Hyperiid amphipods (Crustacea: Peracarida) in relation to a cold-core ring in the Gulf of Mexico

The species composition, distribution, and abundance of the hyperiid amphipods collected in March 1993 across a Gulf of Mexico cold-core ring (CCR) were analyzed. Day and night samples were collected by oblique tows (100 m to surface) with a plankton net. Hyperiids were represented by 56 species, 21 of which have not been recorded previously in gulf waters. The local oceanic community differs from that reported from adjacent neritic and neritic-oceanic areas of the Northwestern Tropical Atlantic. Overall, hyperiids were more abundant within (59% of total catch in org./1000 m³) than outside the CCR (41%). All inside CCR stations were sampled at night. Night outside vs. night inside CCR hyperiid faunas showed important differences in terms of species richness, composition and density. Cluster analysis indicated that day sta. 5 on the edge but outside the CCR was more similar to those stations inside the CCR (nighttime samples) than to the other daytime samples. Moreover, all the stations outside the CCR were clustered together independently of their day or night origin. It is suggested that the differences found were more related to differential conditions related to the CCR than to diel vertical migration. The analysis of three congeneric pairs with inverse CCR-related abundance and with known or inferred migratory patterns strengthened the idea that these pairs are probably separated by thermal preferences; also, their vertical migratory patterns seem to be abnormal inside the CCR. A relatively higher concentration of immature stages inside the CCR supports the idea that the enriched CCR waters constitute areas of increased production. Furthermore, this higher productivity enhances the chances of hyperiids to find their hosts, the gelatinous zooplankters, which are also more abundant inside the CCR.     

Distribution of cephalopod paralarvae in relation to the regional oceanography of the western Iberia

The present study analyses the distribution of cephalopod paralarvaeoff the Portuguese coast. The effects of temporal and physicalvariables on Loligo vulgaris, Octopus vulgaris, sepiolid andommastrephid abundances are analysed with generalized linearmodels. Their distribution patterns are discussed in relationto mesoscale features, including currents, thermal fronts andcoastal upwelling cross-shelf transport, prevailing in the westernIberia upwelling system. Paralarvae of the neritic species occurduring a considerably extended period of the year with two orthree abundance peaks within the highly productive upwellingsystem of the western Portuguese coast and contrasting withthe Gulf of Cadiz area. Temperature and upwelling were shownto be the most important variables in modulating seasonalityand distribution of these paralarvae. The influence of the physicalenvironment is particularly pronounced for the paralarvae ofO. vulgaris, following distinct patterns according to the oceanographyof the western Iberia and the Gulf of Cadiz systems. The paralarvaeof oceanic species, which in many cases have their northernlimit of distribution at these latitudes, were mainly foundin the southern part of the sampling area. The distributionof these species indicates that the prevailing oceanographicfeatures of the Gulf of Cadiz system, especially fronts, togetherwith temperature act as boundaries to geographic dispersal,contributing to an area of high cephalopod biodiversity in thesouthern Portuguese waters.     

The relationship between community respiration and ETS activity in the ocean

We have studied the relationship between community respiration(R) and enzymatic activity of the electron transport system(ETS) in upper ocean microbial communities (<225 µm)from different oceanic regions. In all except one of the regions,R and ETS were significantly positive correlated. This supportsthe hypothesis that ETS can be widely used to estimate planktonrespiration in natural marine communities (Packard, T.T., Adv.Aquat Microbiol, 3,207–261, 1985). A regression equationwas obtained between all the R and ETS data studied, to deriverespiration from ETS activity. This equation yields a mean errorin the prediction of ±34%, similar to the errors obtainedapplying the equations at each area, but lower than the errorobtained when using the mean R:ETS ratio to determine respiration(±45%). Our results suggest that the use of the ETS-Ralgorithm, along with measurements of ETS activity in seawater,facilitates the estimation of seawater respiratory oxygen consumptionon the mesoscale. This means that by using this approach onecould extend our knowledge of oceanic respiration over largetemporal and spatial scales, and begin to use respiration, notonly productivity, in addressing carbon balance problems inthe upper ocean.     

