Seasonal Dynamics of Phytoplankton and Planktonic Protozoan Communities in a Northern Temperate Humic Lake: Diversity in a Dinoflagellate Dominated System

Species diversity and richness, and seasonal population dynamics of phytoplankton, planktonic protozoa, and bacterioplankton sampled from the epilimnion of Crystal Bog in 2000, were examined in order to test the hypothesis that these groups diversity and abundance patterns might be linked. Crystal Bog, a humic lake in Vilas County, Wisconsin, is part of the North Temperate Lakes Long-Term Ecological Research Site. Phytoplankton and planktonic protozoa were identified and enumerated in a settling chamber with an inverted microscope. Bacterial cells were enumerated with the use of fluorescence 4, 6-diamidino-2-phenylindole (DAPI)-staining procedures, and automated ribosomal intergenic spacer analysis (ARISA) was used to assess bacterioplankton diversity. Bacterial cell counts showed little seasonal variation and averaged 2.6 × 10⁶ cells/mL over the ice-free season. Phytoplankton and planktonic protozoan numbers varied by up to two orders of magnitude and were most numerous in late spring and summer. Dinoflagellates largely dominated Crystal Bog throughout the ice-free period, specifically Peridiniopsis quadridens in the spring, Peridinium limbatum in summer, and Gymnodinium fuscum and P. quadridens in fall. Brief blooms of Cryptomonas, Dinobryon, and Synura occurred between periods of dinoflagellate domination. The dominant dinoflagellate, Peridinium limbatum, was calculated to have a growth rate of 0.065 day^–1 and a doubling time of 10.7 days. Heterotrophic nanoflagellates (HNFs) were a consistent component of the planktonic protozoa; seasonal patterns were determined for three genera of HNFs (Monosiga, Bicosoeca, and Desmarella moniliformis). Three genera of ciliates (Coleps, Strobilidium, and Strombidium) comprised the greater part of the planktonic protozoa in Crystal Bog. The number of species of planktonic protozoa was too low to calculate a diversity index. Shannon–Weaver diversity indices for phytoplankton and bacterioplankton in the epilimnion followed very similar seasonal patterns in this lake, supporting the hypothesis that in freshwaters, diversity patterns of these groups are linked.     

Sources and fates of dissolved organic carbon in lakes as determined by whole-lake carbon isotope additions

Four whole-lake inorganic ¹³C addition experiments were conducted in lakes of differing trophic status. Inorganic ¹³C addition enriched algal carbon in ¹³C and changed the C-DOC by +1.5‰ to +9.5‰, depending on the specific lake. This change in C-DOC represented a significant input of algal DOC that was not completely consumed by bacteria. We modeled the dynamics in C-DOC to estimate the fluxes of algal and terrestrial carbon to and from the DOC pool, and determine the composition of the standing stock. Two experiments in lightly stained, oligotrophic lakes indicated that algal production was the source of about 20% of the DOC pool. In the following year, the experiment was repeated in one of these lakes under conditions of nutrient enrichment, and in a third, more humic lake. Algal contributions to the DOC pool were 40% in the nutrient enriched lake and 5% in the more humic lake. Spectroscopic and elemental analyses corroborated the presence of increased algal DOC in the nutrient enriched lake. Natural abundance measurements of the C of DOC in 32 lakes also revealed the dual contributions of both terrestrial and algal carbon to DOC. From these results, we suggest an approach for inferring the contribution of algal and terrestrial DOC using easily measurable parameters.     

Secchi disk — chlorophyll relationships in a lake with highly variable phytoplankton biomass

In meseutrophic Lake Constance mean euphotic phytoplankton chlorophyll concentrations vary about 100-fold over the year. Concomitant fluctuations in euphotic depth (Z_eu) and Secchi depth (Z_s) are related to each other in a non-linear fashion that as a rough approximation can be expressed by Z_eu 5 Z_s.Secchi depth is to a great extent a function of beam attenuation of light which depends on the inherent optical properties of the water and is highly sensitive to light scattering from particles. Euphotic depth, by contrast, is a function of the vertical light attenuation coefficient which also depends on absorption and scattering, but is less sensitive to the latter than beam attenuation. Algal cells both absorb and scatter light and therefore influence Secchi depth and euphotic depth, however, in different fashions.Whenever the lake is clear due to scarce phytoplankton, scattering is small and beam attenuation only exceeds vertical light attenuation by a relatively small factor. As a consequence, the ratio of euphotic depth to Secchi depth is small (1.5–2.5). When the lake is turbid due to high algal density, enhanced scattering from algal cells and detrital particles causes beam attenuation to rise more than vertical light attenuation, thus leading to high ratios of euphotic depth to Secchi depth (3–5). The relatively close relationships between Secchi depth and chlorophyll in Lake Constance are due to (1) high influence of chlorophyll concentration on water transparency, (2) co-variation of phytoplankton and other suspended particles, and (3) limited variation of cellular chlorophyll contents.     

