Molecular systematics of six Calanus and three Metridia species (Calanoida: Copepoda)

The discrimination of species of the copepod genus, Calanus (Copepoda; Calanoida), is problematical-especially in regions of sympatry. Although the species of Calanus exhibit exceptional morphological similarity, they are quite distinct in genetic character. The DNA base sequences of the mitochondrial large subunit (16S) ribosomal RNA (rRNA) gene unambiguously discriminated C. finmarchicus (Gunnerus 1765), C. glacialis (Jaschnov 1955), C. marshallae (Frost 1974), C. helgolandicus (Claus 1863), C. pacificus (Brodsky 1948), C. sinicus (Brodsky 1965), and C. hyperboreus (Kroyer 1838). Sequence differences among Calanus species for this gene portion range from 7.3% (between C. glacialis and C. marshallae) to 23.9% (between C. glacialis and C. sinicus). Differences among conspecific individuals were approximately 1 to 2%. [These sequence data were determined between April and November 1993; the sequenced domain is similar to that published previously in Bucklin et al. (1992) but are derived from analysis of additional individuals.] Statistical analysis of the sequence data using a variety of tree-building algorithms separated the taxa into one group of species corresponding to the C. finmarchicus group (C. finmarchicus, C. marshallae, and C. glacialis) and another ungrouped set of species corresponding to the C. helgolandicus group (C. helgolandicus, C. pacificus, and C. sinicus). The C. helgolandicus group may be older than the C. finmarchicus group, making the tree topology less reliable in this area. Calanus hyperboreus was an outlier; Nannocalanus minor (Claus 1863) was the outgroup. Similar analysis of Metridia species confirmed that M. lucens (Boeck 1864) and M. pacifica (Brodsky 1948) are distinct species; M. longa (Lubbock 1854) was still more divergent. These sequence data will allow the design of simple, molecular tools for taxonomic identifications. Diagnostic characters, assayed by rapid molecular protocols, will enable biological oceanographers to answer important questions about the distribution and abundance of all life stages (as well as patterns of reproduction) of morphologically similar species, such as those of Calanus.     

Short-term changes in feeding and digestion by the copepodCalanus pacificus

The planktonic marine copepodCalanus pacificus exhibits an enhanced feeding rate, or hunger response, when exposed to food following short periods of starvation. In a scries of laboratory experiments with copepods collected from the main basin of Puget Sound, Washington, during 1982 and 1984, we measured maximum ingestion rate, assimilation efficiency, and digestive enzyme activity to determine the time scales over which the feeding behavior ofC. pacificus responds to increases in food. These laboratory results were then compared to field studies of diel fluctuations in digestive enzymes and gut fluorescence ofC. pacificus in Dabod Bay, a fjord of Puget Sound, during September, 1980, and the closely relatedC. marshallae off the Washington coast, in August, 1981. Laboratory experiments demonstrated that the hunger response ofC. pacificus lasts approximately 6 h before ingestion rate returns to a steady state level of about one-half maximum. On the order of 12h of starvation were required to induce the maximum ingestion rate of the hunger response. Digestive enzyme activities did not change over these time scales. Assimilation efficiency peaked within a few hours of the onset of feeding, with low initial rates possibly related to the period of starvation prior to feeding. These results were consistent with diel patterns observed in the field. The hunger response ofC. pacificus appears to be controlled by processes within the gut, and our results are discussed in relation to recent studies of the digestive processes of calanoid copepods.Contribution No. 1772 from the School of Oceanography, University of Washington, Seattle, Washington 98195, USA     

Life history and population dynamics ofKinbergonuphis simoni (Polychaeta: Onuphidae)