Variability in the macrozooplankton community off the Antarctic Peninsula

Summary This paper presents an analysis of zooplankton net sampling surveys carried out during four expeditions to the Antarctic Peninsula region. Cluster analysis documented two to three site groupings for the epipelagic zone and one mesopelagic site cluster below 200 to 300 m depth. Analysis of species dominance, constancy, diversity and evenness indices did not allow clear designation and separation of communities in terms of these parameters.Computation of a rank correlation matrix for each season allowed the characterization of species groups. There were no perfect indicator species in the very strict sense. The main differences in the composition of the zooplankton between the site clusters were due mainly to changes in abundance rather than to presence or absence of particular species. However, the interpretation of the complex species and site groupings led to the conclusion that we can define three distinct communities: an oceanic, a neritic, and a mesopelagic community beneath 200 to 300 m. A so-called transitional cluster represents a mixing zone created by frequent occurrence of species from both the oceanic and neritic community. The location of the described oceanic and neritic community sites seem to be relatively stable with minor latitudinal changes during the seasons, while occurrence and abundance of most species changes with the time of the year. The usefulness of particular species (e.g. Euphausia superba) as indicator species also change during the year     

Zooplankton community structure and copepod egg production in coastal waters of the central Great Barrier Reef lagoon

We describe zooplankton community structure and copepod eggproduction in the vicinity of the coastal boundary zone of theGreat Barrier Reef lagoon, Australia. The abundance and eggproduction rate of constituents of the zooplankton assemblagecharacteristic of the coastal zone rapidly increase subsequentto events such as flooding and upwelling. Our sampling spannedtwo summer monsoonal seasons, the first of which (1990-91) wasvery wet. The second monsoonal season (1991-92) was very dryand was characterized by intrusive upwelling events from theCoral Sea. Chlorophyll a concentrations did not rise in thewet year, probably because of light limitation, but did riseas a result of upwelling. Terrestrial run-off in the wet yearhad a greater apparent effect on zooplankton abundance patternsthan did upwelling in the dry year, except where coastal trappingallowed sufficient time for increases in zooplankton abundanceto occur. Egg production rates by the copepods Acrocalanus gibberand Acrocalanus gracilis showed haphazard spatial differences.Nitrogen-specific egg production ranged between 0.03 and 0.21day^–1 for A.gibber, and between 0.13 and 0.41 day^–1for A.gracilis. The egg production rate by A.gibber was foodlimited for most of the year and showed a poor correlation withtemperature. ³Present address: Department of Biological Sciences, FloridaTech, 150 W University Boulevard, Melbourne, FL 32901, USA     

Inter-annual variation in summer zooplankton community structure in Prydz Bay, Antarctica, from 1999 to 2006

Inter-annual variations in zooplankton community structure in Prydz Bay were investigated using multivariate analysis based on samples collected with a 330-μm mesh, 0.5-m² Norpac net during the austral summer from 1999 to 2006. Two distinct communities, an oceanic and a neritic community, were consistently identified in all surveys. Oceanic communities had higher diversity and were indicated by species such as Haloptilus ocellatus, Heterorhabdus austrinus, Thysanoessa macrura, Rhincalanus gigas, Scolecithricella minor and Oikopleura sp.. Neritic communities were indicated by Euphausia crystallorophias and Stephos longipes and were characterized by fewer but more abundant species. In 1999 and 2006, a transitional community was also distinguished near the continental shelf edge, where ice coverage was more extensive than either the oceanic or neritic regions. Significant inter-annual variations in community structure (mainly involving species abundance rather than species composition) were found in both oceanic and neritic communities, being more obvious in the latter. The timing and amplitude of sea ice retreat (polynya appearance), and its effect on food availability, had strong influences on zooplankton community structure. In oceanic communities during years with earlier ice retreat, the extra time available for phytoplankton blooms to accumulate resulted in a higher proportion of large copepods (Calanoides acutus, Calanus propinquus, Metridia gerlachei) (especially the younger copepodites) in the zooplankton assemblage. In neritic communities, zooplankton such as the ice krill E. crystallorophias, and large copepods (C. acutus, C. propinquus, M. gerlachei), also showed higher abundance and earlier developmental stages in years with larger polynya. On the other hand, in years with later ice retreat, smaller polynya, and less time for phytoplankton blooms to form, the abundance of large copepods was lower and older age classes were more common.     

