Small-scale pattern of a California current zooplankton assemblage

The small-scale vertical (10s of meters) and horizontal (100s to 1000s of meters) distribution of zooplankton in the California Current near Guadalupe Island, Baja California, Mexico was studied. Vertical distributions were sampled using a vertically-towed Longhurst-Hardy Plankton Recorder (LHPR) which gave a sequence of samples, each integrated over about 5 m, from 250 m to the surface. Because of the sampling biases of the LHPR, details in vertical structure of less than 15 to 20 m were not considered. The pattern of the vertical tows and the variability of the integrated counts of species were used to infer horizontal distribution. Four series of 8 tows each were taken around noon and midnight over a 2-day period. Four of the 8 tows in each series were randomly positioned within 2000 m of a parachute drogue (first day) or a fixed geographic position (second day); four were replicate tows taken at the drogue or the fixed position. Sixty-seven taxonomic categories were counted. The replicate tows, separated by no more than a few hundred meters, gave more similar vertical profiles for species than did the random tows, with separations of 100s to 1000s of meters. The night replicate tows showed less variability in depth distribution than did either the night random or any of the day tows, leading to the hypothesis that the vertical distributions observed were generated by interactions of the organisms' diel behavior with internal waves. Variability of abundance estimates using the integrated counts was the same for both replicate and random tows, indicating that horizontal patches may be smaller than 100 m. No evidence was found for a day-night change in patch size, or for a consistent overlapping of patches of different species. Replicate tows gave more similar estimates of community structure (relative proportions of species) than did random tows. Overall day community structure was more similar between tows than night structure. Similarities in species' proportions of any random tow to the replicate tows or to other random tows of a series decreased with increasing distance between the tows being compared. This decrease was greater for the night samples, suggesting that community structure is more heterogeneous at night.This study is based in part on a dissertation submitted in partial fulfillment of the requirements for the Ph.D. degree at the University of California, San Diego, and was supported by National Science Foundation Grant GB 12413 and the Marine Life Research Program of the Scripps Institution of Oceanography.     

Larval fishes in the zooplankton community of the North Pacific Central Gyre

Patterns of species structure are described for the larval fish assemblage in the North Pacific Central Gyre. About 30,000 larvae, primarily of mesopelagic fish species, were identified for 7 cruises. Samples were collected using Isaacs-Kidd plankton trawls on 6 cruises over a 4 1/2 year period; stratified samples were taken with an opening/closing bongo net on one late-summer cruise. Data on absolute and relative abundances, size ranges and summertime depth distributions of over 150 species of larvae are presented. The depth distribution and abundance of the ichthyoplankton are compared to those of the total zooplankton community. The larval assemblage is also examined with respect to the known adult mesopelagic fish assemblage. Seasonal changes in ichthyoplankton species structure occur but, within seasons, species structure is remarkably constant from year to year. Despite seasonal changes in species abundance and rank order of abundance, a constant cumulative frequency structure was found to exist in the ichthyoplankton, both between seasons and between years. The implications of this result are discussed in light of similar findings in a terrestrial community and with respect to possible mechanisms of regulation.     

Latitudinal variation in aspects of the biology of Cyclothone braueri and C. microdon (Pisces: Gonostomatidae) in the eastern North Atlantic Ocean

Samples from a series of RMT-8 tows along 20°W in spring and autumn of various years were used to study the variability in aspects of the breeding biology of two species of Cyclothone: C. braueri and C. microdon in the eastern North Atlantic Ocean. Variables compared within each species included length-frequency distribution, maximum size, size at maturity, sex ratio, egg size and fecundity. Populations of both species at lower latitudes were generally smaller, reached maturity at a smaller size, and probably spawned earlier in the year and over a longer period than populations at higher latitudes. In both species, fecundity was highest at 40°N in the spring. The results suggested that C. microdon may not spawn at the northern extent of its range.     