The relationship between life-history characteristics and population dynamics were investigated in the onuphid polychaeteKinbergonuphis simoni (Santos, Day and Rice) between 1982 and 1987. The studied population is located in Upper Tampa Bay, Florida, USA. This worm attains sexual maturity at 5 to 10 mo of age, depending on temperature. Several consecutive broods are produced during a female's life time, with 7 to 26 young per brood. Adults die in June–July after the breeding season. Generations of breeding individuals do not overlap and the replacement of generations occurs in July–August. Only one extended breeding season is experienced per life time. Life span does not exceed 2 yr. Field population density in 1982 was high in fall and spring (2000 to 9000 individuals/m²), low in summer (1600 individuals/m²), and slightly depressed in winter (2500 to 4000 individuals/m²). Changes in population density may be explained by seasonal, temperature-controlled changes in instantaneous birth and death rates. Birth rates are high in fall and spring, while death rates are high in early summer. Death of juveniles contributes to the winter density decrease.     

Seasonal changes in abundance and development of<Emphasis Type="Italic"> Calanus pacificus</Emphasis> (Crustacea: Copepoda) in the Oyashio–Kuroshio Mixed Region

Seasonal changes in abundance and development of Calanus pacificus Brodsky were investigated by analyzing samples of different depth strata (0–150 m and 0–1000 m) collected monthly in the Oyashio–Kuroshio Mixed Region. Copepodite stage 5 (C5) emerged from dormancy and matured in early summer. A new generation appeared in July and developed into C5 during summer to autumn. Some of the summer generation arrested development at C5 and persisted below 150 m depth until the following early summer. Although the remainder matured and reproduced in October, a new generation was not observed at the surface during winter. These results suggest that C. pacificus shows two different life-cycle patterns, i.e. one generation annually, with overwintering C5 in deep waters, and two generations annually, with surface development during autumn to winter. The complex life-cycle patterns may be an adaptation to the highly fluctuating surface environment in the Oyashio–Kuroshio Mixed Region.Communicated by T. Ikeda, Hakodate     

Seasonal polyphenism in life history traits: time costs of direct development in a butterfly

Insects with two or more generations per year will generally experience different selection regimes depending on the season, and accordingly show seasonal polyphenisms. In butterflies, seasonal polyphenism has been shown with respect to morphology, life history characteristics and behaviour. In temperate bivoltine species, the directly developing generation is more time-constrained than the diapause generation, and this may affect various life history traits such as mating propensity (time from eclosion to mating). Here, we test whether mating propensity differs between generations in Pieris napi, along with several physiological parameters, i.e. male sex pheromone synthesis, and female ovigeny index and fecundity. As predicted, individuals of the directly developing generation—who have shorter time for pupal development—are more immature at eclosion; males take longer to synthesise the male sex pheromone after eclosion and take longer to mate than diapause generation males. Females show the same physiological pattern; the directly developing females lay fewer eggs than diapausing females during the first days of their life. Nevertheless, the directly developing females mate faster after eclosion than diapausing females, indicating substantial adult time stress in this generation and possibly an adaptive value of shortening the pre-reproductive period. Our study highlights how time stress can be predictably different between generations, affecting both life history and behaviour. By analysing several life history traits simultaneously, we adopt a multi-trait approach to examining how adaptations and developmental constraints likely interplay to shape these seasonal polyphenisms.     

Longevity of subitaneous and diapause eggs of Centropages hamatus (Copepoda: Calanoida) from the northern Gulf of Mexico

