Showing papers in "Estuarine Coastal and Shelf Science in 1990"

Seasonal variability of stable carbon and nitrogen isotope ratios of organisms in a North Pacific Bay

Abstract: A survey of δ13C and δ15N of organic carbon and nitrogen in net phytoplankton, zooplankton, juvenile demersal fish and benthic fauna and sediments was made during the spring production cycle in Auke Bay and Fritz Cove marine embayments located in southeastern Alaska. The δ13C and δ15N values of net phytoplankton (>73 μm) increased during the spring bloom from −20·6±0·1%. (δ13C) and + 3·3±0·6%. (δ15N) during the prebloom stage to −18·1±0·2%. (δ13C) and +6·9±0·1%. (δ15N) following the bloom peak. Phytoplankton δ13C and δ15N values were significantly correlated with each other and increased with tempearture [+0·71%. per °C (δ13C) and +0·58%. per °C (δ15N)]. Phytoplankton and δ15N values were also correlated with a decline in ambient nitrate concentration suggesting that nitrate was the primary nitrogen source fueling the spring bloom. Net macrozooplankton (>505 μm) δ13C and δ15N values varied during the spring production cycle in a manner similar to phytoplankton, from −21·2%. (δ13C) and +7·2%. (δ15N) during the prebloom stage to −18·2%. (δ13C) and +8·2%. (δ15N) following the bloom peak. The δ13C values of macrozooplankton (> 165 μm) were, on average, 0·5%. (Fritz Cove) to 0·7%. (Auke Bay) heavier than their presumed phytoplankton food. Macrozooplankton were also on average +3·2%. enriched with 15N over phytoplankton. Values of δ13C for juvenile Auke Bay flathead sole (80–100) mm) averaged −17·2±0·2%. and for juvenile pollock (120–130 mm) averaged −17·8±0·2%., which were 3·1%. and 2·5%., respectively, more positive than phytoplankton and 2·4%. and 1·8%., respectively, more positive than macrozooplankton (> 165 μm). Sole δ15N values averaged +14·9±1·5%. and for pollock +13·8±1·9%., 7·1%. and 6·0%., respectively, more positive than zooplankton. The δ13C values of Auke Bay sedimentary carbon (−20·8±0·6%.) were slightly depleted in 13C compared to Auke Bay net phytoplankton and δ15N values of sedimentary nitrogen (+5·9±1·0%.) were slightly enriched in 15N over phytoplankton suggesting that phytoplankton supply the major portion of both carbon and nitrogen to Auke Bay sediments. Benthic invertebrates exhibited a wide range of δ13C (−20·8 to −14·3%.) and δ15N (+7·5 to +12·8%.) values. Much of the variability is explained by the trophic positions of the animals. The δ13C and δ15N values of adult Auke Bay Macoma nasuta exhibited significant seasonal shifts in δ13C and δ15N which were negative and opposite to the positive seasonal shifts in δ13C and δ15N of phytoplankton and zooplankton. Meiofauna (>98% nematodes) did not exhibit a seasonal shift in δ13C and δ15N as did phytoplankton, zooplankton and Macoma nasuta.

Wetland soil formation in the rapidly subsiding Mississippi River Deltaic Plain: mineral and organic matter relationships.

Abstract: The elevation of submerging coastal marshes is maintained by vertical accretion of mineral and organic matter. Submergence rates currently exceed 1·0 cm year −1 in the Mississippi Deltaic Plain and are expected to increase. Mineral matter-organic matter relationships were examined in surface profiles of Mississippi Deltaic Plain soil from both Active Delta Zone marsh (which receives freshwater and mineral sediment from the Atchafalaya or Mississippi Rivers) and Inactive Delta Zone marsh (which relies on rainfall for freshwater and on reworked sediments for mineral matter) to gain insights into marsh soil structure and formation. Mineral and organic matter accounted for 4–14% of soil volume. The remainder was pore space and was occupied by water and entrapped gases. Organic matter occupied more volume than mineral matter in all but saline marsh soil. The regular influx of mineral matter to active fresh marsh resulted in active fresh marsh soil containing twice as much mineral and organic matter as inactive fresh marsh soil. Within the Inactive Delta Zone, the volume of mineral and organic matter increased from fresh (inland) to saline (seaward) marshes. Saline marsh soil required 1·7 times as much mineral matter as brackish marsh soil to vertically accrete at similar rates, possibly as a result of soil bulk density requirements of the dominant saline marsh plant, Spartina alterniflora . Vertical accretion rates were highest in the Active Delta Zone, probably as a result of increased mineral matter availability and delivery. Current, best estimates of the combination of mineral and organic matter required (g m −2 year −1 ) to maintain marsh surface-water level relationship are fresh marsh: organic matter = 1700 + 269 x , mineral matter = 424 x ; brackish marsh: organic matter = 553 + 583 x , mineral matter = 1052 x ; saline marsh: organic matter = 923 + 601 x , mineral matter = 1798 x , where x = the rate of submergence (cm year −1 ).

