Sediment ammonium availability and eelgrass (Zostera marina) growth

The interaction of sediment ammonium (NH ₄ ⁺ ) availability and eelgrass (Zostera marina L.) growth, biomass and photosynthesis was investigated using controlled environment and in-situ manipulations of pore water ammonium concentrations. Sediment diffusers were used to create pore water diffusion gradients to fertilize and deplete ammonium levels in sediments with intact eelgrass rhizospheres. Between October, 1982 and September, 1983 controlled environment experiments using plants from shallow (1.3 m) and deep (5.5 m) stations in a Great Harbor, Woods Hole, Massachusetts, USA eelgrass meadow along with in-situ experiments at these stations provided a range of sediment ammonium concentrations between 0.1 and 10 mM (adsorbed+interstitial NH ₄ ⁺ ). The results of the in-situ experiments indicate that nitrogen limitation of eelgrass growth does not occur in the Great Harbor eelgrass meadow. A comparison of NH ₄ ⁺ regeneration rates and eelgrass nitrogen requirements indicates an excess of nitrogen supply over demand and provides an explanation for the lack of response to the manipulations. Results of controlled environment experiments combined with in-situ results suggest that sediment ammonium pool concentrations above approximately 100 mol NH ₄ ⁺ per liter of sediment (interstitial only) saturate the growth response of Zostera marina.     

Predictive modelling of eelgrass (Zostera marina) depth limits

Empirical models relating secchi depths to maximum depth limits of eelgrass (Zostera marina L.) can describe basic differences in depth limits between areas or time periods exhibiting large differences in secchi depth. However, these models cannot predict the precise depth limit at a particular site at any specific time. In this study we aim to improve the ability of regression models to predict maximum depth limits by: (1) assuming that eelgrass depth limits respond to changes in secchi depth with a temporal delay of 1–2 years, (2) including other water-quality variables in addition to secchi depth, and (3) taking into account that factors regulating depth limits may vary between years and between sites. We were not able to improve the models by introducing a systematic delay in the response of depth limits to changes in secchi depths. The reason for this failure is likely to have been the systematic nature of our approach, since some sites responded with a delay, while others did not. The explanatory power of the models increased when additional water-quality variables were added in a multiple regression model. Where secchi depth alone explained 58% of the variations in depth limits, addition of winter [NH₄⁺] and maximum water depth as independent variables increased the explanatory power to 71%. These models applied to data from one specific year, but when data from several years (1989–1998) were included, only 35% of the variation in depth limits could be explained by the three factors. More detailed analyses showed that the regulation of eelgrass depth limits varied considerably between years and between sites, and the models were further improved by taking this information into account. Our results confirmed previous studies by showing light to be the most important parameter in the regulation of eelgrass depth limits, but also revealed a complexity in the regulation of depth limits not expressed in earlier studies. Limited colonisation potentials may delay the response to improved light conditions, and hypoxia/anoxia and indirect effects of nutrients may prevent eelgrass from attaining the depth limit that light levels would allow. The power to predict depth limits on the basis of secchi depths can therefore be improved by taking site-specific information on eelgrass growth conditions into account.Communicated by M. Kühl, Helsingør     

13C label identifies eelgrass (Zostera marina) Carbon in an Alaskan estuarine food web

The food web of Izembek Lagoon, Alaska draws most of its carbon from eelgrass (Zostera marina) and phytoplankton. The¹³C:¹²C ratios of these primary producers are sufficiently different to enable their contributions to consumers to be estimated from consumer¹³C:¹²C ratios. Although the technique is conceptually simple, carbon inputs from other sources and isotope fractionations occurring in the food web limit its precision. Isotopic data nevertheless helps to establish the major carbon fluxes through the community and to assess the importance of eelgrass carbon to individual animals. It is particularly useful when dealing with detritus food chains, where direct observations of animal feeding habits are difficult to make. The Izembek community draws much of its carbon from eelgrass. Detritus food chains provide the major pathway for assimilation of eelgrass carbon by the community, but grazers are also important. Eelgrass carbon is more important to benthic animals than to the eelgrass epibiota and the fishes, which depend mainly on phytoplankton carbon.Publication No. 381 of the Institute of Marine Science, University of Alaska.     