Mesozooplankton community indicates climate changes in a shelf area of the inner Bay of Biscay throughout 1988 to 1990

The mesozooplankton community was monitored at both coastaland offshore sites of the Basque shelf (inner Bay of Biscay)during 1988–1990 This was considered of interest becausethe local climate change toward dry conditions was found tobe responsible for changes in shelf-water properties. In additionto seasonal changes, interannual variations in mesozooplanktonabundance and composition were evident, and primarily relatedto the year-to-year increase in water temperature and salinity.The increasing trends in copepod dominance, coupled with theincreasing abundance of species with oceanic affinity (e.g.Euchaeta hebes), were presumably a response to the increasein oceanic features in the shelf area. In the same way, thesubstantial year-to-year increase in the abun dance of specieswith summer-autumn development was attributed to the progressivewarming. Among these species, the case of Temora stylifera isdiscussed specifically because this species was very scarcein 1988, but showed unusual high abundances for the study area,and dominated the summer-autumn assemblage in 1990. We thinkthat T.stylifera can be a key species in monitoring climate-oceanologicalchanges affecting the Bay of Biscay. Results are also discussedin the context of the long climate-oceanological series forthe Bay of Biscay, and its biogeographical status.     

Essential krill species habitat resolved by seasonal upwelling and ocean circulation models within the large marine ecosystem of the California Current System

Due to their global distribution, high biomass and energy content, euphausiids (krill) are important prey for many mid and upper trophic level marine organisms. Understanding drivers of krill habitat is essential for forecasting range shifts, and to better understand the response of krill predators to climate change. We hypothesized that the distribution and abundance of krill species derived from ecosystem surveys in spring/summer relates to geomorphic features, coastal upwelling during the preceding winter, and spring mesoscale oceanographic conditions. To test this hypothesis, we used boosted regression trees with environmental data and ocean model output to quantify the habitat associations of two primary krill species (Euphausia pacifica and Thysanoessa spinifera) in the central California Current Ecosystem from 2002 to 2018. Models confirmed the neritic distribution of T. spinifera and pelagic, outer slope association of E. pacifica (deviance explained ~35%). Distribution of these species were influenced by depth and bottom rugosity; chlorophyll-a concentrations and increased winter upwelling conditions; and spring surface currents and wind stress. Thysanoessa spinifera and E. pacifica abundance responded negatively (positively) to warm (cold) climate events, confirming known relationships. As an independent evaluation of krill models, observations of krill predator (seabirds, marine mammals) distribution indicated they were present within habitats of predicted high krill species abundance. Our framework indicates species-specific habitat relationships for these foundational forage species and their negative response to large-scale climate variations, such as El Niño and marine heatwave conditions. The approach can be easily transferred to other ecosystems or krill species that respond to similar ocean and climate forcing.     

Zooplankton annual cycle in a Mediterranean coastal area

The annual cycle of the zooplankton community in the SaronikosGulf (Aegean Sea, Greece) was studied over a period of 2 years.The sampling scheme included monthly hauls at three stationsdifferentiated according to depth and neritic character. Maximumvalues of total zooplankton abundance were found in summer monthsup to early autumn and partially in spring. Copepods dominatedduring most of the year (Clausocalanus furcatus, Paracalanusparvus, Temora stylifera, Ctenocalanus vanus, Oithona similis,Oithona plumifera), while cladocerans (Penilia avirostris) wereabundant in summer months and in September. No important fluctuationswere detected between the 2 years of study, while monthly oneswere more significant in the more neritic station during thewinter–spring period. Correspondence analysis showed thatthe seasonal evolution of zooplankton is related to environmentalparameters: temperature, hydrography expressed in open sea influenceand topography.     

Day/night differences in the grazing impact of marine copepods

Day/night differences in the removal rate of phytoplankton can occur as a result of increased copepod grazing rates at certain times of the day and diel vertical migration of animals. We conducted shipboard grazing experiments and fine-scale vertical zooplankton sampling to resolve these behaviors. Day/night feeding differences were compared in the center of several warm-core Gulf Stream rings, under conditions of no lateral water mass exchange, in the mesohaline portion of Chesapeake Bay and when following drogues in the Chesapeake Bay plume. Day/night variations in copepod biomass in the surface mixed layer were greater in neritic waters as compared to the open ocean stations. Day/night differences in weight-specific copepod filtration rates varied less than biomass. At the neritic stations copepod grazing was often higher at night, whereas at the oceanic stations day/night grazing rates were similar or daytime grazing rates were highest. The night/day ratio of zooplankton grazing impact on the phytoplankton community (the product of zooplankton biomass and their weight-specific grazing rate) averaged 4.8 in the Chesapeake Bay plume and 1.6 in warm-core Gulf Stream rings. Our results suggest that at lower food levels, there often are less day/night differences in the removal rate of phytoplankton by the copepod community.     