Linking richness, community variability, and invasion resistance with patch size

The influence of community dynamics on the success or failure of an invasion is of considerable interest. What has not been explored is the influence of patch size on the outcomes of invasions for communities with the same species pool. Here we use an empirically validated spatial model of a marine epibenthic community to examine the effects of patch size on community variability, species richness, invasion, and the relationships between these variables. We found that the qualitative form of the relationship between community variability and species richness is determined by the size of the model patch. In small patches, variability decreases with species richness, but beyond a critical patch size, variability increases with increasing richness. This occurs because in large patches large, long-lived colonies attain sufficient size to minimize mortality and dominate the community, leading to decreased species richness and community variability. This mechanism cannot operate on smaller patches where the size of colonies is limited by the patch size and mortality is high irrespective of species identity. Further, invasion resistance is strongly correlated with community variability. Thus, the relationship between species richness and invasion resistance is also determined by patch size. These patterns are generated largely by an inverse relationship between colony size and mortality, and they depend on the spatial nature and patch size of the community. Our results suggest that a continuum of possible relationships can exist between species richness, community variability, invasion resistance, and area. These relationships are emergent behaviors generated by the individual properties of the particular component species of a community.     

Patch study of mortality,growth and feeding of the larvae of the southern gadoid Macruronus novaezelandiae

A discrete dense patch of eggs and larvae of hoki (Macruronus novaezelandiae) within the hoki spawning grounds off Westland, New Zealand, was sampled to examine prey selectivity by larvae and to obtain estimates of larval mortality and growth. The patch was tracked using a free-drifting drogue, and surveys of the horizontal distribution of larvae before and after the patch study indicated that the drogue had successfully followed the patch. Modal analysis of the size-frequency distributions of hoki larvae revealed up to six cohorts within the patch at any one sampling time, and a growth rate of 0.21 mm standard length per day. The daily mortality coefficient for larvae within the patch was 0.19, although this is considered to be an overestimate. Differences in the mean length between cohorts suggest that hoki have a synchronised, diel spawning periodicity, and results of a simple cellular design model revealed that ten continuous days of spawning were required to yield the observed size structure of the hoki larvae population within the patch. Diet analysis of larvae in the patch showed that copepods of the genus Calocalanus are actively selected, and are especially important in the diet of early-stage larvae. Based on aspects of larval diet, morphology, and rates of mortality and growth, it is hypothesised that hoki larvae are adapted to a low-food environment, and that predation is likely to be more important as a source of mortality than starvation.     

Influence of sea surface temperature on reproductive period and size at maturity of brown shrimp (<Emphasis Type="Italic">Farfantepenaeus californiensis</Emphasis>) in the Gulf of California

Reproductive period and size at maturity of the brown shrimp Farfantepenaeus californiensis were analyzed for correspondence to sea surface temperature (SST) in three zones of the Gulf of California. Mature females from fishing areas in the north (Puerto Peñasco), center (Guaymas), and south (Mazatlán), and monthly SST were examined. Average SST for 1983–2000 decreased from Mazatlán (26.2±0.2°C) to Puerto Peñasco (22.6±0.3°C). The seasonal variation in SST between coldest and warmest months was 7.8°C in Mazatlán and 11.4°C in Puerto Peñasco. The size of shrimps at maturity was inversely correlated to SST, increasing from Mazatlán (121 mm total length) to Puerto Peñasco (154 mm total length). The reproductive period near Mazatlán is year-round. Guaymas and Puerto Peñasco have one period of high intensity. We conclude that warmer water and low seasonal variability allow brown shrimp to reproduce more frequently, but reach smaller size at maturity.Communicated by P.W. Sammarco, Chauvin     

Hyperiid amphipods from the zooplankton community of the North Pacific central gyre

The hyperiid amphipods have been examined from an extensive series of depthstratified bongo net tows taken at 28°N; 155°W in the North Pacific central gyre, an open-ocean ecosystem. Seventy-nine samples from 6 depth ranges produced 14, 581 individuals belonging to 13 families, 42 genera, and 83 species. Data on depth distributions, rank orders of abundance, sex ratios, and vertical migration are presented. Most of these species are shown to be reproductively active in the gyre, and there appears to be no seasonality to their reproductive behavior. No species endemic to the gyre are yet known.     

