documenting the geographical boundaries of the estuary and determining estuarine health by comparing the present state of the estuary with a predicted reference condition with the use of an Estuarine Health Index. The importance of the estuary as an ecosystem is taken from a national rating system and together with the present health is used to set an Ecological Reserve Category for the estuary. This category represents the level of protections afforded to an estuary. Freshwater is then reserved to maintain the estuary in that Ecological Reserve Category. The Reserve, the quantity and quality of freshwater required for the estuary, is determined using an approach where realistic future river runoff scenarios are assessed, together with data for present state and reference conditions, to evaluate the extent to which abiotic and biotic conditions within an estuary are likely to vary with changes in river inflow. Results from these evaluations are used to select an acceptable river flow scenario that represents the highest reduction in freshwater inflow that will still protect the aquatic ecosystem of the estuary and keep it in the desired Ecological Reserve Category. The application of the Reserve methodology to the Mtata estuary is described.     

A computational procedure to assess the distribution of constriction sizes for an assembly of spheres

The effectiveness of filters to counteract internal erosion in earth structures is particularly related to their ability to capture fine particles moving under seepage flow through the porous material. More precisely, fine particles are likely to be trapped by the narrowest paths between pores: the constrictions. This paper proposes a methodology to compute the constriction size distribution of model granular filters taking into account the relative density of the material. The approach is based upon probabilistic methods which adopt stated simple geometric packing arrangements to represent the solid structure in the extreme density states. Two new models are proposed for the design of the constriction size distribution according to the type of filter grading: continuously graded or gap-graded materials. The models require the usual material characteristics: the grading curve, and the minimum and maximum void ratios for this material. Calibrated on the basis of statistical analyses over numerical assemblies of spheres generated by a discrete element method, the proposed new models constitute a promising tool to significantly improve the modeling of filtration processes in granular materials.     

Feeding of underyearling walleye surfperch,Hyperprosopon argenteum,in a Northern California Bay

A study of the feeding of underyearling walleye surfperch (Hyperprosopon argenteum) was based on fish collected in seven areas of Humboldt Bay, California, during September and October 1969. Crustaceans were found in all stomachs containing food, but the type of major food consumed varied with distance from the entrance channel. Amphipods were universally consumed, with cypris stages more important in areas with high tidal exchange, and terrestrial moth flies (Psychoda spp.) and harpacticoid copepods more important in backwater areas. Collections indicate daylight feeding and probably limited diurnal movements of underyearling walleye surfperch, a different pattern of feeding behavior than walleye surfperch of inshore waters of southern California.