Feeding electivity of two epiphytic chironomids in a subtropical lake

Summary Feeding electivity of two epiphytic orthoclad chironomid species, Psectrocladius sp. and Thienemanniella cf. fusca, was investigated using gut content analysis of larvae collected from Typha stems in Lake Padgett, Florida. Electivity values, computed using Strauss's Food Selection Index, were strongly positive for Cosmarium and negative for Oedogonium and Bulbochaete. Diatoms either were avoided or grazed in proportion to their abundance in the environment. Patterns of electivity were similar for both chrionomid species during all seasons studied. Pairwise food preference experiments were conducted in the laboratory using the algal species, Cosmarium impressulum, Navicula pelliculosa, Selenastrum capricornuatum, Oedogonium cardiacum, and Synedra sp. Preferences were hierarchical and consistent for both larval genera. Preference ranking for the algae offered were CosmariumSynedra>Navicula>Oedogonium>Selenastrum. Larvae seemed to be feeding electively based upon algal genus, size, position in the epiphytic community, and possibly extracellular chemistry.     

Abundance, Annual Survival, and Recruitment of Unexploited and Exploited Lake Charr, Salvelinus Namaycush, Populations at the Experimental Lakes Area, Northwestern Ontario

We studied abundance, annual survival, and recruitment of nine lake charr, Salvelinus namaycush, populations at the Experimental Lakes Area, Ontario, for periods of 9–24 years. We used the Jolly–Seber mark-recapture method to evaluate abundance and annual survival in all populations, and total catches of individual year classes to evaluate recruitment. Seven populations were unexploited and unaffected by whole-lake experiments. One population was exploited prior to mark-recapture study and another was affected by experimental acidification. Abundance ranged from 8 to 24 fish ha^–1 in the unaffected populations. Annual survival ranged from 78% to 93%yr^–1 in all populations except the exploited population. This population may have been additionally affected by northern pike, Esox lucius, predation. Yearly recruitment was relatively constant in all populations and related to lake charr abundance. Lake charr abundance did not recover in populations affected by exploitation or acidification. Unexploited lake charr populations were characterized by relatively constant abundance, high annual survival, and relatively constant annual recruitment.     

Priorities for the conservation of avian biodiversity in China based on the distribution patterns of endemic bird genera

Ten endemic Chinese bird genera are currently recognized: Nipponia, Tetraophasis, Chrysolophus, Crossoptilon, Ithaginis, Pseudopodoces, Rhopophilus, Kozlowia, Urocynchramus and Latoucheornis. Of these genera, Nipponia, Ithaginis, Pseudopodoces, Rhopophilus, Kozlowia, Urocynchramus and Latoucheornis are monotypic, while Tetraophasis, Chrysolophus and Crossoptilon are polytypic. In this paper, we first set up a database of the geographic distribution and suitable habitat requirements for each species of these endemic genera. Secondly we produced a modified habitat-based prediction of their distribution using GIS (ArcView 3.1) software overlap analysis. We then explored the spatial–temporal distribution patterns of different genera. By comparing the richness of the endemic genera in distributed areas, we identified centers of endemic avian biodiversity. These are: the northern and eastern Hengduanshan Mountains, and the Qinling, Dabashan and Minshan Mountain regions. Finally, we identified conservation hotspots with a higher priority based on these centers. These results may provide a valuable tool for identifying and conserving areas with high endemic biodiversity and methods for researching the biogeography of endemic genera, the evolutionary history of fauna and species differentiation.     