The fish and prawn communities of a Malaysian coastal mangrove system, with comparisons to adjacent mud flats and inshore waters

Abstract: The fish and prawn communities of four coastal habitats in Selangor, Malaysia: (1) coastal mangrove swamps; (2) mud flats, including subtidal regions; (3) near inshore waters; and (4) far inshore waters, were examined with respect to species composition, abundance, diversity, similarity and sexual maturity. The number of fish species sampled for these habitats were 119, 70, 58 and 92 species respectively. For prawns, the number of species were 9, 16, 8 and 14 respectively. The mangrove community comprised 63–99% and 100% juvenile fish and prawn respectively. Despite a high species richness, it was low in fish species diversity due to the dominance (70%) of six species of fish of low economic value. In contrast, the prawn community was poor in species, and was dominated by threePenaeus spp. of high economic value. The intertidal mudflat community is largely transient and comprised mainly of species from both the mangrove and subtidal habitats. Fifty-six per cent of the fish population and 62·6% of the prawn population were juveniles. Many of the inshore fish and prawn species were also common species found in mangrove and mudflat habitats. Similarity coefficients indicate that the inshore fish and prawn communities are more similar to those of mud flats than mangroves. However, maturity studies indicate that only five species of fish are true migrants, that the majority of the fish species (juveniles and adults) are ubiquitous with a distribution that extends several nautical miles offshore. Results of this study indicate that tropical coastal mangroves function more importantly as feeding grounds than as nursery grounds for juveniles of commercially important fish species. Mangroves and mud flats are utilized during flood tides by many periodic foragers from the inshore waters. However, this study confirms that coastal mangroves and mud flats are important nursery areas for commercially important prawn species.

An assessment of metal contamination of sediments in the humber estuary, U.K.

Abstract: It is difficult to make an overall assessment of the degree of metal contamination in estuarine and marine sediments. This is a consequence of variations in analytical procedures between studies and the presence of an unknown natural background in the sediment. Measurement of total (rather than extractable) metal and normalization of concentrations as ratios to an element associated with clays provides a solution to the first difficulty. Expressing these values as enrichment factors relative to pre-industrial sediments from the same environment solves the second. Levels of anthropogenic enrichment of intertidal sediments in the Humber Estuary have been assessed relative to a baseline provided by sediments deposited in the Humber approximately 5000 years B.P. A sample of consolidated Holocene mud estimated to be at least 100 years old confirmed the appropriateness of this baseline. Normalization relative to Rb, which is not anthropogenically enriched, was the most suitable way to adjust for grain size. Levels of Ti, Fe, P, V, Cr, Mn, Ni, Cu, Zn, As, Y, Nb and Pb are elevated above this baseline. The most marked enrichments (between 3·5- and 6-fold) are of P, As, Pb, Cu and Zn. Normalized concentrations were spatially rather uniform with two exceptions. A single sample from the north bank showed elevated levels of Pb, Cu, Zn and Cr. An area receiving effluents from an industrialized zone on the south bank, including two titanium dioxide processing factories, showed high levels of a number of elements, particularly Nb. It is suggested that Nb may be a valuable tracer for effluents from the sulphate process of TiO 2 extraction.

Plant biomass and nutrient flux in a managed mangrove forest in Malaysia

Abstract: This paper summarizes previously reported and new data. Data on biomass and nutrient content in different components of the mangrove trees are presented and estimates of the flux of these are attempted. As a first step to determining the quantitative relationship between the export of material and the areal extent of mangroves, the biomass and nutrients contained in the mangrove trees and the release of these to the ecosystem annually were determined for the 40 800-ha Matang Mangrove Forest Reserve—a managed mangrove forest in Malaysia. The total standing biomass of the Matang Mangrove Forest is estimated to be 8·26 milion tonnes (of dry matter). Biomass released annually from the mangrove trees in the Matang system is 1 015 980 tonnes. Of these, 559 500 tonnes or 55% is in the form of dead trees, 396 840 tonnes (39%) is in the form of small litter and 59 640 (6%) in the slash left behind after thinning and harvesting. The amounts of macro-nutrients (N,P,K, Ca, Mg and Na) released annually are 12 210, 11 870 and 2690 tonnes through litter, dead trees and slash respectively. The fate of these materials is discussed. Using the figure of 50% export, the export of biomass and nutrients from the Matang Mangroves through leaf litter alone is estimated as 158 300 and 5100 tonnes annually or 3·9 and 0·1 tonne ha−1 year−1 respectively.