Uptake and translocation of phosphorus in eelgrass (Zostera marina)

The uptake and translocation of phosphorus in eelgrass (Zostera marina L.) was studied in two-compartment chambers using ³²P and autoradiography. Eelgrass was able to take up phosphorus both with leaves and rootrhizomes. Only a small amount (less than 4%) of the total amount of phosphorus taken up from either source was translocated within the plants during an incubation time of 120 h. Release of translocated phosphorus from the plant tissues to the surrounding water was not detected. The autoradiographs showed that the translocated phosphorus was localized mainly in the actively growing plant tissues, i.e. the stem and young leaves, and nodes of the rhizomes and young roots. The uptake of phosphorus in leaves and the bidirectional translocation in the plants was significantly lower in the dark. The uptake of phosphorus in rootrhizomes was unaffected by light-conditions. The concentration of phosphate in the water of one compartment did not affect the phosphorus uptake by the plant parts in the opposite compartment. However, the translocation was significantly reduced when both compartments were supplied with identical phosphate concentrations. The results of the present investigation indicate that the quantitative significance of eelgrass in the cycling of phosphorus between sediment and water may have been greatly overestimated in earlier studies. The amount of phosphorus taken up by root-rhizomes and subsequently translocated to leaves is probably insignificant during most of the growing season in comparison to the amount taken up by the leaves from the surrounding water. However the uptake of phosphorus by root-rhizomes may be significant in the nutrition of eelgrass during periods with low phosphate levels in the water.     

Ammonium regeneration and assimilation in eelgrass (Zostera marina) beds

Regeneration and assimilation of ammonium in the water column and in sediments of eelgrass (Zostera marina L.) beds of Izembek Lagoon and Crane Cove, Alaska, USA and Mangoku-Ura, northeastern Japan, were investigated by using a ¹⁵N isotope dilution technique. In the water column of Mangoku-Ura, ammonium was regenerated at a rate of 12 nmol l^-1 h^-1 and assimilated at a rate of 74 nmol l^-1 h^-1. The ammonium regeneration rate in sediments ranged from 2 to 150 nmol g^-1 h^-1, and with one exception, exceeded ammonium assimilation in sediments (0.3 to 77 nmol g^-1 h^-1). The ammonium regeneration in the water column was of little significance for the nitrogen supply to the eelgrass bed ecosystem. Net ammonium production (regeneration minus assimilation) in the sediment of Izembek Laggon met nitrogen demand for eelgrass growth, suggesting that ammonium regeneration in the sediments was very important for the nitrogen cycle in the eelgrass bed ecosystem.     

Inhibition of the growth of micro-algae and bacteria by extracts of eelgrass (Zostera marina) leaves

The effects of the water-soluble fraction of dead leaves of the eelgrass Zostera marina L. on the growth of 8 species of micro-algae (pennate and centric diatoms, dinoflagellates, and a green flagellate) and a bacterium were studied on agar plates and in liquid culture. The extracts of leaves which had been dead from a few days to 2 wk inhibited growth and often killed cells in all test organisms. Extracts were lethal even at concentrations equivalent to as little as 0.25 mg dry leaf ml^-1, but inhibition decreased when extracts were prepared from leaves aged in the laboratory for 35 d (loss of anti-bacterial activity) or 90 d (loss of anti-algal activity). Extracts of leaves which had aged and dried several months in the field had no effect, except at very high concentrations (13 mg dry leaf ml^-1) when the lag phase of growth was prolonged several days in a culture of the chlorophyte Platymonas sp. The active fraction in eelgrass leaves may be important in controlling initial growth of micro-organisms on eelgrass detritus, and it could determine the composition and activity of the epiphytic community on living leaves.     

