J. O. Blanton

Interactions between behaviour and physical forcing in the control of horizontal transport of decapod crustacean larvae

We summarize what is known of the biophysical interactions that control vertical migration and dispersal of decapod larvae, asking the following main questions: How common is vertical migration in decapod crustacean larvae? What is the vertical extent of the migrations? What are the behavioural mechanisms that control vertical migrations? How does vertical migration interact with the physics of the ocean to control the dispersal of larvae? These questions are analysed by first giving a synopsis of the physical processes that are believed to significantly affect horizontal transport, and then by describing migration patterns according to taxon, to ecological category based on the habitat of adults and larvae, and to stage within the larval series. Some kind of vertical migration has been found in larval stages of virtually all species that have been investigated, irrespective of taxonomic or ecological category. Most vertical migration schedules have a cyclic nature that is related to a major environmental cyclic factor. Tidal (ebb or flood) migration and daily (nocturnal and twilight) migration are the two types of cyclic migration that have been identified. In general, all species show some type of daily migration, with nocturnal migration being the most common, whereas tidal migrations have only been identified in species that use estuaries during part of their life cycle. Moreover, there are several examples indicating that the phasing and extent of migration both change throughout ontogeny. Reported ranges of vertical displacement vary between a few metres in estuaries and several tens of metres (sometimes more than 100 m) in shelf and oceanic waters. Vertical movements are controlled by behavioural responses to the main factors of the marine environment. The most important factors in this respect are light, pressure and gravity, but salinity, temperature, turbulence, current and other factors, also influence behaviour. Many of these factors change cyclically, and the larvae respond with cyclic behaviours. The type of response may be endogenous and regulated by an internal clock, as in the case of some tidally synchronised migrations, but in most cases it is a direct response to a change in an environmental variable, as in diel migration. The reaction of the larvae to exogenous cues depends both on the rate of change of the variable and on the absolute amount of change. A series of dispersal types, involving different spatial and temporal scales, have been identified in decapod larvae: retention of the larval series within estuaries; export from estuarine habitats, dispersal over the shelf, and reinvasion of estuaries by the last stage; hatching in shelf waters and immigration to estuaries by late larvae or postlarvae; complete development on the shelf; and hatching in shelf waters, long-range dispersal in the ocean, and return to the shelf by late stages. In all of these cases, vertical migration behaviour and changes of behaviour during the course of larval development have been related to particular physical processes, resulting in conceptual mechanisms that explain dispersal and recruitment. Most decapod larvae are capable of crossing the vertical temperature differences normally found across thermoclines in natural systems. This ability may have significant consequences for horizontal transport within shelf waters, because amplitude and phase differences of the tidal currents across the thermocline may be reflected in different trajectories of the migrating larvae.

The Relationship of Upwelling to Mussel Production in the Rias on the Western Coast of Spain

We have calculated an upwelling index for each month over a 17-year period (1969-1985) for a point off the western coast of Spain. We interpret April through September values of the index to indicate the flux of nitrate-rich water into the Spanish Rias. The index representing the 6-month upwelling series has been correlated with an index representing the conditions of mussels grown during that season on rafts in Ria de Arosa. Two seasons represent extreme upwelling conditions over the 17-year sampling period: 1977 when the upwelling index was the highest, and 1983 when it was the lowest. A comparison of the condition of mussels during these years showed that meat content was double in 1977. We suggest, by this study, that long range forecasts of synoptic scale weather patterns could be used to predict the potential nutritional value of mussels harvested in the rias of Spain.

The intrusion of Gulf Stream water across the continental shelf due to topographically-induced upwelling

Abstract Summer bottom temperatures along the continental shelf between Cape Hatteras and Cape Canaveral are abnormally low in regions where isobaths diverge. The regions are north of capes and shoals, which force the flow of shelf water to change vorticity and induce upwelling. Gulf Stream Water intrudes across the bottom during summer to replace the upwelled water, and accounts for the colder and more stratified water over the northern Florida and the North Carolina shelves.

Microbial distribution and abundance in response to physical and biological processes on the continental shelf of southeastern U.S.A.