Near-shore hypoxia in the Chesapeake Bay: Patterns and relationships among physical factors

Abstract: Near-shore, shallow waters in the Chesapeake Bay periodically experience episodes of anoxia or severe hypoxia during summer. In order to examine the severity and temporal pattern of hypoxia, and environmental factors that may lead to such episodes, dissolved oxygen, salinity and temperature were measured at 15-min intervals during the summers of 1987 and 1988 in a western shore oyster bed. Bottom dissolved oxygen concentrations averaged lower at a 4-m site than at a 2-m site. At the 4-m site, dissolved oxygen concentrations dropped below 2 mg l−1 during approximately 40% of days and below 1 mg l−1 during approximately 10% of days each summer. However, diel fluctuations in oxygen concentrations were sufficiently large that even on days of the most severe oxygen minima, dissolved oxygen concentrations always reached or exceeded a level tolerable by most estuarine organisms during some part of the day. During episodes of severe hypoxia on the western shore of the Chesapeake Bay, oxygen minima (1) coincided with increased salinities and ebb tides, (2) were preceded by winds from the S-SSE (minima of ⩽0·5 1 mg l−1) or SW (minima of 0·6-1·1 mg l−1), and (3) were reached during 22.00−06.00 hours. Severe hypoxia at the study site therefore appeared to result from intrusions of bottom water, which were most effectively driven by southerly winds. Tidal currents were required to provide the final force that brought deep water close to shore. The diel pattern of intrusions is most likely caused by winds, which were often either too strong or had too weak a southerly component to permit intrusions (without near-shore mixing and reaeration) to occur during afternoon-early evening hours, and the diel periodicity of tides. Short-term fluctuations in temperature and salinity were not as great as fluctuations in dissolved oxygen when compared to seasonal fluctuations of each parameter. For many organisms, short-term fluctuations in temperature and salinity may therefore be less likely to influence the habitability of areas such as the near-shore Chesapeake Bay than are similar duration fluctuations in dissolved oxygen concentrations.

Factors affecting the hydrochemistry of a mangrove tidal creek, Sepetiba Bay, Brazil

Abstract: We studied the porewater chemistry, and spatial and temporal variation of mangrove creek hydrochemistry. Except for nitrate porewater, the concentrations of nutrients we analysed were higher than for creek water. Groundwater is a source of silica and phosphate, whereas total alkalinity and ammonium are related to mangrove porewater migration to the creek. Open bay waters contribute chlorine, dissolved oxygen and elevated pH. The results also suggest that nitrate is related to nitrification inside the creek. During flood tides, salinity, chlorine, dissolved oxygen and pH increase, whereas total alkalinity decreases. This pattern is reversed at ebb tides. Silica, phosphate, nitrate and ammonium show an erratic behaviour during the tidal cycle. Tidal dynamics, precipitation events and nitrification inside the creek were identified as major control factors and an estimate of tidal exchanges indicate that the system is in an equilibrium state.

Natural isotopic composition of dissolved inorganic nitrogen in the Chesapeake Bay

Abstract: The natural abundances of 15N in the dissolved inorganic pools of nitrogen in the Chesapeake Bay were measured in the spring and fall of 1984. Changes in the δ15N of NH4+ and the combined pool of (NO3− + NO2−) reflected both seasonal and short-term changes in the estuarine nitrogen cycle. In the spring, oxidation of NH4+ at the head of the bay in the region of the turbidity maximum and in localized regions throughout the bay, led to elevated values of δ15N in the NH4+ pool. The δ15N of the (NO3− + NO2−) pool tended to increase toward the south, enabling an estimate of the isotopic fractionation factor for the consumption of NO3− to be derived; the estimate (1·0070), is similar to literature values of the fractionation factor for NO3− uptake by phytoplankton, supporting previous research suggesting that phytoplankton uptake is the major sink for NO3− in the bay. Denitrification led to elevated values of δ15N in the (NO3− + NO2−) pool in deep water. Over the course of the summer, the δ15N of NH4+ increased throughout the bay. A significant correlation was found between the δ15N of the NH4+ pool and the concentration of NO2− both above and below the pycnocline during the fall cruise, suggesting that the increase in the δ15N of the NH4+ pool was due to the oxidation of NH4+. In the fall, changes were also observed in the δ15N of both the NH4+ and (NO3− + NO2−) pools which were consistent with the occurrence of NH4+ oxidation. From these changes, a fractionation factor for NH4+ oxidation between 1·0120 and 1·0167 was derived, which is similar to values reported in the literature.

Subaerial exposure and changes in the stability of intertidal estuarine sediments

Abstract: The surface cohesive strength of intertidal sediments at six sites on two estuaries throughout the period of a single diurnal subaerial exposure was investigated. Stations at different tidal heights were examined at each site. Hourly measurements were made using a cohesive strength meter which provides an ordinal measure of sediment stability. The sediment was most easily eroded immediately after the emersion of the mudflat and there was little difference between the initial stability of the mud at different sites or stations. As the subaerial exposure continued the sediment of mid-shore and high-shore stations began to increase in stability. No significant change was found at low-shore stations before the return of the tide. The surface density of diatom populations was also intermittently measured at each site. The greatest increase in sediment stability occurred at high-shore stations and this was most extreme where there were also dense populations of epipelic diatoms present at the surface of the sediment. This supports laboratory evidence that the extracellular products of diatom populations of sufficient density increase the stability of intertidal sediments.