Metabolic adaptation of Zostera marina (eelgrass) to diurnal periods of root anoxia

The temperate seagrass Zostera marina L. is common in coastal marine habitats characterized by the presence of reducing sediments. The roots of this seagrass grow in these anoxic sediments, yet eelgrass is highly productive. Through photosynthesis-dependent oxygen transport from leaves to roots, aerobic respiration is supported in eelgrass roots only during daylight; consequently, roots are subjected to diurnal periods of anoxia. Under anoxic root conditions, the amino acids alanine and -amino butyric acid accumulate within a few hours to account for 70% of the total amino acid pool, while glutamate and glutamine decline. Little ethanol is produced, and the pool size of the organic acid malate changes little or declines slowly. Upon the resumption of shoot photosynthesis and oxygen transport to the roots, the accumulated -amino butyric acid declines rapidly, glutamate and glutamine pools increase, and alanine declines over a 16-h period. These adaptive metabolic responses by eelgrass to diurnal root anoxia must contribute to the successful exploitation of shallow-water marine sediments that have excluded nearly all vascular plant groups. A metabolic scheme is presented that accounts for the observed changes in organic and amino acid pool sizes in response to anoxia.     

Utilization of growth parameters of eelgrass,Zostera marina,for productivity estimation under laboratory and in situ conditions

Allometry was used for monitoring aboveground growth of the marine angiosperm Zostera marina L. Dry weight was regressed with leaf length and width, allowing estimation of aboveground net productivity and biomass of individual plants. At the termination of the experiment, rhizome productivity of the same plants was determined by harvesting. Plants in shaded and unshaded seawater tanks were monitored from June until September, 1976; in situ plants were also monitored at Point Judith Pond, Rhode Island, USA. Unshaded plants had shorter leaves, a lower net productivity, lower biomass, and a lower aboveground-torhizome productivity ratio than shaded plants. Unshaded plants had a higher rate of rhizome branching and the resulting new shoot formation than in situ plants.     

Development of epiphytic communities on eelgrass (Zostera marina) along a nutrient gradient in a Danish estuary

The effect of nutrient enrichment on epiphyte development was examined by following the seasonal development of epiphyte biomass on eelgrass (Zostera marina L.) at four localities along a nutrient gradient in Roskilde Fjord, Denmark between March and December 1982. In the most nutrient-poor area, epiphyte biomass followed a distinct bimodal seasonal pattern with maxima in spring and early fall. Low nutrient availability and a high rate of eelgrass leaf renewal kept epiphyte biomass at a low level throughout the summer period. Unlike phytoplankton, the epiphytic community was not stimulated by nutrient enrichment during spring, however, from May through August, the biomass of both components increased exponentially with increasing concentrations of total N in the water. Along the nutrient gradient, phytoplankton biomass increased 5- to 10-fold, while epiphyte biomass increased 50- to 100-fold. Thus differences in nutrient conditions among study sites were more clearly reflected by epiphytes than phytoplankton.Contribution No. 419 from the Freshwater-Biological Laboratory, University of Copenhagen     

Size and estimated age of genets in eelgrass, Zostera marina, assessed with microsatellite markers

We examined the spatial distribution of genotypes in a perennial population of eelgrass, Zostera marina L., at two spatial scales. We mapped and sampled 80 ramets in a subtidal area of 20 × 80 m, and an additional 15 ramets in two 1-m² sub-quadrats. Ramets were genotyped for seven polymorphic microsatellite loci. Among a total number of 54 genotypes detected, 12 occurred more than once. The ramets of ten of these genotypes occurred in clusters and represented genets based on their expected multi-locus genotype frequencies. The size distribution of genets was uneven with estimated ramet numbers ranging from 2 to 5000. Whereas some areas displayed a high genet diversity, which may indicate past disturbances, large genets (≥10 m²) predominated in other locations of the sampled plot. Spatial heterogeneity in clone distribution was also obvious at the smaller sampling scale (15 ramets sampled within 1 m²) where the clonal diversity (genets identified/ramets sampled) was 0.24 in one quadrat, and 0.077 in the other. Ramets belonging to the largest clone were maximally 17 m apart, which corresponds to a genet age of 67 yr based on annual rhizome growth rates. We conclude that the spatial arrangement of clones in seagrasses allows inferences about the past demography and the disturbance regime at a given site which may prove useful for coastal zone management of ecologically valuable seagrass meadows. Received: 15 September 1998 / Accepted: 14 November 1998     

