Vertical distribution of first stage larvae of the blue crab,Callinectes sapidus,at the mouth of Chesapeake Bay

The vertical distribution of stage I blue crab larvae, near the mouth of Chesapeake Bay, was examined over four diurnal cycles. Each of two stations was occupied for 30 hours twice during the summer of 1979.On each of the four cruises, peak larval abundance occurred after a night time high slack tide, suggesting a synchronized hatch of blue crab larvae. 90–99% of all larvae collected were taken in the neuston layer. The apparent timing of the hatches to coincide with the beginning of an ebb tide and the concentration of larvae in the neuston layer strongly suggests seaward transport of these early stage larvae and the probability of offshore development.     

Synoptic distribution of crab larvae near the mouth of Delaware Bay: Influence of nearshore hydrographic regimes

In this paper we describe results of a study designed to test the hypothesis that coastal regions with weak subtidal flow (i.e., coastal null zones) may serve as retention areas for estuarine larval forms. Our investigation assessed the distribution of 3 taxa of crab larvae (Callinectes sapidus, Uca spp., and Hexapanopeus angustifrons) within a 200-km² region encompassing the mouth of Delaware Bay (ca. 39° N, 75° W). Previous studies had shown that larvae of C. sapidus and Uca spp. are exported to the coastal ocean, while larvae of H. angustifrons are retained within the estuary. In the present investigation, we conducted simultaneous plankton tows at 3 stations during peak spawning season. Samples were collected from a depth of 1 m every 30 min throughout a complete tidal cycle. One station was located 15 km within the bay and was subjected to strong flow at tidal frequency. A second station was located within a southward-flowing coastal current near the southern terminus of the bay at Cape Henlopen. A final station was located in a coastal area of weak subtidal flow near Cape May at the northern terminus of the bay. Results provide a unique synoptic view of larval distributions in 3 distinct hydrographic regimes in the mouth of a major estuary. The coastal-current station was characterized by low concentrations of newly hatched C. sapidus and Uca zoeae, while the null-zone station had high densities of both early and advanced larval stages of these two taxa. In contrast, the station located within the bay had few C. sapidus or Uca zoeae and was dominated by both early and advanced stages of the mud crab H. angustifrons. These data provide clear evidence for the retention of exported larval forms in a coastal null zone associated with the circulation of a large estuary and thus are consistent with our hypothesis.     

The distribution of brachyuran crustacean megalopae in the waters of the York river, lower Chesapeake Bay and adjacent shelf: Implications for recruitment

Vertical and horizontal distributions are described for megalopae of 11 brachyuran taxa common to the lower Chesapeake Bay and adjacent shelf. Three distribution patterns are apparent from horizontal distribution.

1. (1) More than 75% of the megalopae of estuarine adults, such as Hexapanopeus angustifrons, Neopanope sayi, Panopeus herbstii, Rhithropanopeus harrisii and Pinnotheres ostreum, are retained in estuarine waters.

2. (2) The megalopae of three estuarine taxa, Callinectes sapidus, Uca spp. and Pinnixa sp., are most abundant on the shelf.

3. (3) More than 90% of the megalopae of the shelf species, Portunus sp., Cancer irroratus and Libinia spp. remained in shelf waters.

Vertical distributions indicate the megalopae of a majority of estuarine crabs are epibenthic when waters are stratified. Exceptions include the megalopae of both Rhithropanopeus which are found slightly shallower in well mixed waters and Callinectes sapidus which are generally most abundant in the neuston. The megalopae of two shelf species, Portunus sp. and Cancer irroratus, reach highest densities in surface waters, while those of Libinia spp. are common in the epibenthos.Megalopal distributions analyzed with respect to water column stratification show megalopae never reached highest densities within the layer of the pycnocline. The megalopae of five species show significant shifts in vertical distributions between stratified and homogeneous water columns.     