Distribution of plankton related to the mesoscale physical structure within the surface mixed layer in the southwestern East Sea, Korea

This study investigated the influence of mesoscale physicalfeatures on plankton distribution, for the first time, in thesouthwestern East/Japan Sea using hydrographic data obtainedin November 2000 and April and October 2001. Three differenttypes of mesoscale physical features were observed in the upperlayer during the three survey periods: (i) the southern frontbetween the East Korean Warm Current (EKWC) and the UlleungWarm Eddy (UWE) (November 2000), (ii) the central part of theUWE with no southern front (April 2001) and (iii) the strongEKWC and related subpolar front (October 2001). The chlorophylla (Chl a) concentration and zooplankton abundance in the surfacemixed layer were affected by the UWE-related nutrient upwellingin the southern front in November 2000 and in the central partof the UWE in April 2001. Further north, the zooplankton abundancewas high around the subpolar front in October 2001, while theChl a concentration was high in the southern part. The spatialheterogeneity of the Chl a was affected by variation of netplanktonabundance, whereas seasonal differences of the Chl a concentrationdepended on the picoplankton. Results of cluster and principalcomponent analysis show that the zooplankton-distribution patternswere associated with the Chl a concentration, water density(     

Diel cycle in the percentage abundance of parthenogenetic females with embryos of different developmental stages in four species of marine cladocerans

The diel cycle in the percentage abundance of parthenogeneticfemales carrying embryos of different developmental stages inthe marine cladocerans Penilia avirostris, Pseudevadne tergestina,Pleopis polyphymoides and Pleopis schmackeri was studied atthe edge of an artificial rocky shore area in a semi-enclosedbay in Hong Kong. Females carried embryos during both day andnight, but females with fully-developed embryos were found predominantlyat night. The diel cycle in the abundance of females with matureembryos was most pronounced in P. tergestina, and less prominent,but still clearly noticeable in P. avirostris, P. schmackeriand P. polyphymoides. The absence or scarcity of females withmature embryos during daytime could be caused by both selectivepredation by visual predators and nocturnal maturation and releaseof embryos. Juveniles of Acanthopagrus schlegeli (black seabream)were the most abundant planktivorous fish in the study areain spring. Stomach content analyses revealed that these daytimepredators fed intensively on marine cladocerans and exhibiteda strong selection for females with mature embryos. On the otherhand, the gradual decline in the percentage of females withadvanced embryos during the latter part of the night, when feedingby visual predators presumably had not yet begun, suggests thatthere was a tendency for nocturnal release of neonates in marinecladocerans.     

Small-scale distribution of a cnidarian population in the western Mediterranean

The neritic area of the coast of Catalonia (north–westMediterranean) has a continental shelf narrower in the northernhalf and broader in the southern half. This area is crossedfrom north to south by a coastal current whose intensity decreasesto the south. Both conditions, together with other local geographicaland hydrographical features, are thought to be the key factorsin determining the distribution of planktonic cnidarian assemblages,along two main variational axes: a north-south axis and a coast-opensea axis. Aiming to find support to this hypothesis, the planktoniccnidarians collected in two seasons of the year 1983 have beenstudied. The first season (April–May) corresponds to thehighest abundance and heterogeneity, and the second (September–October),to the epoch when the cnidarian population is lower, but constant,along the coastal area. In a global principal component analysisthe factor that best explains the group distribution in thearea is the coastal character, as given by the bottom depthof the sampling station. The number of individuals is greaternear the coast (due to the dominance of a euryhaline species,Muggiaea atlantica), but the species number increases towardsthe open sea (due to the addition of more oceanic species, suchas Narcomedusae and Trachymedusae and the effect of a permanenthydrographic front at the shelf-slope boundary). The secondfactor is the latitudinal axis. The northern half of the coastis under the influence of waters favouring a greater numberof species and individuals. On the contrary, the southern halfshows a less marked influence of the north–south current,their cnidarian assemblages being more constant over time. Althoughthe global cnidarian population follows this general pattern,the most abundant and frequent species (M.atlantica and Aglaurahemistoma) show a broad distribution over all the studied areabecause they are rather insensitive to hydrographical variations.     