Reintroduction of eelgrass (Zostera marina) in the Dutch Wadden Sea; review of research and suggestions for management measures

Eelgrass (Zostera marina L.) in the Dutch Wadden Sea historically covered an area varying from 65–150 km² in the eulittoral as well as the sublittoral zones. At present, this area comprises less than 1 km² eulittoral eelgrass stands, with an associated decrease in habitat diversity. The causes for this decline are presumably connected with the ‘wasting disease’ and the closure of the former Zuiderzee in the early 1930s resulting in increased tidal range and increased currents. After a slight recovery of the eelgrass populations on the intertidal flats a definite decline started in the early 1970s, possibly connected to increased turbidity. The present water quality and turbidity do not negatively influence eelgrass growth up to a depth of at least 0.6m below Mean Sea Level. Based on mesocosm experiments and field experiments it is concluded that re-establishment of eelgrass should be possible in sheltered bays and on unexposed tidal flats. The most suitable depths for a reintroduction are those between 0 and 20–40 cm below mean sea level.     

Labyrinthula sp., a marine slime mold producing the symptoms of wasting disease in eelgrass,Zostera marina

Coastal ecosystems along the eastern United States are presently threatened by a recurrence of the wasting disease of eelgrass, Zostera marina L. Using Koch's postulates, a species of the marine slime mold, Labyrinthula, is identified as the causal microorganism of this disease. Our disease tests for pathogenicity performed on eelgrass, using four Labyrinthula spp., indicate only one species produces the disease symptoms identical to those found associated with the wasting disease. The pathogenic Labyrinthula sp. has morphological characteristics that distinguish it from the other three species. Identification of Labyrinthula spp. is difficult because species described in the literature are not clearly characterized or identifiable. Tests at various salinities demonstrate that disease symptoms appear infrequently at salinities of 10%. or less.     

Estimates of outcrossing rates and of inbreeding depression in a population of the marine angiosperm Zostera marina

Despite the great diversity of pollination and fertilization mechanisms observed in marine plants, little is known about the causes or maintenance of this variation. In this study, I estimated outcrossing rates and levels of inbreeding depression in Zostera marina L. (eelgrass), providing the first empirical test of hypotheses about the evolution of breeding systems in plants with submerged flowers. This study also addressed temporal separation of female and male flowering (dichogamy) in eelgrass as a mechanism promoting an outcrossing mating system, and whether the mating system in eelgrass is related to the degree of dichogamy in the field. Outcrossing rates (0<t<1) estimated from two polymorphic allozyme loci indicate that the Z. marina population in intertidal and subtidal habitats in False Bay, Washington, USA, was highly outcrossing in both 1991 (intertidal t=0.905, subtidal t=1.0) and 1992 (intertidal t=0.775, subtidal t=0.611). The outcrossing rates were positively associated with the degree of dichogamy in 1992; intertidal plants exhibited a greater temporal separation of female and male flowering and a higher outcrossing rate than did subtidal plants. Inbreeding depression at seed set was estimated from hand pollinations (self- and outcross) on 20 reproductive individuals from the False Bay population. Averaged across all maternal parents, a greater proportion of outcrossed flowers set seed than selfed flowers; i.e., inbreeding depression was detected. Plants exhibited genetic variation for inbreeding depression, detected as a significant pollination treatment × maternal family interaction in a log-likelihood analysis. By the end of the seed-maturation period (7 mo after intial seed set) some families showed outbreeding depression, i.e., greater fitness in progeny derived from selfing than in progeny from outcrossing. The inbreeding depression in the False Bay population may be an important selective factor contributing to the maintenance of dichogamy and an outcrossing mating system, as proposed for aquatic plants.     