Estuary/Ocean exchange controlled by spring-neap tidal mixing

Abstract: This paper documents the role played by tidal mixing and wind forcing in modulating the export of freshwater from an estuarine system. Nineteen years of daily salinity data collected at lighthouses on the shores of the Strait of Georgia and Juan de Fuca Strait have been analysed and related to local wind, runoff and the strong tidal currents. Seaward freshwater export is found to be greatest at neap tides, at which time it may be considerably enhanced by favourable winds. It thus appears that the gravitational circulation of the estuary system is controlled by vertical mixing due to the tidal currents. The origin and impact of the baroclinic monthly and fortnightly ‘density tides” generated in the process are discussed. It is found that the combined influence of the wind and tidal currents may be characterized by a parameter resembling a Froude number, providing some insight into the dynamics of the mixing processes at work in the system.

Recruitment, growth and residence time of fishes in a tropical Australian mangrove system

Abstract: Twenty fish species accounted for > 96% of the catch by numbers in mangrove habitats in Alligator Creek, in tropical Queensland, Australia. The timing of recruitment, residency status, the period of residence and growth of fish during the time they spent in the mangrove habitat was assessed by examining gonad maturity and following changes in size-frequency plots for each species over 13 months. Five species were permanent residents, completing their life-cycles in mangrove swamps; eight were ‘long-term’ temporary residents, being present for not, vert, similar 1 year as juveniles before moving to other near-shore habitats; and seven were ‘short-term’ residents or sporadic users of the mangrove habitat. Amongst the latter group, four species lived in the mangrove habitat for between 1 and 4 consecutive months, while three engraulid species appeared to move rapidly, and often, between mangrove and other near-shore habitats. One of the resident species spawned and recruited throughout the year, but recruitment for most species was highly seasonal, being concentrated in the late dry season (October) to mid wet season (February) period. Temporary resident species dominated the fish community in the wet season (December–April), but resident species comprised more than 90% of total fish numbers in the mid dry season (August) after temporary residents left the mangroves in the early dry season. Several species had more than one peak of recruitment during the wet season. The cohort of 0 + agedLeiognathus equulus which recruited in December grew more rapidly and remained in the mangroves for a shorter period than the cohort which recruited later in the wet season (February). Only nine of the 20 species examined are strictly dependent on mangrove-lined estuaries, the remaining 11 are captured in significant numbers in other near-shore habitats. Only four of the 20 species are of direct commercial importance in Australia, but most are major prey for several valuable, commercial species harvested both within mangrove habitats and in adjacent shallow shelf habitats.

Physical characteristics of the Bahia Blanca estuary (Argentina)

Abstract: Based on temperature, salinity and current velocity and direction data, the physical characteristics of the Bahia Blanca estuary are described. Data were gathered in vertical profiles made in longitudinal as well as in hourly surveys. Freshwater runoff averages 2 m3 s−1; however, peak floods may reach 10–50 m3 s−1. The temperature distribution is quite homogeneous in the estuary. Based on the salinity distribution, the estuary can be divided into two sectors: an inner one showing partially mixed characteristics with a strong tendency to become sectionally homogeneous during runoff conditions similar to the historical averages, and an outer sector which is sectionally homogeneous. Salinity values in the inner sector may be larger than those observed in the inner continental shelf. This is initiated by the restricted circulation in the inner estuary and added to by the tidal washing of back-estuary salt flats and by evaporation processes. Analysis of the residual circulation shows a marked difference in the direction of mass transport. In the deeper regions of the sections (northern flank) the flow reverses with depth, being headward near the bottom. However, net transport is landward in the shallower parts.

A persistent upwelling off the Central Cantabrian Coast (Bay of Biscay)

Abstract: North Atlantic Central Waters were detected near the surface in some locations off the Central Cantabrian Coast (Bay of Biscay). This feature suggests an upwelling process taking place in the season of the thermal stratification which is persistent throughout the summer. The exact mechanism of formation is unknown but the wind driven currents are probably the main force. The prevailing winds off the Cantabrian coast are northeast during summer, generating westward superficial currents that produce an Eckman transport offshore. Factors such as shape of the coast and slope topography maybe modulators. Although the upwelling was subsuperficial, our results suggest that cold and nutrient rich waters reached the surface previously. The biomass distribution of phytoplankton varied according to the upwelling and different populations could be recognized. In stations not affected by upwelling phytoplankton occured mainly on top of the thermocline, whereas in those affected by upwelled waters it was concentrated near the surface. The mean size and composition of the populations were also different, and two distinct types could be recognized and related to nutrient concentrations.

Spatial autocorrelation analysis of meiofaunal and microalgal populations on an intertidal sandflat: Scale linkage between consumers and resources

Abstract: Spatial autocorrelation analysis procedures were used to characterize the spatial pattern of meiofaunal and microalgal populations on an intertidal sandflat located in Barnstable Harbor, Massachusetts. Simple correlation analyses between the abundance of meiofaunal groups and microalgal biomass did not reveal significant correlations. However, the results of autocorrelation analyses suggested that total meiofauna, nematodes, copepods, ostracods, Chl α, and pheopigments exhibit significant spatial autocorrelation. Further analysis, using correlograms, showed that these groups share nearly identicial spatial patterns and have similar patch sizes, suggesting a common spatial linkage between these grazers and microalgal resources.