Multigene phylogeny of Synura (Synurophyceae) and descriptions of four new species based on morphological and DNA evidence

We used phylogenetic analyses based on multiple gene sequences (partial nr SSU and LSU rDNA, partial pt LSU rDNA, psaA and rbcL) from 148 strains (including three outgroups) and scale ultrastructure to examine phylogenetic relationships among species of the colonial genera Synura and Tessellaria. The phylogenetic tree based on the combined dataset was congruent with ultrastructural characteristics of the scales. Synura was divided into three major clades, two including species in section Synura, and one representing section Peterseniae. One clade, consisting of seven strains of S. uvella (section Synura), diverged at the base of the genus. The second clade consisted of the remaining species belonging to the section Synura. The third clade, containing organisms in the section Peterseniae and characterized by scales possessing a keel, was monophyletic with strong support values. Based on our findings, S. uvella needs to be in a separate section from other spine-bearing species, and we therefore propose new sectional ranks; Synura, Peterseniae, Curtispinae (presence of body scales with slender spines, tubular scales and caudal scales). We further propose four new species based on phylogenetic analyses and unique scale characters: S. longitubularis sp. nov., S. sungminbooi sp. nov., S. soroconopea sp. nov. and S. lanceolata sp. nov. Lastly, we propose a new genus name, Neotessella, to replace the invalid use of the name Tessellaria.     

Population ecology of the temperate reef fish Pseudolabrus celidotus Bloch & Schneider (Pisces: Labridae). I. Factors influencing recruitment

Recruitment of Pseudolabrus celidotus Bloch & Schneider was measured at 10 reef areas over four seasons (1977–1980) in north-eastern New Zealand. These encompassed six different habitat types, defined on the basis of their major physical and biological features. Recruitment differed among habitats and years, but spatial differences were more pronounced than temporal changes. Shallow broken rock (SBR) habitats dominated by macrophytic brown algae consistently had the highest recruitment. Six areas within SBR habitats were censused over three seasons (1979–1981). Even within this one habitat type, most of the variation in numbers was explained by spatial effects. Furthermore, the differences in recruitment between sites remained fairly constant because changes between years were similar in magnitude and direction.An adult removal experiment established that recruitment was independent of adult numbers. Macroalgae affected recruitment as was demonstrated by an algal removal experiment, which resulted in a marked reduction in recruitment, and an algal addition (via urchin removal) experiment which resulted in an increase. Juvenile densities increased as an exponential function of algal biomass per unit area, but the relationship was dependent on depth. Recruitment reached higher levels in shallow habitats (< 8 m) than deeper ones, despite a similar range of algal biomass.It is suggested that there are several factors operating at the time of or shortly after settlement, which result in spatial differences in recruitment on a very small scale. The observed temporal variation may be due to a different set of factors operating prior to settlement, and may have effects measurable over a considerably broader spatial scale.     

Environmentally‐mediated consumer control of algal proliferation in Florida springs

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An ancient divergence among the bacteria

Summary The 16S ribosomal RNAs from two species of methanogenic bacteria, the mesophileMethanobacterium ruminantium and the thermophileMethanobacterium thermoautotropbicum, have been characterized in terms of the oligonucleotides produced by digestion withT ₁ ribonuclease. These two organisms are found to be sufficiently related that they can be considered members of the same genus or family. However, they bear only slight resemblance to typical Procaryotic genera; such asEschericbia, Bacillus andAnacystis. The divergence of the methanogeinc bacteria from other bacteria may be the most ancient phylogenetic event yet detected — antedating considerably the divergence of the blue green algal line for example, from the main bacterial line.     

The effect of non-algal turbidity on the relationship of Secchi depth to chlorophyll a

Data from four reservoirs representative of different trophic states and with different apparent optical properties were analyzed to determine the relationship of Secchi depth to algal biomass as measured by chlorophyll a. In the eutrophic reservoir Secchi depth was determined partially by the chlorophyll a content (r² = 0.31) but only when chlorophyll a data from bloom conditions are included. In the two mesotrophic reservoirs, Secchi depth was entirely determined by non-algal turbidity. In the oligotrophic reservoir, Secchi depth was determined neither by chlorophyll a nor non-algal turbidity and was probably determined by dissolved color. When data from the four reservoirs were pooled (N = 205), 53% of the variation in Secchi depth was explained by: SD = 2.55–0.52 ln (Turbidity) + 0.005 (Chlorophyll a). It is apparent that attempts to estimate algal biomass for trophic state classification or other management practices from Secchi depth data are inappropriate even where moderate amounts of non-algal turbidity are present.     