The summer zooplankton community at South Georgia: biomass,vertical migration and grazing

Zooplankton abundance and biomass were determined during January 1990 at two stations to the north-west of South Georgia using a Longhurst Hardy Plankton Recorder (LHPR). At both shelf and oceanic station sites, zooplankton biomass, (excluding Euphausia superba), was found to be ca. 13 g dry mass m^–2. Copepods and small euphausiids dominated the catches. These estimates are over 4 times higher than values generally reported for the Southern Ocean and may reflect firstly, the high productivity of the study area, secondly, the time of year, summer, when biomass for many species is maximal, and thirdly, the high sampling efficiency of the LHPR. Principal components analysis disclosed similarities and differences between adjacent depth strata in terms of abundance, biomass and species composition. At both stations most variability occurred in the mixed layer (0–60 m) and thermocline (60–120 m) with depth horizons below this being more homogeneous. Diel migrations were observed for most taxa with abundance increasing in the mixed layer at night. At the oceanic station, species and higher taxa belonging to the mesopelagic community were generally well spread throughout this domain and, with the exception of Pleuromamma robusta and Metridia curticauda, showed little evidence of migration. The grazing impact of the epipelagic community (copepods and small euphausiids) was estimated to remove 3–4% of the microbial standing stock day^–1 and a conservative 25% and 56% of daily primary production at the oceanic and shelf stations respectively.     

Aggregation of avian predators and zooplankton prey in Otago shelf waters, New Zealand

In Otago shelf waters surface swarms of krill (Nyctiphanes australis),hyperiid amphipods (Parathemisto spp.) and galatheid crab larvae(Munida gregaria) provide an abundant summer food source forplanktivores. We tested the hypothesis that aggregation of avianplanktivores depends upon the spatial distribution of theirprey. Gulls (Larus scopulinus, L.bulleri, L.dominicanus), sootyshearwaters (Puffinus griseus) and white-fronted terns (Sternastriata) showed significantly aggregated distributions. Thedistribution of birds sitting on the sea surface was correlatedwith the abundance pattern of krill but was not correlated withthe distribution of smaller hyperiid amphipods. The distributionof flying red-billed gulls (L.scopulinus), black-billed gulls(L.bulleri) and sooty shearwaters was correlated with the krilldistribution but black-backed gulls (L.dominicanus) were not.Stomach contents of black-billed gulls were dominated by krill,in contrast to sooty shearwaters, which ate a higher proportionof Minida, and black-backed gulls, which contained Munida andfish. No amphipods were found in bird stomachs. There was nosignificant correlation between bird distributions and the hydrographicregime, water depth or distance offshore. The distribution ofprey rather than hydrographic regime was a more important determinantof bird distributions at this spatial scale (2.6–12 km)and location.     

Individual foraging site fidelity in lactating New Zealand fur seals: Continental shelf vs. oceanic habitats

Wide‐ranging marine central place foragers often exhibit foraging site fidelity to oceanographic features over differing spatial scales (i.e., localized coastal upwellings and oceanic fronts). Few studies have tested how the degree of site fidelity to foraging areas varies in relation to the type of ocean features used. In order to determine how foraging site fidelity varied between continental shelf and oceanic foraging habitats, 31 lactating New Zealand fur seals (Arctocephalus australis forsteri 1 ) were satellite tracked over consecutive foraging trips (14–108 d). Thirty‐seven foraging trips were recorded from 11 females that foraged on the continental shelf, in a region associated with a coastal upwelling, while 65 foraging trips were recorded from 20 females that foraged in oceanic waters. There were no significant differences in the mean bearings (to maximum distance) of individual's consecutive foraging trips, suggesting individual fidelity to foraging areas. However, overlap in area and time spent in area varied considerably between continental shelf and oceanic foragers. Females that foraged on the continental shelf had significantly greater overlap in consecutive foraging trips when compared to females that foraged in oceanic waters (overlap in 5 × 5 km grid cells visited on consecutive trips 55.9%± 20.4% and 13.4%± 7.6%, respectively). Females that foraged on the continental shelf also spent significantly more time within the same grid cell than females that foraged in oceanic waters (maximum time spent in 5 × 5 km grid cells: 14%± 5% and 4%± 2%, respectively). This comparatively high foraging site fidelity may reflect the concentration of productivity associated with a coastal upwelling system, the Bonney Upwelling. Lower foraging site fidelity recorded by seals that foraged in oceanic waters implies a lower density/larger scale habitat, where prey are more dispersed or less predictable at fine scales, when compared to the continental shelf region.     