Dynamics, flushing and trapping in Hinchinbrook channel, a giant mangrove swamp, Australia

Abstract: Hinchinbrook Channel is a 44-km long tidal channel connected at its northern and southern ends to the continental shelf of the Great Barrier Reef. The channel drains 164 km 2 of mangrove swamps. It also drains the Herbert River. The dynamics within the channel and the flushing of the fringing mangrove swamps were studied both in the field and using numerical models. The dominant currents are tidal semi-diurnal and are due to two nearly identical tidal waves entering the channel at its norhtern and southern openings and meeting in the middle. The mangrove swamps contribute significantly to the tidal dynamics. When the Herbert River discharge is small, net residual currents are negligible even in the presence of wind and the residence time of water is about 2 months. Flushing of the fringing mangrove swamps is largely controlled by the trapping effect in the mangrove swamps, and is slow with an e -fold residence time of about 54 days. On the northern end, the mangrove-fringed channel is wide and shallow, and a barotropic, coastal boundary layer is formed, due to shallow water effects and trapping in mangrove swamps. This results in a long-term (2 weeks) trapping of water along the mangrove-fringed coast in calm weather. Thus, coastal waters are continuously exchanged and mixed with mangrove swamp waters, but mixing between coastal and offshore waters is much slower.

A time-dependent, three-dimensional model of the Delaware Bay and River system. Part 1: Description of the model and tidal analysis

Abstract: A three-dimensional, time-dependent numerical model is used to simulate the dynamics and thermodynamics of Delaware Bay, River and adjacent continental shelf. This study describes the first attempt to model an estuary and the contiguous shelf as a coupled hydrodynamic and thermodynamic system. Here, in Part 1, a description of the model, boundary conditions and forcing information is provided. Numerical results are compared with surface elevation data at several locations throughout the Bay and the River, as well as with the observations collected by the National Ocean Service during their 1984–1985 circulatory study. It is shown that a vertically-integrated, two-dimensional version of the model predicts realistic amplitudes but with some phase error. The full three-dimensional model reduces the phase error but underpredicts the tidal range; this is due to the higher values of horizontal viscosity required by the three-dimensional model. The model accounts for non-linear, shallow-water effects and reproduces the observed amplification of the high-frequency tidal components from the mouth of the Bay to the head of the River at Trenton.

Seasonal abundances of planktonic ciliates and microflagellates in mesohaline Chesapeake Bay waters

Abstract: Ciliate, heterotrophic microflagellate (hflag) and autotrophic microflagellate (aflag) abundances are reported for mesohaline Chesapeake Bay waters based on samples gathered from April through October 1985–1987. Total water column averages for ciliate and microflagellate abundances were typical of eutrophic marine systems. Ciliate density ranged from 17·2 cells ml−1 in April to 1·8 cells ml−1 in September; hflag ranged from 3·7 × 103 cells ml−1 in June to 1·1 × 103 cells ml−1 in October. In spring the majority of ciliate and hflag standing stocks (70% and 64%, respectively) were located in bottom and transition waters; during summer months the majority (approximately 85% of both groups) were in surface and transition waters. During fall, ciliate stock was concentrated (72%) in surface waters and hflag were relatively evenly distributed in the three water column zones. Ciliate and microflagellate numbers were not directly related to chlorphyll α concentration except in the bottom layer, where simultaneous declines accompanied anoxia. Ciliate concentrations correlated with total numbers of microflagellates and hflag abundance, but not aflag density. We discuss the relative importance of predation and food availability in regulating ciliate and hflag concentrations in mesohaline Chesapeake Bay waters.

Mixing, trapping and outwelling in the Klong Ngao mangrove swamp, Thailand

Abstract: The Klong Ngao estuary in Thailand is a 7-5-km long tidal creek facing the Andaman Sea and drains 11.5 km 2 of mangrove swamps. Physical processes in the estuary differ greatly from the wet season to the dry season. In the dry season, vertical homogeneity prevails and the swamp behaves like an evaporation pond. Salt and water are trapped upstream, longitudinal gradients result and, through tidal dispersion, nutrient outwelling may result for SiO 2, possibly NO 2 and NO3, but not PO 4. The outflow is trapped in a coastal boundary layer. In the wet season, short-lived local floods generate a strong stratification in salinity and episodical flushing of the estuary and may make measurements of nutrient budgets inconclusive. The Klong Ngao mangrove swamp traps land-derived sediments in the wet season.