Copepod resting eggs are abundant in the seabed of many bays and estuaries where they provide a potential source of recruits for growth of planktonic populations. In the northeastern Gulf of Mexico the copepod Centropages hamatus (Lillejeborg) occurs in the water column only during the late fall, winter and early spring. The species produces subitaneous and diapause eggs, and both egg types have been found in the seabed. We determined the longevity of these two egg types to ascertain their potential for contributing to the growth of the planktonic population and for sustaining a persistent egg bank. Eggs were collected from females and incubated in the laboratory under temperature and oxygen conditions chosen to simulate field conditions. The diapause eggs were also exposed to sulfide. The total hatching success of subitaneous eggs in two experiments declined from highs of 78 and 97% to zero after 60 and 90 d of exposure to anoxia. The total hatching success of diapause eggs that were exposed to anoxia for 90 d however was typically greater than 80%. Some diapause eggs hatched after being incubated under anoxia for 437 d. Diapause eggs survived longer at ambient field temperatures when incubated under anoxia (437 d) compared to normoxia (118 d). Exposure to sulfide did not result in greater mortality of diapause eggs compared to anoxia alone. Diapause eggs that were incubated at ambient field temperatures did not hatch when exposed to normoxia until the temperature dropped to <20 °C. The results of this study suggest that C.␣hamatus sustain a short-term reserve of subitaneous eggs in the seabed that provides recruits for the current year's population. The greater longevity of diapause eggs suggests that they sustain the seasonal reappearance of the species year after year in the northeastern Gulf of Mexico. However, the contribution of diapause eggs of C. hamatus from the Gulf of Mexico to a persistent egg bank is questionable since hatching ceased after 437 d. Received: 30 July 1997 / Accepted: 18 January 1998     

Preliminary rearing experiments on the larvae of Sergestes lucens (Penaedia,Natantia, Decapoda)

Sergestes lucens Hansen, a mesopelagic shrimp fished commercially in Suruga Bay, Japan, was successfully reared from egg to post-larval stage V under laboratory conditions. Chaetoceros ceratosporum and Artemia nauplii were found to be satisfactory food in the laboratory during rearing. Growth, mortality, food preference, and feeding and swimming activities during the various developmental stages were investigated. Temperature changes greatly affected the speed of development and the mortality of the larvae. The optimum temperature range for larval development was 18° to 25°C. The growth rate (length) of larval stages was as rapid as 0.16mm/ day at 20 °C and 0.21 mm/day at 23 °C. The larvae first started feeding on phytoplankton at elaphocaris stage I, and then gradually became predators in the post-larval stages. It is suggested that the critical period for the species occurs in the elaphocaris stages. Environmental data, vertical distribution of the species, and data obtained from laboratory experiments suggest that the fluctuation in the abundance of S. lucens is greatly influenced by the water temperature at around 50 m from June to August. Feeding mechanisms observed in the post-larval stages are described.     

Calanoid copepod eggs in sea-bottom muds. V. Seasonal changes in hatching of subitaneous and diapause eggs of Tortanus forcipatus

A warm-water neritic calanoid copepod, Tortanus forcipatus, has been found to lay two physiologically different types of eggs (subitaneous and diapause eggs), which are separately spawned in response to different seasonal environmental conditions. Subitaneous eggs are produced when the planktonic populations are at their numerical maxima, and are only shed until late summer. After peak spawning, true resting eggs are spawned which undergo diapause for 1 to 3 months. The eggs recover from their diapause around mid-winter, but hatching does not take place until the water temperature at the sea bottom exceeds 15°C in early summer.     

Physioecology of zooplankton. I. Effects of phytoplankton concentration,temperature, and body size on the growth rate of Calanus pacificus and Pseudocalanus sp.

Changes in dry weight and in weight-specific growth rates were measured for copepodite stages of Calanus pacificus Brodsky and Pseudocalanus sp. cultured under various combinations of phytoplankton concentration and temperature. Mean dry weight of early copepodites was relatively unaffected by either food concentration or temperature, but mean dry weight of late stages increased hyperbolically with food concentration and was inversely related to temperature. The food concentration at which maximum body weight was attained increased with increasing temperature and body size, and it was considerably higher for C. pacificus than for Pseudocalanus sp. This suggests that final body size of small species of copepods may be determined primarily by temperature, whereas final body size of large species may be more dependent on food concentration than on temperature. Individual body weight increased sigmoidally with age. The weight-specific growth rate increased hyperbolically with food concentration. The maximum growth rate decreased logarithmically with a linear increase in body weight, and the slope of the lines was proportional to temperature. The critical food concentration for growth increased with body size proportionally more at high than at low temperature, and it was considerably higher for C. pacificus than for Pseudocalanus sp. Because of these interactions, early copepodites optimized growth at high temperature, even at low food concentrations, but under similar food conditions late stages attained higher growth at low temperature. The same growth patterns were found for both species, but the rates were significantly higher for the larger species, C. pacificus, than for the smaller one, Pseudocalanus sp. On the basis of findings in this study and of analyses of relationships between the maximum growth rate, body size, and temperature from other studies it is postulated (1) that the extrapolation of growth rates from one species to another on the basis of similarity in body size is not justified, even for taxonomically related species; (2) that the allometric model is inadequate for describing the relationship between the maximum weight-specific growth rate and body size at the intraspecific level; (3) that the body-size dependence of this rate is strongly influenced by temperature; and (4) that species of zooplankton seem to be geographically and vertically distributed, in relation to body size and food availability, to optimize growth rates at various stages of their life cycles.Contribution No. 1127 from the Department of Oceanography, University of Washington, Seattle, Washington 98195, USA     