Estuarine retention of larvae of the crab Rhithropanopeus harrisii

Larvae of estuarine organisms continually face possible export from the parent estuary. Retention of larvae of the estuarine crab Rhithropanopeus harrisii was investigated in the upper Newport River estuary, North Carolina. All of the developmental stages occurred in the same area of the estuary with similar horizontal distributions, and the concentrations of intermediate and late stages were not greatly reduced from those of the first larval stage. This was strong evidence for the continuous retention of larvae in the upper estuary.To determine mechanisms by which retention might be effected, field studies of the vertical distributions and migrations of these larvae were made. The four zoeal stages had similar but complex vertical migration patterns, which varied from study to study. These migrations centered on the depth of no net flow, reducing longitudinal transport during development. Cross-spectral analysis of the larval migrations and the environmental cycles of light, salinity and current speed revealed that each of these external cycles affected larval depth. Megalopae of R. harrisii also migrated vertically, but they were present in much lower concentrations than the zoeal stages, an indication of a change to benthic existence in this final larval form.     

Ra and Ra in the mixing zones of the Pee Dee River-Winyah Bay, Yangtze River and Delaware Bay Estuaries

²²⁶Ra and ²²⁸Ra have non-conservative excess concentrations in the mixing zones of the Pee Dee River-Winyah Bay estuary, the Yangtze River estuary, and the Delaware Bay estuary. Laboratory experiments, using Pee Dee River sediment, indicate desorption of ²²⁶Ra to increase with increasing salinities up to 20‰. In Winyah Bay desorption from river-borne sediments could contribute almost all of the increases for both isotopes. Desorption adds only a portion of the excess ²²⁸Ra measured in the Yangtse River and adjacent Shelf waters and Delaware Bay. In the Yangtze River the mixing zone extends over a considerable portion of the Continental Shelf where ²²⁸Ra is added to the water column by diffusion from bottom sediments, while ²²⁶Ra concentrations decrease from dilution. Diffusion of ²²⁸Ra from bottom sediments in Delaware Bay primarily occurs in the upper part of the bay (< 22‰ water) where fine grained sediments predominate. A diffusive flux for ²²⁸Ra of 0·33 dpm cm⁻² year was determined for Delaware Bay.     

226Ra and 228Ra in the mixing zones of the Pee Dee River-Winyah Bay,Yangtze River and Delaware Bay Estuaries

²²⁶Ra and ²²⁸Ra have non-conservative excess concentrations in the mixing zones of the Pee Dee River-Winyah Bay estuary, the Yangtze River estuary, and the Delaware Bay estuary. Laboratory experiments, using Pee Dee River sediment, indicate desorption of ²²⁶Ra to increase with increasing salinities up to 20‰. In Winyah Bay desorption from river-borne sediments could contribute almost all of the increases for both isotopes. Desorption adds only a portion of the excess ²²⁸Ra measured in the Yangtse River and adjacent Shelf waters and Delaware Bay. In the Yangtze River the mixing zone extends over a considerable portion of the Continental Shelf where ²²⁸Ra is added to the water column by diffusion from bottom sediments, while ²²⁶Ra concentrations decrease from dilution. Diffusion of ²²⁸Ra from bottom sediments in Delaware Bay primarily occurs in the upper part of the bay (< 22‰ water) where fine grained sediments predominate. A diffusive flux for ²²⁸Ra of 0·33 dpm cm^?2 year was determined for Delaware Bay.     

Retention of crab larvae in a coastal null zone

Alongshelf transport in the southern Middle Atlantic Bight is forced by buoyancy-driven currents originating in three large estuaries along the bight. These currents are strongest in the coastal ocean near the southern terminus of each estuary, while the analogous region on the northern side is characterized by weak subtidal flow. We used a combination of field observations and numerical modeling to test the hypothesis that these regions of weak subtidal flow are coastal null zones that serve as retention areas for larvae. The field study consisted of a four-day, shipboard investigation of the distribution of blue crab larvae (Callinectes sapidus) near the mouth of Delaware Bay (39°N, 75°W) in late summer, 2004. Hydrographic surveys of the study site were conducted with a hull-mounted, surface-measuring system. Results showed a sharp boundary between the null zone and the buoyancy-driven current to the south. Blue crab larvae were collected in surface plankton tows along a 30-km transect that encompassed these two areas. Stations with higher densities of larvae were clustered in the null zone during both ebb and flood tides. A numerical model was used to examine the physical mechanisms responsible for the observed distribution. Model results agreed with the field survey and showed that simulated larvae are aggregated in the null zone. The simulations also demonstrated that larvae spawned within the null zone have a much greater probability of settling in juvenile nursery habitat within the bay. The close agreement between field and model results provides consistent support for the hypothesis that coastal null zones associated with the buoyancy-driven circulation of large estuaries may allow retention of larvae in the vicinity of the natal spawning population.     