Phytoplankton seed populations of the cape peninsula upwelling plume, with particular reference to resting spores of Chaetoceros (bacillariophyceae) and their role in seeding upwelling waters

Abstract: Phytoplankton seed populations of newly upwelled water collected off the Cape Peninsula in the southern Benguela upwelling system were examined. The similar composition of the phytoplankton along the entire base of the plume implies a common source of water during each individual upwelling event. Greater variability in the composition and abundance of phytoplankton of different upwelling events was attributed to varying offshore phytoplankton developmental processes, the success of the seeding strategies of various species and variation in circulation patterns. Such variability was found to be instrumental in determining the species composition and the time required for bloom development. The prevalence of intact diatom resting spores, especially of the genus Chaetoceros, in newly upwelled waters demonstrated the ability of spores to utilize the mixing and current patterns in order to seed upwelling systems. Higher sinking rates would appear to be the key advantage to spore formation, rapidly removing the population from an inhospitable environment following nutrient depletion, thus preventing offshore advection and maintaining the population close to the centre of upwelling. The germination of diatom resting spores following incubation of newly upwelled water provided convincing evidence of the initiation of diatom blooms by resting spores in upwelling systems. These resting stages are therefore of selective advantage in seeding upwelling systems, increasing the chances of a species being represented when conditions are favourable for growth. Thus spore-forming diatom species have been found to be among the most abundant and persistent phytoplankton species in the southern Benguela.

Use of a mangrove estuary as a nursery area by postlarval and juvenile banana prawns,Penaeus merguiensis de Man, in Northern Australia

Abstract: In the Embley River, Gulf of Carpentaria, Australia, the largest catches of the commercially important banana prawns, Penaeus merguiensis , were made on mangrove-lined, steeply sloping mud banks. The upstream limit of distribution of P. merguiensis was found to coincide with the distribution of broad bands of fringing mangrove forests but, except in the wet season, was not related to salinity levels. Although some postlarval P. merguiensis settled on all habitat types in the estuary, large catches were only taken on the mangrove-lined banks. Catches of both postlarvae and juveniles in the upstream reaches of a small creek were almost five times higher than those in the river near the creek mouth. Moreover, prawns in the 2 to 4 mm carapace length (CL) size class were poorly represented in the river but were abundant in catches in the small creek. This suggests that either the survival rate of postlarvae is highest in the upper reaches of the small creeks, or that the small prawns are migrating from the main river into the creek. As prawns increase in size above 5 mm CL it appears that they take part in daily tidal migrations from small creeks to the river and begin a gradual migration from the creeks to the river.

Nitrogenous nutrient transformations in the spring and fall in the Chesapeake Bay

Abstract: Measurements of the standing stock of phytoplankton, bacterioplankton and particulate carbon and nitrogen, and dissolved pools of inorganic nitrogen were made at a series of stations located in the main channel of the Chesapeake Bay during cruises in the spring and fall of 1984. On each cruise, we conducted two transects of the long axis (N-S) of the bay; comparison of these transects revealed that the horizontal and vertical distribution of O2, chlorophyll a, and dissolved inorganic nitrogen can change dramatically on a time scale of days. Experiments were carried out with 15N-labelled substrates (NO2−, NO3−, and NH4+) at a subset of the stations. These experiments were designed to measure the fluxes of nitrogen between the dissolved pools as well as the flux into particles. During the spring cruise, a surprisingly high rate of uptake of NO2− by particles was found. In the fall, uptake by particles followed previously reported patterns more closely. The flux of nitrogen between dissolved inorganic pools was often greater than the flux into particles. The rates of individual reactions varied spatially, and significant activity throughout the water column was often found. The occurrence of typically anaerobic reactions in well-oxygenated waters suggests that anaerobic microzones may be an important site for nitrogen transformations in the bay. Our results also indicate that the rates of a number of microbially-mediated processes were greatly enhanced by storm-induced mixing of the water column during our fall cruise. Such transient enhancements may have a large, and usually undocumented, effect on the nitrogen cycle.

Effect of mangrove detrital outwelling on nutrient regeneration and oxygen fluxes in coastal sediments of the central Great Barrier Reef lagoon

Abstract: The effect of tidal outwelling of mangrove detritus on sediment nutrient chemistry, nutrient regeneration and oxygen fluxes in a coastal area of the central Great Barrier Reef lagoon was examined. Organic carbon and total nitrogen concentrations ranged from 0·2 to 3·9% and 0·01 to 0·18% by sediment dry weight (DW), respectively, and were highest at stations receiving the greatest quantities of mangrove litter. Total phosphorus concentrations ranged from 0·013 to 0·048% by DW, but did not relate to outwelling. C:N:P. ratios ranged from 29:6:1 at the site receiving the least amount of detritus to a high of 397:17:1 at the station receiving the most litter. The effect of outwelling on dissolved inorganic nutrients in porewaters was less clear, with concentrations closely related to sediment type and obscured by contributions from continental runoff. Ammonium (range: 10–310 μM) and silicate (range: 10–315 μM) were in greatest concentration at most sites followed by phosphate (range: 1–12 μM) and nitrite and nitrate (0·07–6·0 μM). Directly measured fluxes of dissolved inorganic nutrients across the sediment-water interface were either undetectable or small [ΣN = −560 to +3503; PO43 = undetectable to +157; Si(OH)4 = undetectable to +6449 μmol m−2 day−1] and not related to outwelling. Addition of mangrove litter to ☐core samples incubated for 10 days in the laboratory did not affect rates of dissolved inorganic nutrient release. Oxygen consumption rates ranged from 8·7 to 60·2 mmol O2 m−2 day−1 and were highest at most sites closest to mangrove forests, in agreement with previous measurements of bacterial productivity. Rates of net community primary production were either undetectable or low (as were chlorophyll α and phaeopigments) ranging from 12 to 77 mg C m−2 day−1. It appears that much of the deposited mangrove litter is highly refractory, but the amount of sedimented material appears to be great enough to significantly enrich bulk concentrations of particulate carbon and nitrogen, and rates of oxygen consumption. Benthic nutrient regeneration contributes only a small proportion to the daily nitrogen (6%) and phosphorus (9%) demand of coastal phytoplankton, implying supplementation from continental runoff and/or high rates of pelagic recycling.