Distribution and structure of heterotrophic protist communities in the northeast equatorial Pacific Ocean

The distribution and structure of heterotrophic protist communities and size-fractionated chlorophyll a were studied during the Korea Deep Ocean Study 98 (KODOS 98) research expedition (July 1998) in the northeast equatorial Pacific Ocean (5–11°N). Areas of convergence and divergence formed at the boundaries of the South Equatorial Current (SEC), North Equatorial Current (NEC), and North Equatorial Counter Current (NECC) during the expedition. Water column physicochemical characteristics significantly influenced the size structure of heterotrophic protist communities. Intense vertical mixing and high nutrient and chlorophyll a concentrations characterized SEC and NECC areas, which were affected by converging and diverging water masses, respectively. Nanophytoplankton dominated in SEC and NECC areas; both areas also had relatively high heterotrophic protist biomasses (average 743 µg C m^–2). NEC areas were characterized by a stratified vertical structure, low nutrient and chlorophyll a concentrations, and picophytoplankton dominance. The heterotrophic protist biomass in NEC areas averaged 414 µg C m^–2; nanoprotists (<20 µm) dominated the community. The nanoprotist biomass comprised 49–54% of the total heterotrophic protist biomass in SEC/NECC areas and 67–72% in NEC areas. The biomass of heterotrophic protists was higher in SEC/NECC areas than in NEC areas, but the relative importance of nanoprotists was greater in NEC areas than in SEC/NECC areas. Heterotrophic dinoflagellates were dominant components of the <20 µm and >20 µm size classes in both water columns. The biomass of heterotrophic protists significantly correlated with the net-, nano-, and picophytoplankton biomass in SEC/NECC areas and with the nano- and picophytoplankton biomass in NEC areas. Heterotrophic protists and phytoplankton also showed strong positive correlation in the study area. The size structure of the phytoplankton biomass coincided with that of heterotrophic protists; the heterotrophic protist biomass positively correlated with the protists prey source. These relationships suggest that the community structure of heterotrophic protists and the microbial food web depended on size classes within the phytoplankton biomass. Microzooplankton grazing and phytoplankton growth rates were higher in SEC/NECC areas than in NEC areas. In contrast, the potential primary production grazed by microzooplankton was relatively high in NEC areas (127.3%) compared with SEC/NECC areas (94.6%). Our results indicate that the relative importance and size structure of heterotrophic protists might vary according to two distinct water column structures.Communicated by T. Ikeda, Hakodate     

Diversity of soft-bottom benthos in a tropical estuary: Gulf of Nicoya,Costa Rica

The community structure of soft-bottom infauna is described for the Gulf of Nicoya, Costa Rica, Central America, with special reference to the relationship between diversity and a tropical estuary. Of the 205 species of invertebrates collected in July, 1980, polychaetes comprised 58.6% by number of species and 68.1% by number of individuals. Density ranged from zero to 8 744 m^-2 with a mean of 1 269 m^-2 per station, and biomass ranged from zero to 10.986 g m^-2 with a mean of 2.010 g m^-2. Numerical analysis indicated considerable faunal homogeneity, characterized by polychaetes. Five major polychaete feeding guilds were recognized. From measures of diversity we concluded that the soft-bottom community of the Gulf of Nicoya was not rich for a tropical estuary. Physical processes (riverine runoff and frontal systems) mainly influenced community structure and function in the lower Gulf whereas biological processes (predation) seemed to prevail in the upper Gulf.Southern California Ocean Studies Consortium Contribution No. 23     

Observations of planktonic bioluminescence in the euphotic zone of the California Current

The distributions of bioluminescence, temperature, salinity, oxygen. pH, and chlorophyll a were measured at 10 m intervals, to a depth of 100 m at a station (33°46N; 119°36W) in the California Current from 17 to 20 July 1982. The distribution of bioluminescence showed a marked day-night change which was consistent over the sampling period. The nighttime maximum was at the surface, and the daytime maximum was between 30 and 40 m. The shapes of the day and night distributions were independent of the absolute intensity of bioluminescence and were also insensitive to advection, as inferred from changing temperature-salinity relationships. The nighttime depth distribution broadened during a period of high wind Day to night differences in the color spectrum at the depth of maximum bioluminescence suggest that the luminescent organisms differed from day to night.     

