Large Estuarine System Research Articles

Size-differential Effects of Vertical Stability on the Biomass and Production of Phytoplankton in a Large Estuarine System

Size-differential effects of vertical stability on the biomass and production of phytoplankton were investigated in the Lower St. Lawrence Estuary and the Gaspé Current, Canada. In summer, when nitrate concentrations were low and the ratio of critical depth to surface mixed layer depth was high, the biomass (chlorophylla) and production were dominated by phytoplankton <5μm. Phytoplankton >5μm were found only where low vertical stability allowed nitrate replenishment in the euphotic zone. In autumn, the ratio of critical depth to mixed layer depth was relatively low (≈1·0). Nitrate concentrations in the euphotic zone were high throughout the system because of the combined effects of low dynamic stability in the Lower Estuary, which resulted in nutrient enrichment of the surface layer, and horizontal advection. Phytoplankton <5μm exhibited low standing stock and little variability relative to phytoplankton >5μm, and appeared to be related to temperature alone. Phytoplankton >5μm accounted for departures from background levels of production and biomass, which were tightly coupled to hydrodynamics. A positive, size-dependent relationship between phytoplankton biomass and the Brunt-Väisälä frequency was found, suggesting that variability in the large size fractions was explained primarily by their residence time in the euphotic zone. It follows that, in summer and autumn, the production of large phytoplankton was primarily governed by variations in the vertical stability of the water column associated with horizontal advection, through their effects on nutrient replenishment and the residence time of diatoms in the euphotic zone.

Transgressive systems tract development and incised-valley fills within a Quaternary estuary-shelf system: Virginia inner shelf, USA

High-frequency Quaternary glacioeustasy resulted in the incision of six moderate-to high-relief fluvial erosion surfaces beneath the Virginia inner shelf and coastal zone along the updip edges of the Atlantic continental margin. Fluvial valleys up to 5 km wide, with up to 37 m of relief and thalweg depths of up to 72 m below modern mean sea level, cut through underlying Pleistocene and Mio-Pliocene strata in response to drops in baselevel on the order of 100 m. Fluvially incised valleys were significantly modified during subsequent marine transgressions as fluvial drainage basins evolved into estuarine embayments (ancestral generations of the Chesapeake Bay). Complex incised-valley fill successions are bounded by, or contain, up to four stacked erosional surfaces (basal fluvial erosion surface, bay ravinement, tidal ravinement, and ebb-flood channel-base diastem) in vertical succession. These surfaces, combined with the transgressive oceanic ravinement that generally caps incised-valley fills, control the lateral and vertical development of intervening seismic facies (depositional systems). Transgressive stratigraphy characterizes the Quaternary section beneath the Virginia inner shelf where six depositional sequences (Sequences I–VI) are identified. Depositional sequences consist primarily of estuarine depositional systems (subjacent to the transgressive oceanic ravinement) and shoreface-shelf depositional systems; highstand systems tract coastal systems are thinly developed. The Quaternary section can be broadly subdivided into two parts. The upper part contains sequences consisting predominantly of inner shelf facies, whereas sequences in the lower part of the section consist predominantly of estuarine facies. Three styles of sequence preservation are identified. Style 1, represented by Sequences VI and V, is characterized by large estuarine systems (ancestral generations of the Chesapeake Bay) that are up to 40 m thick, have hemicylindrical wedge geometries, and occur within large, coast-oblique trending depressions (paleo-estuaries). Style 1 is dominated by fluvial through estuary-mouth depositional systems (Seismic Facies 1–4). Style 2 sequence preservation, represented by Sequences III and II, is dominantly an inner shelf and shoreface succession with a seaward-thickening tabular wedge geometry that does not exceed 15 m in thickness. These shoreface and inner shelf depositional systems of the upper transgressive systems tract (Seismic Facies 9) and highstand systems tract (Seismic Facies 7 and 11) are not associated with paleo-estuaries. Style 3 sequence preservation is represented by Sequence I, the Holocene Sequence. It consists of lower transgressive systems tract fluvial-estuarine, lagoonal, and tidal-inlet fill deposits (Seismic Facies 1–6, and 8) overlain by upper transgressive systems tract shelf and shoreface sands (Seismic Facies 9). Style 3 has a crenulated wedge geometry, and is thickest beneath and seaward of the modern Chesapeake Bay mouth. It thins northward and landward onto Late Pleistocene interfluvial highs on the basinward side of the southern Delmarva Peninsula.