Seagrass wrack-induced dune formation on a tropical coast (Banc d'Arguin, Mauritania)

Abstract: The Banc d'Arguin (Mauritania) is a shallow water area with mudflats bordering on the Sahara desert. Mass depositions of seagrass litter were observed on some parts of the shore. Soil profiles of the low dune formations separating the flood debris zone from the desert showed that layers of seagrass detritus, in various stages of decay, extended under the dune formations. The tentative mechanism of dune formation, involving the interacting processes of beaching of seagrass litter and wind-blown transport of desert sediment in a seaward direction, is described. This role in dune formation adds a new element to the significance of seagrass vegetation for the coastal environment.

Metal concentrations in tissues of Spartina alterniflora (Loisel.) and sediments of Georgia salt marshes

Abstract: The concentrations of eleven metals (Al, Cd, Cr, Cu, Fe, Hg, Mn, Mo, Ni, V and Zn) were determined in salt marsh sediments from seven locations in two industrial/port cities and one relatively unimpacted region of the Georgia coast. In addition, six of these elements (Al, Cu, Fe, Hg, Mn and Zn) were measured in the above- and below-ground tissues of the salt marsh plants Spartina alterniflora from the same locations and in Spartina cynosuroides at one site. The sedimentary metal concentrations of Cr, Cu, Hg, V and Zn were higher in the industrial/port sites by less than a factor of ten relative to the other areas, and the remaining elements had similar sedimentary concentrations at all locations. Tissue concentrations of elements in S. alterniflora varied little between sites. Elemental ratios and concentration factor calculations for plant tissues indicated that Al and Fe were not actively taken up, but that internal concentrations of Cu and Hg appeared to be controlled by the plants.

Qualitative and numerical analyses of the effects of river inflow variations on mixing diagrams in estuaries

Abstract: The effects of river inflow variations on alkalinity/salinity distributions in San Francisco Bay and nitrate/salinity distributions in Delaware Bay are described. One-dimensional, advective-dispersion equations for salinity and the dissolved constituents are solved numerically and are used to simulate mixing in the estuaries. These simulations account for time-varying river inflow, variations in estuarine cross-sectional area, and longitudinally varying dispersion coefficients. The model simulates field observations better than models that use constant hydrodynamic coefficients and uniform estuarine geometry. Furthermore, field observations and model simulations are consistent with theoretical ‘predictions’ that the curvature of propery-salinity distributions depends on the relation between the estuarine residence time and the period of river concentration variation.

The relationship between nitrogen fixation and tidal exports of nitrogen in a tropical mangrove system

Abstract: Various components (sediments, algal mats, decomposing logs and algal-covered prop roots) of a tropical mangrove forest showed low to moderate nitrogen fixation (acetylene reduction) rates. Measurements carried out in March, May and October 1989 revealed negligible or inconsistent seasonal variation in activity for all components. Prop roots with their associated algae showed much greater activity during night-time, typical of many substrates with associated cyanobacteria, whereas all other components had constant activity over 24 h periods. Extrapolation of the nitrogen fixation rates for each component to a ‘whole-forest’ basis, using field estimates of the densities of each component, indicated that sediments, prop roots and decomposing logs contributed approximately 3·5, 1·6 and 1·0%, respectively, of the nitrogen requirements for forest net primary production (FPPN). Blue-green algal mats on bare saltpan areas showed no significant nitrogen-fixing activity (in excess of that for the bare sediments in the same area). The total nitrogen contribution of the various components (6% of FPPN) closely matches previous esimates of the net nitrogen loss from the system through tidal fluxes of particulate and dissolved materials. The possible relative importance of other nitrogen input and loss mechanisms for this mangrove system is also discussed.