Occurrence patterns of mesopelagic fish larvae in Sagami Bay, central Japan

The species composition, seasonal abundance, and vertical distribution of mesopelagic fish larvae are described based on discrete depth sampling from the surface down to 1000 m depth during four cruises at a fixed sampling station in Sagami Bay. The abundances of total mesopelagic fish larvae in April, July, September, and December were 65.7, 13.6, 118.9, and 17.2 individuals per 10 m² sea surface, respectively. Twenty species or types of mesopelagic fish larvae belonging to 10 families were collected. Diaphus garmani, Lipolagus ochotensis, Diogenichthys atlanticus, Sigmops gracile, and Maurolicus japonicus were the five most abundant larvae and accounted for 43.1, 14.5, 7.4, 6.3, and 5.9% of the total mesopelagic fish larvae, respectively. These five species showed clear seasonal changes in abundance, i.e. L. ochotensis, D. atlanticus, and S. gracile larvae mainly occurred during winter— spring; D. garmani and M. japonicus were collected during summer—autumn. No obvious diel vertical migration was found in these larvae. The larvae of D. garmani and M. japonicus were concentrated in the 25–50 and 50–100 m depth layers, respectively. The transforming stage of L. ochotensis, S. gracile, and D. atlanticus occurred at 400–1000 m depth, while their larvae (<8 mm standard length) occurred in the upper 100 m layer, indicating that metamorphosis of these species takes place in the 400–1000 m layer. Based on the occurrence of mesopelagic fish larvae and oceanographic processes in Sagami Bay, with the exception of D. garmani and M. japonicus, most larvae are considered to originate from the Kuroshio region where their main spawning grounds are formed.     

Paradigm shifts in community ecology: Open versus closed units,challenges and limits of connectivity studies

Due to the presence of the complex life cycles involving a benthic adult and a pelagic larval phase, the study of benthic community dynamics cannot ignore investigations of the processes occurring in the water column. Current investigations focus mainly on larval dispersal from an evolutionary and a biogeographic perspective, taking into account also population connectivity, conservation planning and coastal management. In the present paper we underline the need to improve knowledge of the main traits of marine invertebrate life cycles, highlighting the limits and challenges of current approaches. Firstly, we summarized the changing approaches within community studies, following the paradigm shifts found in recent marine ecological research, from supply‐side ecology to connectivity, and involving the concepts of open and closed populations. Secondly, we analysed the main larval traits influencing dispersal, paying particular attention to pelagic larval duration in light of the few available data for connectivity studies. The difficulty in estimating many of the main traits of larval ecology make numerical simulation fundamental for a better understanding of the relationship between propagule dispersal and seawater dynamics, both being highly variable. We conclude that some essential biological information is still lacking for the proper integration of the modeling approaches. Thus it is necessary to further investigate the life‐cycle traits and physiological and ecological characteristics of each species, an approach known as autecology or natural history. All too frequently modern ecologists ignore such reductionist approaches, although they are essential for a full understanding of processes, such as connectivity and metapopulation dynamics.     

Mnemiopsis leidyi (Ctenophora) in Narragansett Bay, 1975–1979: Abundance,size composition and estimation of grazing