Suitability of estuarine nursery zones for juvenile weakfish (Cynoscion regalis): effects of temperature and salinity on feeding,growth and survival

Juvenile weakfish, Cynoscion regalis (Bloch and Schneider, 1801), exhibit significant spatial diffrences in growth rate and condition factor among estuarine nursery zones in Delaware Bay. The potential influence of temperature and salinity on the suitability of estuarine nursery areas for juvenile weakfish was investigated in laboratory experiments by measuring ad libitum feeding rate, growth rate and gross growth efficiency of juveniles collected in Delaware Bay in 1990 (40 to 50 mm standard length; 1.4 to 2.1 g) in 12 temperature/salinity treatments (temperatures: 20, 24, 28°C; salinities: 5, 12, 19, 26 ppt) representing conditions encountered in different estuarine zones during spring/summer. Feeding rates (FR) increased significantly with temperature at all salinities, ranging from 10 to 15% body wt d^-1 at 20°C to 33–39% body wt d^-1 at 28°C. Specific growth rates (SGR) ranged from 1.4 to 9.4% body wt d^-1 (0.3 to 1.5 mm d^-1) and gross growth efficiencies (K ₁) varied from 13.6 to 26.4% across temperature/salinity combinations. Based on nonlinear multiple regression models, predicted optimal temperatures for SGR and K ₁ were 29 and 27°C, respectively. Salinity effects on SGR and K ₁ were significant at 24 and 28°C where predicted optimal salinity was 20 ppt. At these warmer temperatures, SGR and K ₁ were significantly lower at 5 than at 19 ppt despite higher FR at 5 ppt. Therefore, maximum growth rate and growth efficiency occurred under conditions characteristic of mesohaline nurseries. This finding is consistent with spatial patterns of growth in Delaware Bay, implying that physicochemical gradients influence the value of particular estuarine zones as nurseries for juvenile weakfish by affecting the energetics of feeding and growth. Laboratory results indicate a seasonal shift in the location of physiologically optimal nurseries within estuaries. During late spring/early summer, warmer temperatures in oligohaline areas permit higher feeding rate and faster growth compared to mesohaline areas. By mid-late summer, spatial temperature gradients diminish and mesohaline areas provide more suitable physicochemical conditions for growth rate and growth efficiency whereas oligohaline areas become energetically stressful. Substantial mortality occurred at 5 ppt and 28°C, providing additional evidence that oligohaline conditions are stressful during late summer. Furthermore, juveniles provided a choice among salinities in laboratory trials preferred those salinities which promoted higher growth rates. The extensive use of oligohaline nurseries by juvenile weakfish despite the potential for reduced growth rate and growth efficiency suggests this estuarine zone may provide a substantial refuge from predation.     

Interactions between two introduced species: Zostera japonica (dwarf eelgrass) facilitates itself and reduces condition of Ruditapes philippinarum (Manila clam) on intertidal flats

Dwarf eelgrass (duckgrass; Zostera japonica) and Manila clams (Ruditapes philippinarum) are two introduced species that co-occur on intertidal flats of the northeast Pacific. Through factorial manipulation of clam (0, 62.5, 125 clams m⁻²) and eelgrass density (present, removed by hand, harrowed), we examined intra- and interspecific effects on performance, as well as modification of the physical environment. The presence of eelgrass reduced water flow by up to 40% and was also observed to retain water at low tide, which may ameliorate desiccation and explain why eelgrass grew faster in the presence of conspecifics (positive feedback). Although shell growth of small (20–50 mm) clams was not consistently affected by either treatment in this 2-month experiment, clam condition improved when eelgrass was removed. Reciprocally, clams at aquaculture densities had no effect on eelgrass growth, clam growth and condition, or porewater nutrients. Overall, only Z. japonica demonstrated strong population-level interactions. Interspecific results support an emerging paradigm that invasive marine ecosystem engineers often negatively affect infauna. Positive feedbacks for Z. japonica may characterize its intraspecific effects particularly at the stressful intertidal elevation of this study (+1 m above mean lower low water).     