Mapping growth and mortality rates of crevice-dwelling organisms onto a perforated surface: The relevance of ‘cover’ to the carrying capacity of natural and artificial habitats

Abstract: A theoretical basis is suggested for examining habitat limitations of artificial surfaces for population enhancement of crevice-dwelling, or territorial organisms such as reef fish or lobsters, that takes into account known growth and mortality rates. This approach is compared with the crevice frequency at size in a ‘natural’ substrate, which, it is assumed, corresponds to fractal expectation. With naturally dissected surfaces, fractal expectation predicts that crevice availability declines with size, so that a ‘bottleneck’, limits the population of larger-sized individuals, but that natural mortality, or rate of loss of individuals displaced from crevices, will also decline with age. The paper considers a surface perforated with circular niches of a limited number of discrete radii, distributed at random over a surface, without overlap, and considers what is the size frequency of hole radii needed to complete the life history of an infaunal organism with a given mortality and growth schedule, and a constant occupancy rate. Determining a priori crevice frequency at size before constructing an artificial surface, and for a given packing density, distributing the crevices over it in two dimensions, is referred to as ‘mapping’ the growth and mortality rates onto the surface. Sample calculations are described either for continuous recruitment or where recruitment occurs as a ‘pulse’ during a specific season, and subsequent cohort growth obeys a common scheduling. It is shown that fractal surfaces limit the production of larger individuals unless migration intervenes, but are more suitable for recruitment enhancement. Using units with a limited size range of perforations is biologically inefficient, and depends on natural recruitment elsewhere for stock replenishment by migration. These calculations are relevant to the design of artificial surfaces, throw light on mechanisms limiting productivity of dissected surfaces, and point to effects of individual niche size on abundance at size of niche-limited organisms. The existence of nursery areas which can support large numbers of small individuals but few adults, and migratory stages in the life history of crevice-dwelling organisms, could be functions of the fractal nature of natural surfaces. An experimental field approach (a ‘niche sampler’) measuring occupancy at size of existing habitats is proposed that could be used prior to designing and installing artificial reefs in a given area to determine the optimal hole frequency at size.

Influence of stratification and irradiance regime on summer phytoplankton composition in coastal and shelf seas of the British Isles

Abstract: The effect of interaction between stratification and irradiance regimes on phytoplankton community structure was investigated in three shelf/coastal regions of the British Isles, each of which displayed ranges of vertical stability and light attenuation. Relationships between vertical stability, light penetration and community structure were indicated by the ordination of dominance (assessed using cell volume) of the main phytoplankton phyletic groups—diatoms, dinoflagellates and microflagellates—on a surface defined by a bulk stratification index (ϕ) and water column depth, scaled by transparency (λ h ). Diatom- and dinoflagellate-dominated communities occupied distinctly different domains on the ϕ - λ h surface, diatoms being favoured in well-mixed water columns with high values for λ h and dinoflagellates dominating where stratification was strong and λ h was low. Microflagellates were not abundant in any of the study areas and showed no clear ordination on the ϕ - λ h surface. The domain of co-dominance of diatom and dinoflagellates on the ϕ - λ h surface was narrow with small changes in the irradiance or stratification regime resulting in a switch to diatom or dinoflagellate dominance. It is suggested that loss of non-motile diatom communities in strongly stratified water columns might be a strong selective force in favour of dinoflagel-lates. However, in water columns with intermediate stratification and optical properties, the outcome of competition may be decided by physiological attributes of the two groups with respect to growth in low and fluctuating irradiances.

Benthic microalgal biomass in sediments of Onslow Bay, North Carolina

Abstract: Sediment samples were collected at stations along cross-shelf transects in Onslow Bay, North Carolina, during two cruises in 1984 and 1985. Station depths ranged from 11 to 285 m. Sediment chlorophyll a concentrations ranged from 0·06 to 1·87 μg g −1 sediment (mean, 0·55), or 2·6–62·0 mg m 2 . Areal sediment chlorophyll a exceeded water column chlorophyll a a at 16 of 17 stations, especially at inshore and mid-shelf stations. Sediment ATP concentrations ranged from 0 to 0·67 μg g −1 sediment (mean, 0·28). Values for both biomass indicators were lowest in the depth range including the shelf break (50–99 m). Organic carbon contents of the sediments were uniformly low across the shelf, averaging 0·159% by weight. Photography of the sediments revealed extensive patches of microalgae on the sediment surface. Our data suggest that viable benthic microalgae occur across the North Carolina continental shelf. The distribution of benthic macroflora on the North Carolina shelf indicates that sufficient light and nutrients are available to support primary production out to the shelf break. Frequent storm-induced perturbations do not favour settling of phytoplankton, an alternative explanation for the presence of microalgal pigments in the sediments. Therefore, we propose that a distinct, productive benthic microflora exists across the North Carolina continental shelf.

Nature of POC transport in a mangrove ecosystem: A carbon stable isotopic study

Abstract: The isotopic composition of particulate organic carbon (POC) was studied during five tidal cycles in a mangrove creek of Sepetiba Bay, Rio de Janeiro, Brazil. The results show that a mixture of organic carbon from mangrove and marine origins is always present in the creek. Mean mangrove contribution to the POC varied from 16% to 100% and was dependent on tidal amplitude. The results suggest that oceanic carbon can be an important component of carbon balance in mangrove ecosystems. Therefore, earlier carbon balance studies from mangroves which did not include measurements of carbon isotopic composition should be interpreted with care.