Surveys of the distribution, abundance and size of the ctenophore Mnemiopsis leidyi were carried out in Narragansett Bay, R.I. over a 5-year period, 1975–1979. Yearly variations were observed in time of initiation of the ctenophore increase and maximum abundance. Biomass maxima ranged from 0·2 to 3 g dry weight m^?3 at Station 2 in lower Narragansett Bay while maximum abundance varied from 20 to 100 animals m^?3. Ctenophores less than 1 cm in length generally composed up to 50% of the biomass and 95% of the numerical abundance during the peak of the M. leidyi pulse. During the 1978 maxima and the declining stages of the pulse each year, 100% of the population was composed of small animals. M. leidyi populations increased earlier, reached greater maximum abundances, and were more highly dominated by small animals in the upper bay than toward the mouth of the bay. The averageclearance rate of M. leidyi larvae feeding on A. tonsa at 22°C was 0·36 l mg^?1 dry weight day^?1, with apparent selection for nauplii relative to copepodites. Predation and excretion rates applied to ctenophore biomass estimated for Narragansett Bay indicated that M. leidyi excretion is minor but predation removed a bay-wide mean of 20% of the zooplankton standing stock daily during August of 1975 and 1976. Variation in M. leidyi predation at Station 2 was inversely related to mean zooplankton biomass during August and September, which increased 4-fold during the 5-year period.     

The composition of phytoplankton within the Chesapeake Bay plume and adjacent waters off the Virginia coast,U.S.A.

Phytoplankton composition differences were used to identify and plot the extent of the Chesapeake Bay plume over the continental shelf. Distinct but different phytoplankton assemblages were noted within the plume for March, June and October 1980. The major constituents in the plume were Skeletonema costatum, a variety of other diatoms and an unidentified ultraplankton component assumed to consist of several cyanophycean and chlorophycean species. The shelf plankton outside of the plume was mainly characterized by the coccolithophores and other phytoplankters that formed different seasonal assemblages than what was within the plume. Patchiness and fluctuations in the shape, size and extent of the plume were noted. A total of 235 species is given, with their presence within the plume and at shelf stations indicated.     

Distribution, abundance, and growth of larval tautog, Tautoga onitis, in Buzzards Bay, Massachusetts, USA

Tautog, Tautoga onitis, is an abundant species of fish in estuaries of the northeastern United States. Planktonic tautog larvae are abundant in summer in these estuaries, but there is little information on rates of growth of tautog larvae feeding on natural assemblages of food in the plankton. We examined abundance and growth of larval tautog and environmental factors during weekly sampling at three sites along a nearshore‐to‐offshore transect in Buzzards Bay, Massachusetts, USA during summer 1994. This is the first study of a robust sample size (336 larvae) to estimate growth rates of field‐caught planktonic tautog larvae feeding on natural diets, using the otolith daily‐growth‐increment method. The study was over the entire summer period when tautog larvae were in the plankton. The sampling sites contrasted in several environmental variables including temperature, dissolved oxygen (DO), and chlorophyll a concentration. There was a temporal progression in the abundance of tautog larvae over the summer, in relation to location and temperature. Tautog larvae were first present nearshore, with a pronounced peak in abundance occurring at the nearshore sites during the last 2 weeks in June. Larvae were absent at this time further offshore. From late June through August, larval abundance progressively decreased nearshore, but increased offshore although never approaching the abundance levels observed at the nearshore sites. The distribution and abundance of tautog larvae appeared to be related to a nearshore‐to‐offshore seasonal warming trend and a nearshore decrease in DO. Otoliths from 336 larvae ranging from 2.3 to 7.7 mm standard length had otolith increment counts ranging from 0 to 19 increments. Growth of larval tautog was estimated at 0.23 mm·day^?1, and length of larvae prior to first increment formation was estimated at 2.8 mm indicating that first increment formation occurs 3–4 days after hatching at 2.2 mm. Despite spatial and temporal differences in environmental factors, there were no significant differences in growth rates at any of three given sites over time, or between sites. Because larval presence only occurred at a narrow range of temperature (17–23.5 °C) and DO (6.5–9.3 mg·l^?1), in situ differences in growth did not appear to be because of differences in larval distribution and abundance patterns relative to these parameters.     