The distribution and abundance of bacteria and phytoplankton on the continental shelf of the southeastern United States were observed in relation to physical processes. Phytoplankton production was influenced by inputs of water of reduced salinity from the estuaries and by inputs of high salinity, low-temperature water from the west front of the Gulf Stream. The distribution of chlorophyll suggests that in each case production is influenced both by inputs of nutrients and by the enhanced vertical stability associated with the stratification of waters of different densities. The standing stock of bacteria on the inner shelf, 106 ml−1, is little changed by the influx of water of reduced salinity. On the outer shelf, where the usual standing stock of bacteria is 105 ml−1, the numbers increase to 106 ml−1 in and above intrusions of Gulf Stream water in which phytoplankton blooms have developed, suggesting that the bacteria respond to products of both phytoplankton and zooplankton production. Adenylate energy charge values in the waters of the southeastern shelf are variable and volatile. At times values of 0.7 to 0.8 are widespread over most of the shelf, while at other times values <0.6 are common, with localized patches of high values. Both autotroph-dominated and heterotroph-dominated microbial communities show these variations.

PHYSICAL TRANSFER PROCESSES BETWEEN GEORGIA TIDAL INLETS AND NEARSHORE WATERS

Abstract: Nearshore continental shelf waters are directly influenced by the many tidal inlets which are spaced 10 to 20 km apart along the Georgia coast. Despite the highly variable and complex nature of the flow near inlets, there are recurring hydrographic features that typify the nearshore region. An innermost zone, adjacent to inlet mouths, contains highly turbid waters ejected from the inlets and marshes along the sounds. Fresher waters override more saline waters in a more or less classic riverine plume, inside of which the water motion is nearly independent of the motion in the surrounding water. Seaward of the zone containing the inlet plumes, a complex array of salinity and turbidity fronts is found that is probably a relict from preceding tidal cycles. The zone extends offshore 10 to 20 km and is defined here as the nearshore zone. In this zone, the fresher and more turbid waters are mixed horizontally and vertically with continental shelf water, resulting in a hydrographic structure similar to a partially mixed estuary. Material from the many inlets enters the nearshore zone where the vertical and horizontal gradients inhibit the exchange in the onshore direction unless tidal and wind-generated currents induce transfer processes that overcome this inhibition. These transfer processes are investigated with a limited amount of time-series data at one point. It is shown that onshore salt fluxes have strong tidal components and are influenced by vertical shear accompanying the presence of vertically stratified water. While these transfer processes occur regularly over each tidal cycle, more efficient and dramatic exchange processes may be associated with the changes in alongshore current direction.

Transport and fate of low-density water in a coastal frontal zone

Abstract We have examined the transport and fate of low-density water in a coastal frontal zone located on the continental shelf of the southeastern United States. About 800 m3 s−1 of freshwater in the frontal zone was lost seaward over an alongshelf distance of 250 km. During moderate to strong southward wind stress events, low-salinity water in the coastal frontal zone is transported seaward to the middle shelf. There, the transport is augmented by alongshelf transport of low-salinity water apparently lost from the frontal zone further upstream. Most of the low-salinity water is lost to the outer shelf south of the center of the bight-shaped coastline. A simple Fickian diffusion model with a diffusion coefficient of O(100) m2 s−1 represented the observed loss. Among the factors contributing to the loss were: (1) decreased alongshelf wind stress in the southern part of the area due primarily to the bight-shaped nature of the coast; and (2) cross-shelf mixing due to the time-dependent flow. Dispersion due to vertical shear in tidal currents could partially account for some of the observed cross-shelf transport of low-density water. Estimates based on analyses of tidal currents indicate that a cloud of freshwater at the coast would spread seaward at a rate of about 2 km per day. This rate is consistent with earlier estimates of the offshore transport of particulate aluminum. During wind relaxation or reversal, the pressure field collapses and briefly bifurcates the alongshelf currents inside the 20 m isobath. Bifurcation is confirmed in observations and models. This mechanism would coincide with an offshore tilt of the coastal frontal zone and would efficiently remove low-density water in surface layers, thus confirming earlier hypotheses.

Influences of physical oceanographic processes on chlorophyll distributions in coastal and estuarine waters of the South Atlantic Bight