Synergistic effects of salinity,temperature and heavy metals on mortality and osmoregulation in marine and estuarine isopods (Crustacea)

The effects of cadmium (3CdSO₄·8H₂O), zinc (ZnSO₄·7H₂O) and lead [Pb (NO₃)₂] on mortality, and cadmium, zinc and mercury (HgCl₂) on osmoregulation, have been recorded for marine and estuarine species of isopods (Crustacea). The marine species studied were Idotea baltica, I. neglecta, I. emarginata and Eurydice pulchra, which were adapted to 100, 80, 60 and 40% sea water (SW) (100% SW э 34‰ S). The estuarine species used were Jaera albifrons sensu stricto and J. nordmanni, which were adapted to 100, 50, 10 and 1% SW. Both groups of isopods have low mortalities in 100% SW with 10 and 20 ppm of cadmium, zinc and lead, but a decrease in salinity caused an increase in the toxicities of these metals and reduced the LT₅₀ values (time, in hours, to 50% mortality). Mortalities at 10°C were generally higher than those recorded at 5°C. Cadmium had no significant effect on the osmoregulation of I. baltica and I. emarginata in 100 and 80% SW at 5°C, but this metal significantly lowered the blood osmotic concentration of I. neglecta in 80% SW. Zinc did not alter the haemolymph osmotic concentration of I. neglecta in 100 and 80% SW, but significantly lowered the blood osmotic concentration of I. baltica in 100% SW. Cadmium, zinc and mercury also significantly altered the osmoregulatory ability of J. albifrons in dilute saline.     

Responses of estuarine crab megalopae to pressure,salinity and light: Implications for flood-tide transport

The megalopal larval stage of many estuarine brachyuran crabs appears to return to adult habitats by undergoing rhythmic vertical migrations which result in saltatory up-estuary transport on flood tides. Larval ascent into the water column during rising tides may be cued by changing hydrologic variables. To test this hypothesis, we investigated the responses of field-caught megalopae of the blue crab Callinectes sapidus and the fiddler crab Uca spp. to constant rates of pressure and salinity change under laboratory conditions. For both genera, pressure changes resulted in increased movement (barokinesis) and upward migration in the test chamber, with C. sapidus megalopae having a lower response threshold (2.8×10^-2 mbar s^-1) than Uca spp. larvae (5×10^-2 mbar s^-1). Similarly, larvae ascended in response to increasing salinity, with C. sapidus larvae being more sensitive. Larvae were negatively phototactic and failed to respond to pressure increases at light levels above 1.0×10¹⁵ and 1.0×10¹³ photons m^-2 s^-1 for C. sapidus and Uca spp. megalopae, respectively. Such responses are thought to explain the low abundances of larvae in the water column during daytime flood tides. Nevertheless, threshold sensitivities to increasing pressure for both genera were above levels experienced during floodtide conditions in the field. Similarly, it is unlikely that increasing salinity is sufficient to induce ascent in Uca spp. postlarvae. However, rates of salinity increase during midflood tide typically reach levels necessary to induce an ascent in C. sapidus megalopae. These results are consistent with the hypothesis that fiddler crab megalopae utilize an endogenous activity rhythm for flood-tide transport, while blue crab megalopae rely upon external cues, especially salinity changes, to time their sojourns in the water column.     

Determination of lethal dissolved oxygen levels for selected marine and estuarine fishes, crustaceans, and a bivalve