Occurrence and recruitment of fish larvae in a northern New Zealand estuary

The occurrence of fish larvae and the effect of diel and tidal variation on catches was studied at about biweekly intervals for a year in Whangateau Harbour, a small well mixed northern New Zealand estuary. Larvae from 31 taxa were identified. The annual pattern of larval occurrence was typical for fish in temperate waters, with a major peak of abundance in early summer. For six taxa, larval densities were significantly greater in night-time than in daytime catches, and analysis of length-frequency distributions suggested that for two species this was due to daytime net avoidance.No significant differences were found between the densities of larvae caught on flood and ebb tides, but changes in length-frequency distributions were significant for two species. Recently hatched larvae of an unidentified goby were found leaving the harbour, where they were probably spawned, while older larvae of this species appeared to be recruiting back in. Larvae of the flounder Rhobosolea plebeia were also apparently recruiting into the harbour. In the absence of a two-layered circulation pattern larvae relied upon tidal transport for recruitment, and probaby ensured their retention by rapidly settling to the bottom.     

A sediment budget for the Choptank River estuary in Maryland,U.S.A.

A sediment budget for the Choptank River, one of the three largest estuaries on the eastern shore of Chesapeake Bay, was developed from measurements of sediment carried in upland runoff, shore erosion, sedimentation, and levels of suspended sediments in estuarine waters. Shore erosion was the major source of sediment (340 × 10⁶ kg y^?1), contributing seven times more sediment than upland runoff. Low relief, the rural character of the Coastal Plain drainage basin, and the susceptibility of poorly consolidated shoreline materials to erosion contributed to the dominance of shore erosion over runoff as a sediment source. Box modelling indicated a net annual flux (14–44 × 10⁶ kg y^?1) of sediment from the Choptank River to Chesapeake Bay. A mass balance estimate of sedimentation, calculated as the difference between total inputs and loss at the mouth of the estuary, (350 × 10⁶ kg y^?1) agreed well with an estimate based on ²¹⁰Pb profiles (340 × 10⁶ kg y^?1) measured along the longitudinal axis of the estuary. Lead-210 sedimentation rates correspond to accumulation rates of 1·5–7·9 mm y^?1.     

Geomorphology of a sand wave in lower Chesapeake Bay,Virginia, U.S.A.

Morphologic and sedimentologic studies of a single sand wave within a sand wave field in the lower Chesapeake Bay suggest that the bedform was originally formed by ebb currents, and is presently in static equilibrium with the circulation pattern. In this report, the concept of solitary sand wave is introduced to describe the state of a sand wave when further evolution of the sedimentary structure is mostly independent of adjacent bedforms. This concept can be applied to several bedforms in the area that are isolated from others by flats. A particular sand wave that is included in this category is discussed.Contribution number 79, Instituto Argentino de Oceanografia.     

胶州湾鱼卵、仔鱼和稚鱼的分布

2003年1月-2004年1月在胶州湾海域逐月进行连续13个航次的调查,对胶州湾鱼卵、仔鱼和稚鱼的种类组成、丰度和主要种类的分布进行了研究。结果表明,采集到的鱼卵均为浮性鱼卵。鱼卵丰度最高值出现在8月(5.02粒/m3),次之为5月(4.97粒/m3)。共采集到仔、稚鱼10种,隶属9科10属,优势种为刺鱼叚虎鱼、斑鱼祭和玉筋鱼。根据出现的季节和出现时间的长短,将仔、稚鱼分为暖季类群、暖季短时类群和冷季类群三种类群。暖季类群较冷季类群占有明显的优势。暖季短时类群的种数、丰度和站位出现率都比较低。仔、稚鱼的丰度、种数的季节变化显著,最大值均出现在6月份,丰度是16.34尾/m3,种数为7种,与海水月平均温度的季节变化节律基本一致。     

Population biology of the portunid crab Callinectes arcuatus Ordway in the Gulf of Nicoya,Costa Rica,Central America

Tropical blue crabs Callinectes arcuatus were collected by trawling in the Gulf of Nicoya on the Pacific coast of Costa Rica, Central America. The gulf population was generally dominated by females. Adult females were common in the upper, more estuarine regions of the gulf during rainy season, but appeared to migrate to the lower gulf during dry season for spawning. Biomass varied with seasonal changes in abundance, but was generally similar to biomass of C. sapidus in Chesapeake Bay. Analysis of size frequency indicated that the population is numerically dominated by adults during January and February and that juveniles are common during the remainder of the year. Extrapolation of available data suggests that female crabs reach maturity in approximately one year after hatching.