Coastal and estuarine waters of the South Atlantic Bight are highly productive, with primary production of 600-700 gC/m 2 /y. While controls and fate of this production are conceptually well understood, the importance of meteorology and physical circulation processes on phytoplankton has not received equivalent attention. Here, we describe the effects of wind stress and tidal currents on temporal and spatial distributions of phytoplankton biomass represented as chlorophyll a (chl a). Moored instruments were deployed and shipboard sampling was conducted in the North Edisto estuary (South Carolina) and adjacent inner shelf waters during four, two-week field studies in May and August 1993, and June and September 1994. Local wind regimes induced upwelling- and downwelling-favorable conditions which strengthened or reduced vertical density stratification in the coastal frontal zone, respectively, and shifted the location of the front. Chl a in shelf waters was more or less homogenous independent of the wind regime, while chl a on the estuary delta was generally vertically stratified. Within the estuary, chl a concentrations were positively correlated with the alongshore component of wind stress; chl a was not correlated with the weaker cross-shelf component of wind stress. Highest chl a occurred during strong downwelling-favorable events. The quick response time to wind forcing (6-12 hrs) implied a direct effect on chl a distributions and not a stimulation of growth processes. The source of the elevated chl a in response to wind forcing was apparently resuspension of settled and epibenthic algal cells. Tidal currents also influenced the vertical distribution and concentration of chl a. Time series sampling on the estuary delta showed that, with increasing velocity of ebb and flood tide currents, the relative contributions of pennate and centric diatoms with attached detritus and sand grains also increased, indicating that tidal resuspension of settled and epibenthic microalgae also occurred. Vertical stratification of chl a (highest concentrations near the bottom) began to degrade upon mixing by tidal currents with velocities as low as 10 cm/sec. Homogenization of 5-7 m water columns was fully achieved at velocities of 20-30 cm/sec. The data document the direct and comparatively immediate (timescales of minutes-hours) impact of tidal and wind energy on concentrations and distribution patterns of phytoplankton in coastal and estuarine waters of the South Atlantic Bight.

The Mobilization and Deposition of Mud Deposits in a Coastal Plain Estuary

Muddy sediments with their potential for containing contaminants are commonly deposited and remobilized by tidal currents in estuarine environments. We examined the mobilization and subsequent redeposition of mud in a coastal plain estuary located in the southeastern United States. Time-series data for salinity, suspended sediment concentrations and quality (percent organic matter and pigment concentrations) were obtained over a 13-hour tidal cycle. We found that fast-settling mud particles are found during the highest tidal current speeds. Particle quality analyses suggest that all the material is of similar origin, and that phaeopigment can be used as a tracer of particles in this system. These particles settle onto the bed when current speeds approach slack conditions. We speculate that the quantity of mud mobilized during neap tide is less than during spring tide resulting in an opportunity for the mud to partially consolidate on the bottom and be removed from resuspension. We further speculate that the muddy sediments are mainly derived from fringing marshes in this estuary.

Autotrophic and heterotrophic abundance and activity associated with a nearshore front off the Georgia Coast, U.S.A.

Abstract The nearshore frontal zone off the coast of Georgia was found to be an area of high phytoplankton and bacterioplankton abundance and activity. Phytoplankton and bacterioplankton populations on the seaward side of the frontal zone had significantly higher photosynthetic and heterotrophic potentials than the nearshore side of the front. Phytoplankton species composition changed across the front, verifying that the front is a barrier to cross shelf mixing. Nearshore, large chain forming diatoms dominated, while smaller single cell diatoms and cyanobacteria dominated the seaward side of the front. Increased bacterioplankton activity was found associated with phytoplankton photosynthetic activity. Light appeared to be the major factor controlling photosynthesis across the frontal zone. Nitrogen, phosphorus and silica were present in similar concentrations, well above levels that would limit photosynthesis, on both sides of the front. Therefore the outflow of nutrients from rivers or estuaries did not influence primary production directly.

Variability of particulate matter and abundant zooplankton off the southeastern United States during spring of 1984 and 1985

Abstract The variability of particulate matter, chlorophyll a and zooplankton often depend strongly on physical processes. In this paper we compare distributions observed on the southeastern shelf of the United States during April 1984 and 1985. April 1984 was characterized by high freshwater runoff, vertical stratification and strong wind stress. In April 1985 all three variables were much lower. While alongshore flow was similar in both years, cross-shelf flow was more pronounced in 1984. The most noticeable effect on zooplankton was their expatriation towards offshore. The near surface, offshore and deeper Ekman flows from northeastward wind events, in conjunction with intermittent sinking, tend to trap most particulate matter that was produced nearshore in the nearshore or middle shelf region. Offshore removal was transient following upwelling wind events during stratification conditions. Zooplankton abundance on the inner and middle shelf was not only a function of near- and offshore production but also of cross-shelf and alongshore displacement. Whereas the circumglobal copepod genus Paracalanus occurs abundantly in summer, spring and autumn on the inner and middle shelf, the cosmopolitan genus Oithona which is abundant during summer is scarce during spring and appears to be affected by temperature and water column stability. The variability of particulate matter and zooplankton during early spring seems to be at least partly determined by atmosphere forcing and the amount of freshwater runoff.