Recruitment,growth, and diel vertical migration of Euphausia pacifica in a temperate fjord

The pelagic crustacean Euphausia pacifica Hansen was sampled with a multiple-sample 1.0 m² Tucker trawl and a multiple-sample 1.0 m² vertical net in Dabob Bay, Washington on 17 dates between May 1985 and October 1987. Size (stage) structure and abundance of the population were determined for each date, while vertical distribution and diel migration were determined for 13 dates. Although internannual variability in both timing and magnitude of events occurred, consistent patterns were discernable. The population produced a large pulse of larvae (2 to 5 mm) in late spring of each year, apparently in response to the vernal phytoplankton bloom. Much lower abundances of larvae occurred during summer and autumn of each year, and larvae were completely absent during winter. Recruitment to the juvenile (6 to 9 mm) and adult (10 mm) stages was strongest during the summer, with abundances of these individuals peaking in summer and autumn. Individual growth rates, determined by modal progression analysis, were calculated for E. pacifica. Rates ranged from zero for some adult cohorts during the winter to 0.12 mm d^-1 for larvae during spring. The latter are among the highest ever reported for this species in the field. The vertical distributions and diel vertical migrations (DVM) of E. pacifica varied seasonally and between size (stage) classes. At night, all size classes were distributed in the surface layer (upper 25 m) irrespective of season or year. During the day, the larger/older stages were always distributed at middepths (50 to 125 m). In contrast, the daytime distribution of the larvae was more variable, being concentrated at the surface during spring and early summer of 1985, and at increasing depths later in the summer and autumn of 1985 and again in spring of 1986. This resulted in invariant DVM in the juveniles and adults, but variable DVM in the larvae, the latter of which is hypothesized to be a response to variable abundances of zooplanktivorous fish.     

Distribution and abundance of eggs of Labidocera aestiva (Copepoda: Calanoida) in the bottom sediments of Buzzards Bay,Massachusetts, USA

Grain-size composition of the sea-bed and density of eggs of Labidocera aestiva in bottom sediments in Buzzards Bay were determined at approximately monthly intervals from March 1983 through April 1984. The results of this study, together with those of Marcus (1984), show that during the fall and winter periods of 1982–1984 the proportion of eggs of L. aestiva occurring in the surficial sea-bottom sediments declined and the proportion of eggs in the deeper sediment layers increased. High positive correlations (r ²=0.72 and 0.92) were observed during the early fall 1983 between egg abundance and the proportion of the total sediments represented by the mud fraction. During late fall, winter, and early spring 1983–1984, the coefficients of determination were much lower. Physical criteria (e.g. sedimentation and transport characteristics) can be used to accurately predict the distribution and abundance of recently spawned eggs on the sea-bottom.     