Fire-climate history and landscape patterns of high burn severity areas on the California southern and central coast

There is mounting evidence that fire size and severity have been growing on the central and southern California coastal landscape over the past several decades. Landsat satellite data was analyzed for the 20 largest fires on the Central California coast since 1984 to determine the relationships between climate/weather conditions at the time of ignition and the size of high burn severity (HBS) areas. The study also examined the relationship between area burned and landscape patterns of HBS coverage, including patch size, edge complexity, perimeter-to-area ratio, and aggregation metrics. Results showed that climate conditions at the time of ignitions have been significant controllers of the total area of HBS and the complexity of HBS patches on the fire landscape. As maximum air temperatures for the month of ignition approached 40^o C, the percentage of HBS to total area burned frequently exceeded 20%. The percentage of HBS to total area burned also exceed 20% when the precipitation total recorded during the previous 12 months was less than 25% of the annual average precipitation. Landscape analysis results showed that, as the total area burned in fires on the Central California coast grows, the edge lengths and areas of HBS patches also grows at a rapid rate. At the same time, the perimeter-to-area ratio of HBS patches decreases gradually and the HBS patches become more aggregated as total burned area grows.     

Feeding of laboratory-reared larvae of the sea bream Archosargus rhomboidalis (Sparidae)

Feeding by larvae of the sea bream Archosargus rhomboidalis (Linnaeus) was investigated from late September, 1972 to early May, 1973 using laboratory-reared larvae. Fertilized eggs were collected from plankton tows in Biscayne Bay, and the larvae were reared on zooplankton also collected in plankton nets. Techniques were developed to estimate feeding rate, food selection, gross growth efficiency, and daily ration. Daily estimates of these were obtained through 16 days after hatching at rearing temperatures of 23°, 26°, and 29°C. Feeding rate increased exponentially as the larvae grew, and increased as temperature was raised. At 23°C larvae began feeding on Day 3, at 26° and 29°C larvae began feeding on Day 2. Feeding rates at initiation of feeding and on Day 16 were, respectively: 23°C, 7.16 food organisms per larva per hour (flh) and 53.78 flh; 26°C, 7.90 flh and 168.80 flh; 29°C, 17.62 flh and 142.07 flh. Sea bream larvae selected food organisms by size. At initiation of feeding they selected organisms less than 100 m in width. As larvae grew they selected larger organisms and rejected smaller ones. The major food (more than85% of the organisms ingested) was copepod nauplii, copepodites, and copepod adults. Minor food items were barnacle nauplii, tintinnids, invertebrate eggs, and polychaete larvae. Mean values for gross growth efficiency of sea bream larvae ranged from 30.6% at 23°C to 23.9% at 29°C. Mean values for daily ration, expressed as a percentage of larval weight, ranged from 84% at 23°C to 151% at 29°C and tended to decline as the larvae grew.This paper is a contribution from the Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, USA     

Effects of patch shape in intertidal algal mosaics: roles of area,perimeter and distance from edge