Euphausiids (Crustacea: Melacostraca) from the southern Mexican Caribbean Sea

The composition, abundance and distribution of euphausiids from the southern part of the Mexican Caribbean Sea (August 1986) were sampled during the ARCOMM I cruise on board the R/V "Justo Sierra" of UNAM. Sampling was done using a Bongo-net (0.5 mm mesh size) performing oblique hauls from 200 m to the surface at 28 stations. Of the total euphausiid numbers, the most abundant species was Stylocheiron carinatum (49%), followed by Euphausia americana (9.8%) and E. tenera with (7.8%). The highest total density and that of the three most abundant species occurred during the night sampling, and probably was related to vertical migration patterns. The Bray-Curtis index revealed three station assemblages, related to the day-night variations of the euphausiid community. Species were distributed mainly in the oceanic area, and were absent in the neritic zones. The local fauna shows a strong affinity for the euphausiid community of the Gulf of Mexico and other areas of the Caribbean Sea. Four species are considered to be new records for the western Caribbean Sea.     

Across-shelf predatory effect of Pleurobrachia bachei (Ctenophora) on the small-copepod community in the coastal upwelling zone off northern Chile (23{degrees} S)

To estimate the predation effect of the predominant ctenophorePleurobrachia bachei on the small-copepod community in the upwellingarea off Mejillones (23°S), northern Chile, a series ofoceanographic cruises and predation experiments were conductedin the austral springs 2000, 2001 and 2002. The daily consumptionrates and predatory effect of P. bachei on the small copepods(in terms of % of standing stock and biomass removed daily)were determined at three stations located in relation to theshelf-break (coastal, shelf-break and oceanic) reaching valuesup to 4.5% per day of the <1500 µm copepod standingstock. Our results indicate that the ctenophores were most abundantat the coastal station, that small copepods dominated the copepodcommunity (being more abundant nearshore), and that the relativefrequency of ctenophores with copepods in their guts was alsohigher near the coast. The predatory effect of P. bachei onthe small-copepod community was also higher in the coastal zone.However, the effect of this predation on the copepod biomassin terms of carbon did not decrease steadily seawards, whichmay be due to the larger sized copepods consumed at the offshorestations. Determinations of predatory effect on the secondaryproduction of the more abundant small-copepod populations (i.e26% daily in 2000) suggest that this single species of Pleurobrachiais modulating the population growth rate of the small copepods,the copepod community size structure, and maybe even the alternanceof key species in the Mejillones coastal upwelling zone.     

Seasonal abundance and vertical distribution of mesopelagic calycophoran siphonophores in Monterey Bay, CA

The seasonal abundance and vertical distribution patterns ofa group of small calycophoran siphonophores (principally Chuniphyesmultidentata and Lensia conoidea) were investigated using aremotely operated vehicle (ROV) deployed in Monterey Bay, California.Abundance was assessed along 295 horizontal transects coveringa depth range of 100–1000 m over a three and a half yearperiod. The vertical distribution of the study animals changedseasonally, coupled to the onset and cessation of upwellingin the bay. While numerical abundance peaked after upwelling,there was no significant difference between seasons. The siphonophoreswere more broadly distributed over the depth range sampled duringthe upwelling or Shallow Mixed Layer (SML) period, than duringthe non-upwelling or Deep Mixed Layer (DML) period. There wereno significant differences in abundance or distribution patternsbetween years except in 1993, when there were significantlymore siphonophores observed during the SML period than duringthe DML period. This may reflect effects resulting from the1992–1993 El Niño event. The abundance of thesesiphonophores was negatively correlated with that of Nanomiabijuga, a physonect siphonophore of similar size and feedingbehavior found in the bay. The siphonophores studied here appearfrom preliminary data to migrate vertically, possibly with twoseparately migrating groups.     

Scaling-up from nutrient physiology to the size-structure of phytoplankton communities

In many community assemblages, the abundance of organisms isa power-law function of organism size. In phytoplankton communities,changes in size structure associated with increases in resourceavailability and total biomass have often been interpreted asa release from grazer control. A metapopulation-like approachis used to scale up from the individual physiological responsesto environmental conditions to community size structure assumingthe community taxonomic composition reflects the species pool.We show that the size scaling of cellular nutrient requirementsand growth can cause (1) the power-law relationship betweencell size and abundance, (2) dominance of small phytoplanktoncells under oligotrophic conditions, and (3) relative increasein abundance of larger phytoplankton cells under eutrophic conditions.If physiological differences associated with the taxonomic compositionof different community size fractions are considered, then themodel can replicate detailed field observations such as theabsence of small, slow-growing Prochlorococcus spp. and therelative dominance of large diatom species in nutrient-rich,upwelling regions of the ocean. This paper was presented in a session on "Size Structure ofPlankton Communities", at the ASLO Summer International Meeting,held in Santiago de Compostela, Spain, between 19 and 24 June,and coordinated by Xabier Irigoien, Roger Harris and Angel Lopez-Urrutia.