Photoperiodic regulation of parturition in the self-fertilizing viviparous polychaete Neanthes limnicola from central California

Seasonally-changing photoperiod controls the timing of parturition in the viviparous, self-fertilizing polychaete Neanthes limnicola (Johnson, 1901) from Watsonville Slough, a central California estuary. During 1987 to 1989, worms in the field gave birth mainly in the spring. Those born in late February from field-collected parents and maintained in the laboratory under in-phase photoperiodic conditions reproduced in 12 to 13 mo, under spring light-regimes. When maintained under light conditions 6 mo out of phase,they required only about 6 to 8 mo to reproduce, giving birth in the fall, but under spring light-regimes. Worms born in the laboratory in fall and then maintained in phase reproduced in the ambient spring, at 6 to 8 mo of age; those maintained out of phase took 12 to 13 mo, giving birth the following fall under spring light-regimes. Photoperiod treatments had no consistent effect on the number of young produced, and age and fecundity were only weakly correlated. Highest fecundities were in salinities of 15 to 20,with lower fecundities at higher salinities. Worms maintained in fullstrength sea water(33 S) showed abnormal development and produced very few or no young. Salinity did not affect timing of parturition. Temperature differences of 3 to 7 C° between treatments had no effect on timing of parturition or number of young produced, and marginal effects on life span. These results indicate that photoperiod regulates the timing of reproduction in N. limnicola in central California, while salinity mainly influences fecundity.     

Life cycles ofCalanoides acutus,Calanus simillimus andRhincalanus gigas (Copepoda: Calanoida) within the Scotia Sea

The life cycles and distribution of three dominant copepods,Calanoides acutus, Calanus simillimus andRhincalanus gigas were studied from the Discovery collections in the Scotia Sea earlier this century.C. simillimus is a sub-Antarctic species which mates in the top 250 m mainly in spring. The rapid development of the summer generation may allow a second mating period and a smaller second generation to appear in late summer.C. simillimus remains in the surface layers for a longer period thanCalanoides acutus orR. gigas, and its depth distribution is bimodal throughout the winter.R. gigas is most abundant in sub-Antarctic waters to the north of the Polar Front. It mates within the top 750 m later in spring, and development seems less synchronised than that of the other two species, with egg laying and the growth season being more protracted. Stages CIII and CIV are reached by the first autumn and further development resumes very early the following spring. It is not clear whether the majority then spawn or whether a further year may be needed to complete the life cycle. The predominantly Antarctic species,C. acutus mates below 750 m in middle to late winter and the summer generation develops rapidly to either CIV or CV. Its lifespan seems typically 1 yr, but some of the CVs which fail to moult and spawn in winter survive into their second summer, and their subsequent fate is uncertain. The cold-water speciesCalanus propinquus is comparatively rare in the Scotia Sea and aspects of its distribution and life cycle are briefly described for comparison. Regional variations in the timing of these events were apparent forC. simillimus and possiblyCalanoides acutus, but were not seen inR. gigas. Their geographic and vertical separation, together with their asynchronous life cycles support the concept of habitat-partitioning of these dominant herbivores.     

Seasonal fluctuations of benthic macrofauna in the apalachicola estuary,Florida, USA: The role of predation

Long-term monitoring studies revealed a negative correlation between the abundance of bottom-feeding fishes (Leiostomus xanthurus and Micropogonias undulatus) and benthic macrofaunal density, suggesting a causal relationship. To test this hypothesis, large (3x3 m) topless predator-exclusion cages (mesh opening=6 mm) of a design that produced minimal physical artifacts were constructed at two shallow mud-bottom oligohaline stations. Two-sided control cages and uncaged control areas were also sampled Experiments coincided with maximum abundances of bottom-feeding fishes in the spring and motile macroinvertebrates (penaeid shrimp) in the fall. During short-term (two-month) exclusion experiments in the spring and fall of 1980, large predators (those excluded by 6-mm mesh) were not a major factor in regulating the densities of most macrofaunal species. Predation may be important during other seasons, although this possibility was not tested. Densities of Leiostomus xanthurus and Penaeus setiferus during 1980 were lower than in previous years. Despite the reduced abundance of bottom-feeding fishes, macrofaunal densities still declined during late winter/early spring. It is postulated that increased river flow and lowered salinities during the late winter/early spring may contribute to the decline of macrofaunal densities by reducing larval recruitment.     