The management of European estuaries: A comparison of the features, controls and management framework of the Tagus (Portugal) and Humber (England)

The paper describes the local, national, European and wider-area framework, statutes, and formal and voluntary mechanisms for managing European estuaries. These aspects are discussed in relation to two large and representative estuarine systems, the Tagus, Portugal, and the Humber, on the English North Sea coast As estuaries are sites of many activities and uses, most of which are encouraged or at least condoned, management has the role of preventing and resolving conflicts between those uses and users. Accepted uses of estuaries include the discharge and dumping of waste materials, fin and shell-fisheries, conservation, land reclamation, natural usage, abstraction by industry, and recreation. Estuarine management is now being carried out within the constraints of local and regional government planning, planning and activities of water pollution control bodies, fisheries control bodies, and navigation and port authorities The Tagus and Humber estuaries support all of the above activities and uses, and have controls within a European legislative framework but have differing histories of management and planning in order to resolve conflicts. In addition the Humber is subject to controls placed on North Sea areas. The paper discusses the relevant national and European legislation (Directives) and accepted practices for management. Furthermore, the paper discusses the formulation and practice of estuarine management plans as used by various bodies (nature conservation, water quality and regional authority). It is of particular note that the lessons from these two estuaries are relevant to many other European estuaries.

Why be choosy? Temporal changes in larval sensitivity to several naturally-occurring metamorphic inducers in the opisthobranch Haminaea callidegenita

In the opisthobranch Haminaea callidegenita Gibson & Chia, time of metamorphic competence varies among sibling larvae over a 2-wk period. Intra-clutch variation in onset of competence occurs in constant presence of inducer and is independent of larval feeding, as these are lecithotrophic larvae. All larvae become competent and metamorphose successfully. This suggests that intra-clutch variability in time of competence may be adaptive in providing flexibility to the larval period, rather than reflecting the slow development of a few “inferior” larvae. Competence occurs throughout both the encapsulated and free-swimming larval phases. As a result, both dispersive larvae and nondispersive juveniles are simultaneously released at hatching, an unusual mode of development termed poecilogony. Hatched veligers are characterized by (1) an initially high rate of metamorphic competence which rapidly decreases with increasing time hatched, (2) a decrease in larval “choosiness”, or an increase in sensitivity to various substrata, and (3) the ability for “spontaneous” metamorphosis occurring in absence of known cue. Competent larvae initially metamorphose rapidly in response to a naturally-occurring inducer found in egg mass jelly and to excess potassium, then gradually become sensitive to certainother substrata [the seagrass Zostera marina L., and the filamentous green alga Chaetamorpha linum (O.F. Müller)]. Older larvae are not simply becoming indiscriminate in responding to these other substrata, as evidenced by their lack of response to biofilmed sediment. This pattern of increasing sensitivity may result in short-range dispersal of most hatched veligers (possibly within or near the large estuarine system inhabited by parents) and longer-range dispersal of a few siblings, thus maintaining the potential for mixing within and exchange among populations.

HABITAT USE PATTERNS AND RANGES OF THE BOTTLENOSE DOLPHIN IN THE GULF OF CALIFORNIA, MEXICO

Abstract: I studied behavior and range patterns of individual bottlenose dolphins during 1984 in the mid‐eastern Gulf of California, Mexico. Dolphin sighting rate was significantly higher in areas close to estuary mouths, 0.306 sightings per hour compared with 0.155 sightings per hour in areas distant from estuary mouths. Dolphins used these estuarine areas to feed; 61% of all behavior observed near estuaries was feeding as compared with 23% elsewhere. Traveling comprised 61% of all behavior observed in areas distant from estuary mouths. Estuaries are sites of large concentrations of nutrients which support great numbers of filter‐feeding zooplankton and fish. Bottlenose dolphins may specialize on esmarine prey, or they may feed in estuarine areas simply because of the abundance of potential prey that these systems support. In either case, data on relative numbers, distribution patterns, behavior and diet indicate that this is a general trend in habitat use for many coastal populations of this species in the Pacific and Atlantic.Ranges of a few individuals spanned a minimum of 65 km of coastline, and animals were not permanent residents of a monitored bay. In contrast, dolphins off the coast of Sarasota, Florida, have been reported to be year‐round residents with smaller ranges. This difference in degree of site fidelity may be related to habitat differences. The west coast of Florida is dotted with numerous and large estuarine systems which may host permanent prey populations and support resident groups of dolphins. The Gulf of California coastline contains few estuaries; most are small and perhaps support prey resources which are ephemeral, requiring dolphins to range over larger distances in search of food.