Shape is considered an important attribute of patches, but little attention has been paid to its influence on the structure and dynamics of rocky shore assemblages. Effects of patch shape were investigated from September 1997 to June 1998 in algal assemblages on intertidal shores at Cape Banks, south of Sydney (NSW, Australia). Three experiments were established, in order to separately investigate the importance of three different characteristics of patch shape, namely area, perimeter and distance of the central zone from the edge. In each experiment, responses for sets of replicate patches that differed on average for only one shape characteristic were compared. Patches of different shapes were produced by scraping off all resident organisms, and subsequent colonisation was recorded. The range of shapes naturally occurring at the study sites was also measured over 1 year, and the shapes of plots used in the experiments were chosen to cover this range of natural variability. Shapes of natural patches within assemblages were very variable. Consistent differences were observed in patch size, geometry and characteristics of the substratum among sheltered and exposed shores and among different times of the year, suggesting that different disturbances influence the dynamics of assemblages at these sites. The results of the three experiments showed that shape may have important effects on patch colonisation. Area, perimeter and distance of the central zone from the edge significantly affected growth of algae in experimental clearings. Responses to different patch characteristics were species specific. Effects of different attributes of patch shape also varied over time. This spatial and temporal variability of responses suggests that colonisation processes of patches of different shapes are complex, probably involving effects that can produce contrasting results. Results are relevant to the design and interpretation of field experiments, as they suggest that the shape of plots may affect the perception of patterns and processes in intertidal assemblages.Communicated by R. Cattaneo-Vietti, Genova     

Re-appraisal of age composition,growth and survivorship of the deep-sea brittle star Ophiura ljungmani from size structure in a sample time series from the Rockall Trough

Samples were taken from a 2 900 m deep permanent station (centred on 54°40N; 12°16W) in the southern Rockall Trough, from November 1975 to September 1980, inclusive. Revision of juvenile size frequencies was necessary after removal of postlarvae of an annual non-viable settlement of Ophiocten gracilis (G. O. Sars) that were mistakenly included in a previous analysis. The present study of the revised data shows that settlement of Ophiura ljungmani (Lyman) occurs in summer when disk-size frequencies are dominated by the newly metamorphosed postlarvae. The likely age composition in larger sizes was suggested from the progression of modes along the size axis in juvenile size classes in consecutive were developed from Gaussian mixtures fitted by constrained computer search using, as starting values, component parameter values suggested from the frequency histograms. Choice between competing solutions was made on the basis both of goodness-of-fit and biological plausibility. However, identification of year-class structure amongst sizes >3 to 4 mm disk diameter remained conjectural owing to the low frequencies of adults in the samples. Comparison of the fitted component-means in the time series suggested a marked seasonal growth pattern, with maximal growth in spring. Probable age composition amongst adult sizes was extrapolated from von Bertalanffy growth parameters estimated from the means of juvenile year classes.Communicated by J. Mauchline, Oban     

Habitat structure and complexity as determinants of biodiversity in blue mussel beds on sublittoral rocky shores

Habitat-forming, ecosystem engineer species are common in most marine systems. Still, much uncertainty exists about how individual and population-level traits of these species contribute to ecosystem processes and how engineering species jointly affect biodiversity. In this manipulative field experiment, we examined how biodiversity in marginal blue mussel beds is affected by blue mussel (1) body size, density and patch context and (2) presence of fucoid and algal structures. In the study area, bladder-wrack (Fucus vesiculosus), filamentous algae and blue mussels (Mytilus edulis) coexist at shallow depths in a variety of patch configurations and offer complex habitats with a high variability of resources. We hypothesized that complexity in terms of mussel bed structure and algal presence determines species composition and abundance. Results from the experiment were compared with macrofaunal communities found in natural populations of both engineering species. Results show that the physical structure and blue mussel patch context are important determinants for species composition and abundance. Results further show that the presence of algal structures positively affects diversity in blue mussel habitats due to increased surface availability and complexity that these algae offer. This study shows that blue mussel beds at the very margin of their distribution have an indisputable function for promoting and maintaining biodiversity and suggest that facilitative effects of habitat-modifying species are important on Baltic Sea rocky shores with fundamental importance to community structure.     