Effects of chronic oil pollution on a salt-marsh grass community

A small, naturally defined saltmarsh in Chesapeake Bay (USA) was repeatedly dosed with a No. 2 fuel oil. The production, decay rates, physical characteristics and hydrocarbon content of the dominant grass species present in the marsh were routinely monitored. Spartina alterniflora was the only grass species displaying effects of the oil dosing; this is attributed to the grass's location in the marsh. A substantial portion of the S. alterniflora in the marsh was killed by the oil dosing; the remaining portion evidenced sublethal effects including delayed development in the spring, increased density, and reduced mean weight per stem. The second annual cohort of shoots, usually produced in late summer and early fall, was suppressed almost entirely. Decay rates of S. alterniflora in the oiled marsh were higher and peaked later than decay rates in a control marsh. Oil which entered the roots and rhizomes of dead S. alterniflora was retained in a relatively undergraded state for at least 7 months after dosing stopped.Virginia Institute of Marine Science Contribution No. 945     

Population biology of hyperbenthic crustaceans in a cold water environment (Conception Bay,Newfoundland). I.<Emphasis Type="Italic"> Mysis mixta</Emphasis> (Mysidacea)

Seasonal population dynamics of Mysis mixta Lilljeborg were studied from December 1998 to November 2000 at a 240 m deep site in Conception Bay, Newfoundland. At this depth, temperature was <0°C and salinity between 32.0 and 34.0 psu year-round. The spring phytoplankton bloom began in early or late March and reached a maximum in late April to mid-May. M. mixta exhibited a highly synchronised life cycle, with spawning and mating occurring in October to November, embryos brooded for ~5 months, and juveniles released during spring bloom sedimentation in April and May. Females were semelparous and died at age 2.5 years, following release of juveniles in spring, whereas the majority of mature males died at age 2 years, following mating in November. The biennial life cycle of this population resulted in the presence of two cohorts in the hyperbenthos at any given time. Variation in density and biomass was low among cohorts but high within cohorts, the latter probably due to the high motility of mysids. Densities in 1999 and 2000 were 242±379 and 544±987 ind. per 100 m³ (mean±SD), respectively. Although growth rates were similar between years, rates measured from changes in dry mass differed both seasonally and among life-history stages (range from –4 to 7 mg month^–1). Annual secondary production was estimated at 29–73 mg C m^–2 in 1999 and 53–205 mg C m^–2 in 2000. The annual P/B ratios were 1.62 and 1.19 in 1999 and 2000, respectively.Communicated by J.P. Grassle, New Brunswick     

Life-history analysis in “physiological” compared with “sidereal” time: An example with an amphipod (Gammarus lawrencianus) in a varying environment

The multivoltine, estuarine amphipodGammarus lawrencianus has four generations per year in an environment where temperatures range seasonally from –1° to 25°C. Temperature-response curves for rates of brood production and development were determined by laboratory experiments and field observation. The life history and population dynamics were observed over a full annual cycle (1981) for a field population located at Rocky Run, Porter's Lake, Nova Scotia, Canada. On a natural (i.e., sidereal) time scale, the generations appear to have very different life histories: the two summer generations have short lives, rapid development and mature at small size (classicr-selection), whereas the overwintering generations have relatively low rates of mortality, slow development and mature at large size (classicK-selection). This pattern (larger size at maturity at lower temperatures) is widespread in aquatic poikilotherms. Similar life-history differences are evident among cohorts of the summer generations that mature at different temperatures. When time is expressed on a physiological scale that removes the effect of temperature on embryonic development and reproductive rate, the variation within and among generations is greatly reduced. In particular, an apparent alternation betweenr- andK-selection largely disappears. Because the generations are temporally isolated, it might be surmised that natural selection acting on the summer generations might antagonize the effects of natural selection acting on the fall and winter generations. However, the scaling of the rates of development, maturation, growth, reproduction and mortality on the physiological time scale derived from the temperature dependence of development and reproductive rate gives a very different and more homogeneous pattern.     