Increased Benthic Algal Primary Production in Response to the Invasive Zebra Mussel (Dreissena polymorpha) in a Productive Ecosystem, Oneida Lake, New York

Increased water clarity associated with zebra mussel (Dreissena polymorpha) populations may favor benthic algal primary production in freshwater systems previously dominated by pelagic phytoplankton production. While zebra mussel-mediated water clarity effects on benthic primary production have been implicated in published reports, few production estimates are available. This study estimates benthic primary production in Oneida Lake, NY before and after zebra mussel invasion (1992), using measured photosynthetic parameters ( , α^B and β) from sampled benthic algal communities. In the summers of 2003 and 2004, primary production was measured as O₂ evolution from algal communities on hard (cobble) and soft (sediment) substrate from several depths. We also backcast estimates of benthic primary production from measurements of light penetration since 1975. Estimates of whole-lake epipelic and epilithic algal primary production showed a significant (4%) increase and exhibited significantly less interannual variability subsequent to the establishment of zebra mussels. We applied our model to two lakes of differing trophic status; the model significantly overestimated benthic primary production in a hypereutrophic lake, but there was no significant difference between the actual and predicted primary production values in the oligotrophic lake. The hypereutrophic lake had higher zebra mussel densities than Oneida (224 vs. 41 per sample respectively). Though total community respiration (measured in total darkness) was factored into our model predictions of production, our model may need modification when heterotrophic respiration is a large portion of total community metabolism.     

Empirical analysis of the removal rate of periphyton by grazers

To establish a general model for the removal rate of periphyton by grazers, we identified 27 publications in which removal rates could be estimated from grazer enclosure or exclosure experiments. When all the measurements obtained under different experimental conditions were extracted, these publications provided 107 data points. Multiple regression of these data showed that periphyton removal rate increased significantly with grazer body mass and food availability, and decreased with grazer crowding. Grazer body mass explained 65% of the variation, while crowding and food availability explained 7 and 6% respectively. Except for the significantly lower removal rate of amphibians, neither taxon of the grazer nor algal composition significantly affected removal rate. Experiments in the laboratory and in outdoor channels tended to give higher removal rates than experiments performed in streams or lakes. A comparison with previous allometric equations predicting the ingestion rate of other invertebrate guilds, and with experiments in which periphyton ingestion rate was measured as incorporation of labelled food, indicated that a large portion of periphyton is removed by the activity of the grazer rather than by direct ingestion. These results could be utilized to predict the impact of grazing on periphyton biomass.A contribution of the Groupe d'Écologie des eaux douces, Université de Montréal     

SILICEOUS SCALE PRODUCTION IN CHRYSOPHYTE AND SYNUROPHYTE ALGAE. I. EFFECTS OF SILICA-LIMITED GROWTH ON CELL SILICA CONTENT,SCALE MORPHOLOGY,AND THE CONSTRUCTION OF THE SCALE LAYER OF SYNURA PETERSENII1

The cells of synurophyte flagellates (algal class Synurophyceae, formerly included in the Chrysophyceae) are enclosed within a regularly imbricate layer of ornamented siliceous scales. Scale morphology is of critical taxonomic importance within this group of algae, and the scales are valuable indicator microfossils in paleolimnological studies. The data presented here demonstrate that scale morphology and the integrity of the scale layer can exhibit extreme variability in culture as a function of the cellular quota of silica under silica-limited growth. Silica-limited, steady-state populations of the colonial flagellate Synura petersenii Korsh. were maintained over a range of specific growth rates (μ= 0.11–0.69 days^?1) and silica cell quotas (Q_si= 0.13–2.40 pmoles Si · cell¹). Scale morphology and the organization of the scale layer became increasingly aberrant as silica stress increased. Under severe stress, scale deposition was completely suppressed so that cells appeared scale-free. This depression of scale deposition was reversible; populations of silica-starved, scale-free cells rapidly regenerated new scale layers when placed in batch culture and spiked with dissolved silica. During recovery from silica stress, cell division was repressed for 24 h while mean cell silica quota increased 25-fold. The first new scales appeared within 2 h after the silica addition, and development of the new scale layer proceeded in an approximately synchronous manner, residting in normal scale layers on virtually all cells after 48 h of recovery in Sirich medium. Silica content of silica-replete Synura cells is comparable to freshwater diatoms of siynilar size, but Synura has much greater potential quota variability than diatoms and no apparent threshold silica requirement. Silica-limited growth kinetics and competition between diatoms and Synura for silica are discussed. The results suggest that morphological variability of siliceous scales in natural populations of synurophyte flagellates may result from silica stress and that the experimental approach developed here has great potential value as a means for circumscribing ecotypic variation in scale morphology. Results also demonstrate that scale production can be uncoupled from cell division, suggesting that cell cycle regulation of silica biomineralization in the Synurophyceae may be fundamentally different from that of diatoms (algal class Bacillariophyceae). This experimental system has application in the future study of the intracellular membrane systems and the regulatory processes involved in silica biomineralization.     