Heat waves,baby booms,and the destruction of kelp beds by sea urchins

Large populations of sea urchins, Strongylocentrotus droebachiensis (Müller), destroyed kelp beds along the Atlantic coast of Nova Scotia in the 1960's and 1970's. The origin of these large sea urchin populations is not understood. We have investigated the potential influence of variable growth and development of the planktonic larvae of sea urchins (in response to temperature and food abundance) on recruitment of benthic juveniles. The adult sea urchins were collected at Sandy Cove, Digby County, Nova Scotia, Canada, in December 1986. Temperature strongly affected larval size and the growth of the echinus rudiment within the range 3° to 9°C, and larvae grew most rapidly at 14°C. Food abundance had a smaller effect on larval growth, and these effects were apparent only at high temperature. Larvae fed the same concentration of two different algal food species grew and developed similarly. Correspondence between spring temperature variation and qualitative variation in sea urchin recruitment, as well as strong temperature effects on larval growth in culture, and the occurrence of a large, positive temperature anomaly in June 1960, all suggest that temperature effects on larval growth and development may have led to intense sea urchin recruitment in 1960 and the appearance of large adult populations 4 to 6 yr later. This result invites further research.     

Succession patterns of phytoplankton blooms: directionality and influence of algal cell size

Using four replicate microcosms in the laboratory, we induced a phytoplankton bloom by enclosing a natural community sampled from Masnou Harbor (N.E. Spain) in November 1987, and examined the pattern of algal succession during the bloom. Good replicability of the temporal patterns of the community biomass and the abundance of most species demonstrated that succession was a directional, non-random process. The successional pathway observed (small flagellates » small centric diatoms » small flagellates) resembled that observed by other authors studying phytoplankton blooms. This pattern differed from previous models of algal succession in that dinoflagellates never comprised a substantial fraction of the community biomass, and in that algal cell size did not tend to increase along the successional sequence. Algal cell size, however, was an important determinant of phytoplankton community structure, since it constrained the density, but not the biomass, achievable by the different species. We suggest that there is not a single, general pattern of phytoplankton succession, but that distinction should be made, at least between seasonal and bloom patterns of phytoplankton succession.     

Relation between surface chlorophyll,integrated chlorophyll and integrated primary production

The relationships between surface chlorophyll and integrated values of chlorophyll and primary production are examined in the California Current and the central North Pacific Ocean. In the California Current, surface chlorophyll is correlated with both integrated chlorophyll and integrated primary production, although there is considerable scatter in the relationships. In the central North Pacific, surface chlorophyll is not correlated with either integrated chlorophyll or integrated primary production. An analysis of closely spaced replicate casts shows that in both areas most of the scatter in the relations between surface values and integrated values is due to spatial or temporal changes in the relations themselves rather than measurement error. The use of surface chlorophyll or fluorescence values as indicators of the biological state of pelagic ecosystems should thus be applied with caution.     

Microplankton population structure in Southern California nearshore waters in late spring

Taxonomic composition, biomass as organic carbon, numerical abundance, and size distribution of the microplankton were determined at 6 Southern California nearshore locations in late May–early June, 1970. Samples were taken at approximately 5 m (10 m at one station) intervals through the upper 40 to 50 m to reveal some of the small-scale differences and levels of variability in the populations. Total microplankton biomass over all euphotic zone samples varied by more than two orders of magnitude (7.6 to 1,200 g C l^-1). Average biomass at comparable sites (n=5) ranged from 48 to 240 g C l^-1; biomass range within stations varied from about 5-fold to 120-fold. Total microplankton numbers varied approximately 22-fold (4.3×10⁵ to 9.5×10⁶ organisms l^-1) over all euphotic zone samples, but the range within stations was always less than an order of magnitude. At comparable stations, nanoplankton biomass had ranges extending from 3.7-fold to 12-fold; its average percentage contribution (±1 SD) to the total microplankton biomass varied from 39±5% to 54±13%. Netplankton biomass showed a similar minimal range, but its greatest range was more than two orders of magnitude. Ranges of abundance of major taxonomic groups within stations varied considerably from about 2-fold to more than three orders of magnitude. The small-scale variability of the populations probably affects the reliability of the microplankton as a food source for pelagic consumers.