Long-term experiments on lifespan, reproductive activity and timing of reproduction in the Arctic copepod Calanus hyperboreus

Several experiments were conducted with starved and fed females of the Arctic copepod Calanus hyperboreus (1) to investigate their lifespan, reproductive period, egg production and egg viability; (2) to study the effect of origin, i.e., Atlantic and Arctic Waters in the Greenland Sea, on the timing of reproduction; and (3) to study the effect of time of collection on the onset of reproductive activity as a first approach to study control mechanisms of the reproductive cycle. Females collected in October produced up to 1,000 eggs and had a maximum lifespan of 164 days without feeding, whereas fed females produced up to ca. 6,000 eggs and survived up to 806 days. These observations support earlier assumptions that females were multiannual-iteroparous, i.e., capable to spawn in successive years, which would be unique for calanoid copepods. In starved females, clutch size decreased significantly with each spawning event. Viable eggs were produced during most of the life time. There was no difference in the timing of reproductive activity between females from the West Spitsbergen Current and the Greenland Sea Gyre. Fed and starved females collected in May and June began to spawn circa 2 and 4 months after collection, respectively, whereas females collected in August and October started spawning at the same time, in the middle of October. This indicates initiation of reproductive activity in the field in August, coincident with the descent into deep waters. Potential cues for the untimely spawning of females collected in spring and ‘unnatural’ feeding in fall experiments are discussed. Their large size, robustness and combination of different types of diapause in their life cycle make C. hyperboreus a good model organism to study diapause control mechanisms.     

Diel feeding behavior of neritic copepods during spring and fall blooms in Akkeshi Bay,eastern coast of Hokkaido,Japan

In situ diel feeding behavior of neritic copepods was investigated using the gut fluorescence method, during spring and fall bloom periods in Akkeshi Bay, on the eastern coast of Hokkaido, Japan. Acartia omorii and Paracalanus sp. were the dominant species during the fall, and Pseudocalanus spp. and A. longiremis during the spring. During both bloom periods, diel rhythms were always observed for the gut pigment contents of these dominant copepods, although there were interspecific differences in the pattern. The maximum gut pigment content was always observed during the night and the minimum during the day. For all species, except Paracalanus sp., the average gut pigment content during the night was significantly higher (p<0.05) than during daytime by factors of between 1.5 and 2.7. There were no significant differences between the gut evacuation rate constants determined during the day and the night, and initial gut pigment content had no effect on the value of gut evacuation rate constants. The instantaneous ingestion rates of individual copepods calculated from gut pigment and the mean value of gut evacuation rate constants followed the same diel rhythms as gut pigment contents. Copepod daily ingestion rates were higher than the daily requirements for respiration during both bloom periods. Estimated daily ration was 40 to 91% of body carbon during the fall bloom, and 17 to 28% during the spring bloom. The higher daily rations during fall were probably due to the difference in in situ temperature (ca. 14°C).     

Seasonal variations in Delaware Bay phytoplankton community structure

Whole-water phytoplankton samples were obtained from 3 stations in Delaware Bay on a once- or twice-monthly basis from June 13, 1974 to May 28, 1975. The flora was composed primarily of small flagellates during the summer and early fall, while diatoms dominated from October to May. Peak cell numbers occurred during fall and early spring blooms. Evenness diversity was lowest during periods of maximum diatom abundance and highest when microflagellates predominated. There was a gradual shift in dominance, except during the early spring Skeletonema costatum bloom. Cluster analysis allowed the separation of the flora into 3 time groups and 8 recurrent species groups. The species composition and community structure of Delaware Bay phytoplankton is compared with other USA east-coast estuaries where comparable sampling techniques have been used.Contribution No. 129 from the Ira C. Darling Center and No. 129 from the College of Marine Studies.