The objective of this study was to provide a database of the incipient lethal concentrations for reduced dissolved oxygen (DO) for selected marine and estuarine species including 12 species of fish, 9 crustaceans, and 1 bivalve. All species occur in the Virginian Province, USA, which is a cold temperate region. The study period was August 1987 to September 1995. Standard bioassay procedures were employed, with most tests being of 4-day duration. Up to eight lethal concentrations (LCs) between LC₀₅ and LC₉₅ were estimated. The study provides four general conclusions about determining lethal thresholds of low DO for these organisms. First, the concentration response curve of most species did not change greatly beyond day 1 of the exposure with the exception of crustacean larvae, which were usually more sensitive on day 4, possibly due to molting. Second, acute LC₅₀ values (1- to 4-day) for low DO were influenced by life-stage and habitat, with pelagic larvae generally being the most sensitive and benthic juveniles the least. Species mean LC₅₀ values ranged from 1.4 to 3.3 mg l^-1 for larvae, 1.0 to 2.2 mg l^-1 for postlarvae, and 0.5 to 1.6 mg l^-1 for juveniles. No intraspecific differences in LC₅₀ were detected between larval stages in crustaceans or with age in larval fishes. The response range between LC₀₅ and LC₉₅ was narrowest for the least sensitive organisms (0.6 mg l^-1), and broadened with sensitivity. The mean LC₁₀:LC₅₀ ratio for all species was 1.32 for larvae and juveniles, and 1.36 for postlarvae. The ratio for postlarvae represents only four species, and hence is not considered different from the other life stages. Third, variability increased with increased species and life stage sensitivity to low DO, and with endpoints of LC₁₅ and below, which reduces the certainty of some of these results. Lastly, no influence of temperatures between 20°C and 30°C was detected in a small set of tests with thermally acclimated crustacean larvae. This data set has been used to describe protection limits for juvenile and adult survival, and for larval recruitment for the case of persistent (₄ h) low DO for estuarine and coastal waters of the Virginian Province, USA.     

High levels of genetic subdivision of marine and estuarine populations of the estuarine catfishCnidoglanis macrocephalus (Plotosidae) in southwestern Australia

The cobblerCnidoglanis macrocephalus (Valenciennes) is an endemic marine and estuarine catfish from southern Australia. Conflicting views on the degree of isolation of the estuarine populations underscore general questions about genetic divergence in coastal species. Although estuaries are widely recognized as ecologically important, little work has been done on their role in favouring genetic divergence. In order to estimate the extent of genetic subdivision among nearshore marine and estuarine populations, electrophoretic variation of enzymes was examined in seven marine and six estuarine populations of cobbler from sites spanning 1500 km along the southwest Australian coastline. Among all populations, the mean standardized variance in allelic frequencies (F _ST) for six polymorphic loci was 0.277, a high value comparable to those of other shallow-water teleosts whose life-history characteristics and habitat preferences restrict their dispersal capability. The pattern of genetic identities between populations showed divergence between west and south coast sites. Within these regional groups, however, there was substantial heterogeneity, much of which was associated with estuaries. Among all six estuarine sites, the averageF _ST was 0.333, 40% higher than the value of 0.237 for the marine sites. Low estimates of the genetically effective number of migrants suggest population subdivision between marine and estuarine environments and between similar habitat types. This study indicates the importance of habitat in affecting the connectedness of populations, even in apparently open marine systems.     

Age structure,growth rates,movement patterns and feeding in an estuarine population of the cardinalfish Apogon rueppellii

The biology of a population of the cardinalfish Apogon rueppellii has been studied over several years (1977–1983) in the Swan Estuary in south-western Australia, using ramples collected monthly from the shallows by beach seine and from various depths by otter trawl. While the life cycle of this species typically lasts for one year, at the end of which time the mean length is 50 to 60 mm, some individuals survive for a further year and attain lengths up to 104 mm. A. rueppellii shows a marked tendency to move offshore into deeper water during the winter months. This tendency is more pronounced in the 1+ than in the 0+ year class and in larger than smaller 0+ individuals. An inshore movement of A. rueppellii in the spring is followed by spawning and by oral brooding by the males, which leads to the recruitment of large numbers of a new 0+ year class on to the banks during the summer. The offshore movement is correlated with changes in salinity and temperature. The larger catches taken by otter trawl during the day than at night indicate that A. rueppellii exhibits a diel pattern of activity. Mean fecundity ranged from 70 in the 45 to 49 mm size class to 345 in the 90 to 94 mm size class. Measurements of fecundity and the number of oral-brooded eggs demonstrated that the majority of the eggs released by the female are collected and incubated by the males. Copepods are ingested in relatively greater amounts by small than by large A. rueppellii, whereas the reverse situation occurs with larger crustaceans, polychaetes and small fish. The presence of greater amounts of copepods in the diet during the day and of amphipods at night probably reflects the diel activity patterns of the prey.