Scales of horizontal patchiness in chlorophyll a, chemical and physical properties of landfast sea ice in the Gulf of Finland (Baltic Sea)

Horizontal variation of first-year landfast sea ice properties was studied in the Gulf of Finland, the Baltic Sea. Several scales of variation were considered; a number of arrays with core spacings of 0.2, 2 and 20 m were sampled at different stages of the ice season for small-scale patchiness. Spacing between these arrays was from hundreds of meters to kilometers to study mesoscale variability, and once an onshore–offshore 40-km transect was sampled to study regional scale variability. Measured variables included salinity, stable oxygen isotopes (¹⁸O), chlorophyll a (chl-a), nutrients and dissolved organic carbon. On a large scale, a combination of variations in the under-ice water salinity (ice porosity), nutrient supply and the stage of ice development control the build-up of ice algal biomass. At scales of hundreds of meters to kilometers, there was significant variability in several parameters (salinity, chl-a, snow depth and ice thickness). Analyses of the data from the arrays did not show evidence of significant patchiness at scales <20 m for algal biomass. The results imply that the sampling effort in Baltic Sea ice studies should be concentrated on scales of hundreds of meters to kilometers. Using the variations observed in the study area, the estimate for depth-integrated algal biomass in landfast sea ice in the Gulf of Finland (March 2003) is 5.5±4.4 mg chl-a m^–2.     

Heterotrophic bacterial activity and primary production in a hypertrophic African lake

The relationship between heterotrophic bacteria and phytoplankton in the epilimnion (0–10 m) of hypertrophic Hartbeespoort Dam, South Africa, was examined by statistically analyzing three years of parallel measurements of heterotrophic bacterial activity (glucose uptake) and phytoplankton particulate and dissolved organic carbon production. Algal biomass ranged between 4.0 and 921.1 mg Chl a m^-3 at the surface. Primary production varied between 69.5 and 3010.0 mg C m^-2h^-1 while algal production of dissolved organic carbon (EDOC) ranged from 2.5 to 219.2 mg C m^-2h^-1. Bacterial numbers reached a summer peak of 44.23 × 10⁶ cells ml^-1 in the first year and showed no depth variation. The maximum rate of glucose uptake, V_max, reached a peak of 5.52 g C l^-1h^-1. V_max, maximum glucose concentration (K_t + S_n) and glucose turnover time (T_t) were usually highest at the surface and decreased with depth concomitant with algal production. At the surface, V_max was correlated to EDOC (r = 0.59, n = 67, p < 0.001) and primary production (r = 0.71, n = 70, p < 0.001). At 5 and 10 m, V_max was correlated to integral euphotic zone (~ 4 m) algal production and bacterial numbers. Glucose turnover time was inversely related to integral algal production (r = -0.72, n = 70, p < 0.001) and less strongly to bacterial numbers. The data indicated that although bacterial numbers and biomass were low relative to algal biomass in this hypertrophic lake, the heterotrophic bacteria attained high rates of metabolic activity as a result of enhanced algal production of available organic carbon.     

Biodegradation of dyes by some green algae and cyanobacteria

The ability of Chlorella vulgaris, Lyngbya lagerlerimi, Nostoc lincki, Oscillatoria rubescens, Elkatothrix viridis and Volvox aureus to decolorize and remove methyl red, orange II, G-Red (FN-3G), basic cationic, and basic fuchsin was investigated. These algae showed different efficiency for colour removal; varied from 4 to 95% according to the algal species, its growth state and the dye molecular structure. Basic cationic and basic fuchsin were the most susceptible dyes for decolorisation and removal by all algae being tested, and up to 82% of methyl red was also removed by N. lincki and O. rubescens. However, the algal activity to decolorize orange II and G-Red was markedly fluctuated and lower. C. vulgaris displayed activity to remove 43.7 and 59.12% while as V. aureus removed 5.02 and 3.25% of the added dyes respectively. The results also showed that treatment of either C. vulgaris or N. Linckia with G-Red or methyl red, respectively, induced the algal azo dye reductase enzyme by 72 and 71% at the same order.     