Stratigraphy and Holocene Evolution of Mobile Bay in Southwestern Alabama

ABSTRACT Mobile Bay is a large (1058 km2) estuarine system in southwestern Alabama. On the basis of borings, vibracores, radiocarbon dating, and high resolution seismic lines, Holocene inundation of the bay has been reconstructed. A paleotopographic map delineates an entrenched river valley that occupied the present-day bay area during the last Pleistocene lowstand. Vertical stacking of Holocene facies seen in vibracores and boring logs records the evolution of the bay during postglacial sea-level rise and the resumption of deposition above the Late Pleistocene exposure surface. Two types of vertical sequences are present in Mobile Bay sediments. The first type is a fining-upward sequence that formed as beach, marsh, and near-shore sediments were covered by open-bay muds. Local progradation of the bay-head delta and the Dauphin Island-Morgan Peninsula barrier complex has produced the second type, a fining then coarsening-upward sequence. A disconformable contact with the pre-Holocene sediments is recognizable on seismic lines by erosional truncation and in cores by coloration, root mottling, and radiocarbon ages greater than 17,500 years before present (Y. B. P.). Bay inundation commenced approximately 7,500 Y. B. P. and proceeded in two phases. The first phase, from 7,500 to 6,000 Y. B. P., was a time of rapid relative sea level rise in which 70% of the bay was inundated. Rapid submergence below normal wave base produced a low energy, open-bay setting in the central part of the bay. In this area, vertical sequences are characterized by thin near-shore and beach deposits ( 5 m). By 6,000 Y. B. P., the bay extended farther to the north and was slightly deeper than the present-day bay. The second phase, from 6,000 Y. B. P. to present, was a time of slow relative sea-level rise. Slow inundation resulted in more time for sediments to be reworked and to accumulate above normal wave base. Thus, vertical sequences from the slowly inundated bay margins contain thick sections of near-shore and beach facies (> 5 m) overlain by thin sections of open-bay mud (< 1 m). Both length and depth of the bay decreased as the bay-head delta prograded and the bay filled. Decreasing length and shallowing of the bay over this time resulted in increasing riverine dominance and a diminishing of the northward intrusion of high salinity Gulf of Mexico waters.

Reproductive biology and growth of the Western School Prown, Metapenaeus dalli, in a large Western Australian Estuary

During the summer, one-year-old western school prawns (Metapenaeus dalli) congregate and spawn in reduced-salinity (&lt; 10 g kg-1) regions of the Murray River, a tributary of the large Peel-Harvey estuarine system in Western Australia. Although some 1 + prawns are found into the following spring, few if any survive or remain in the system until the next spawning season. Growth is highly seasonal and females attain a greater carapace length than males by the end of the first year of life (18 v. 14 mm). Changes in the incidence of spermatophore deposition, gonadosomatic index (GSI) and ovarian stage during the spawning period are described. Although the timing of recruitment of O+ prawns could be related to the sharp peak in the incidence of spermatophore deposition and GSI in 1985-86, such relationships were not so clearly defined in 1987-88 when the incidence of spermatophore deposition remained above 9% for over twice the length of time.

The influence of turbidity on juvenile marine fishes in estuaries. part 1. field studies at Lake St. Lucia on the southeastern coast of Africa

Lake St. Lucia, the largest estuarine system in Africa (325 km 2), was chosen as the field study area for a 3.5-yr ((1980)–83) investigation into relationships between water turbidity and estuarine fish distribution. The variety of habitats, from clear water, open sandy shores to shallow muddy substrata and turbid waters, together with high species diversity (108 species) rendered the area suitable for this study. The relationships between fish distribution and environmental factors were monitored by monthly seine netting of fishes at seven sites representative of the range of conditions in St. Lucia. Simultaneously, water turbidity, salinity, and temperature were recorded. The possible influences of substratum type and food availability were also investigated by using recently published data on invertebrate benthos and Zooplankton distributions. Published data were also used to determine the diet of the common fish species. The results showed that the distribution of juveniles of the 20 commonest fish species were statistically correlated only with water turbidity, water temperature, and food availability. The correlation with temperature was related to seasonal not spatial temperature patterns. Turbidity and food type influences were difficult to separate but exceptions were the anchovy Thryssa vitrirostris (Gilchrist and Thompson) and the sole Solea bleekeri Boulenger which occurred only in turbid water despite the widespread occurrence of their prey, and Genes acinaces Bleeker, G. rappi (Barnard), and G. fllamentosus Cuvier, all of which occurred only in clear water although the greatest densities of their bivalve prey were in turbid waters. Similarly, the sparids Rhabdosargus holubi (Steindachner) and R. sarba (Forsskal) were distributed according to turbidity and not their preferred foods. Principal component analysis with a minimum spanning tree plot and a canonical correlation test showed that the fish fauna could be divided into five groups according to their occurrence in various turbidities. These were: clear water species (e.g. Gerreidae) in < 10 NTU, clear to partially turbid species (e.g. Liza dumerilii (Steindachner) and L. macrolepis (Smith)) in < 50 NTU, intermediate turbidity species (e.g. Valamugil cunnesius (Valenciennes) and Leiognathus equula (Forsskal)) in 10–80 NTU, turbid-water species (e.g. Elops machnata (Forsskal) and Thryssa vitrirostris) in > 50 NTU, and species indifferent to turbidity (e.g. Acanthopagrus berda (Forsskal) and Teraponjarbua (Forsskal). It is, therefore, suggested that turbidity plays a significant roˆle, either singly, or in combination with other variables in determining the distribution of juvenile marine fishes in estuaries.

Blue-green algae and fish population changes in a eutrophic estuary

Evidence is presented which indicates that dense blooms of the blue-green alga Nodularia spumigena have affected fish and crab populations in a large estuarine system in south-western Australia. For example, the numbers of fish were generally very low at sites in which the chlorophyll a level, an excellent indicator of Nodularia density during the late spring and summer, was above 100 μgl −1. Moreover, commercial fishermen have recorded greatly reduced catches in Nodularia-affected areas and dead fish and crabs were found in regions where Nodularia was very dense. While the effects of this blue-green alga apparently led to death in the case of some bottom-living species in the most affected parts of the system, more active species moved into regions where Nodularia was virtually absent.

An assessment of the angling fishery for yellowfin bream, Acanthopagrus australis (Günther), in Moreton Bay, Australia

Yellowfin bream, Acanthopagrus australis, support an important angling fishery in Moreton Bay, a large estuarine system on the east coast of Australia. An analysis of the records of catches by anglers indicates that abundance and mean size of yellowfin bream have changed little in Moreton Bay during the period 1945 to 1980. Over the past 5 years substantial increases have occurred in the total angling effort for yellowfin bream at surf bar spawning areas in Moreton Bay. This has resulted in a decrease in mean catch per unit effort but no apparent decrease in total yield. Reclamation and development projects which cause degradation of the surf bar spawning areas have the greatest potential to cause reduction in abundance of yellowfin bream in Moreton Bay.

Brent geese Branta bernicla bernicla and their food in the solent, Southern England

The Solent, a large estuarine system on the central south coast of England, is an important wintering ground for the dark-bellied brent goose Branta bernicla bernicla: peak winter counts there rose from 300 in 1951–1952 to 23 000 in 1979–1980. Until 1973–1974 the geese remained in the intertidal zone, where they fed on eelgrass Zostera and green algae Enteromorpha and Ulva lactuca. The increase in goose number coincided with the spread of these plants in parts of the Solent. The spread of algae has been linked with increased discharges of sewage. However, only part of the annual production of Zostera and algae is available to geese because of autumn and winter die-back, storm damage and other factors. From 1973 to 1974 geese fed on permanent pasture, playing fields and autumnsown cereals, the last being especially important in 1978–1979 and 1979–1980. The evidence suggests that in some estuaries terrestrial feeding is increasingly attractive even though the intertidal zone could still sustain most of the goose-days spent in the area. The habit appears to be stimulated by large numbers of first winter birds in the population. The geese have evoked great public interest and generated concern for estuarine conservation. However, terrestrial feeding conflicts with farming. Nature conservation organisations have encouraged a policy of scaring from valuable crops by proven methods, whilst seeking to provide alternative terrestrial refuges.

Movements and migrations of yellowfin bream, Acanthopagrus australis (Günther), in Moreton Bay, Queensland as determined by tag recoveries

Yellowfin bream were tagged and released in three non‐spawning areas and two spawning areas in Moreton Bay, a large estuarine system on the east coast of Australia. A total of 194 fish were recaptured. Juvenile fish made only small‐scale movements (less than 6 km) from the release site. Small‐scale movements were also recorded for adult fish, although large‐scale movements from 10 to 90 km were also recorded. The large‐scale movements appear to be associated with the migration of adult fish to or from surf bar spawning areas. Some adult fish remained in feeding areas during the spawning season (May‐August). There was no indication from tag recoveries of any movements of yellowfin bream outside Moreton Bay and the results indicate that adult yellowfin bream in Moreton Bay may be regarded as a unit stock for the purpose of fisheries management.