The ecology of Lake Nakuru (Kenya)

Summary Abiotic factors, standing crop and photosynthetic production were studied in the equatorial alkaline-saline closed-basin Lake Nakuru (cond. 10,000–160,000 S). Meteorological conditions and abiotic factors offer suppositions for a high primary productivity: mean solar radiation is 450–550 kerg·cm^-2·s^-1, with little seasonal variation, regular winds circulate the lake every day and nutrient concentrations are usually high (>100 g P–PO₄·l^-1). Oxygen concentrations near sediments were <1 gO₂·m^-3 for at least 6 h·d^-1 in 1972/73, resulting in a release of 45 mg P–PO₄·m^-2·d^-1. Attenuation coefficients vary from 3.6–16.5 according to algal densities and mean depth from 0–400 cm. Algal biomass was 200 g·m^-3 (d.w.) in 1972/73, due to a lasting Spirulina platensis bloom (98.5% of algal biomass). In 1974 algal biomass suddenly dropped to 50 g·m^-3 (d.w.). Spirulina and several consumer organisms almost vanished, but coccoid cyanobacteria, Anabaenopsis and diatoms increased. Several causes for this change in ecosystem structure are discussed. The use of the light/dark bottle method to measure photosynthetic production in eutrophic alkaline lakes is discussed and relevant experiments were done. Oxygen tensions of 2–35 gO₂·m^-3 do not influence primary production rates. Net photosynthetic rates (mgO₂·m^-3·h^-1; photosynthetic quotient=1.18) reached 12–17.7 in 1972/73 and 2–3 in 1974, but vertically integrated rates were only 1–1.4 in 1972/73 and 0.8 in 1974, and daily net photosynthetic rates (gO₂·m^-3·24 h^-1) 3.5 in 1972/73 and 1 in 1974. 50% of areal rates were produced within the 10 most productive cm of the depth profile. The disproportion between high algal standing crops and relatively low production rates is due to self-shading of the algae, reducing the euphotic zone to 35 cm in 1972/73 and 77 cm in 1974. Efficiency of light utilization is 0.4–2%, varying with time of day and phytoplankton density. In situ efficiencies show an inverse relationship to light intensities. Photosynthetic rates of L. Nakuru remain within the range of other African lakes (0.1–3 gO₂·m^-2·h^-1). The relation of O₂ produced/Chl a of the euphotic zone is 50% lower then in tropical African freshwater lakes and conforms to lakes of temperate regions.     

The Distribution of Nearshore Fishes in Kelp and Eelgrass Communities in Prince William Sound, Alaska: Associations with Vegetation and Physical Habitat Characteristics

The nearshore (less than 20m depth) demersal fish community in Prince William Sound, Alaska, is dominated by Pacific cod, Gadus macrocephalus, pricklebacks (mostly Arctic shanny Stichaeus punctatus), gunnels (mostly crescent gunnels Pholis laeta), a variety of greenlings (Hexagrammidae) and sculpins (Cottidae). During summer, the spatial distribution of fishes, over scales of 100's of m to 10's of km, varied by habitats characterized by different vegetation types. Juvenile Pacific cod and greenlings were numerically dominant in eelgrass, Zostera marina, beds. Pricklebacks and sculpins were dominant in areas with an understory of the kelps Agarum cribrosum and Laminaria saccharina. Greenlings and sculpins were the most abundant demersal fishes in more exposed sites with a canopy of Nereocystis luetkeana and an understory of L. bongardiana. Measured habitat variables, including vegetation type, slope, vegetation biomass, and substratum type, explained a significant proportion of the variation in the presence or absence of most fishes. The relative importance of different habitat characteristics varied between taxonomic groups of fishes. Vegetation type explained a significant proportion of variation for cod, rockfishes, and ronquils. Juvenile cod were closely associated with eelgrass, while rockfish and ronquils were associated with kelps. Pricklebacks and rockfishes were more frequently observed on steeply sloped shorelines, while ronquils were more often found at sites with higher biomass of vegetation. Within A. cribrosum habitats, more greenlings and sculpins were present at sites where algal biomass was higher. Also, sculpins were more abundant in deeper water and gunnels were more abundant in shallow water within this habitat. These associations may not have been causative. However, evidence suggests that some differences between fish communities in eelgrass and Agarum beds may have been causally related to vegetation characteristics. The possible roles of different vegetation types as refugia from predators or as sources of prey are discussed.     

Stoichiometric relationships among producers,consumers and nutrient cycling in pelagic ecosystems

Most ecosystem models consolidate members of food-webs, e.g. species, into a small number of functional components. Each of these is then described by a single state variable such as biomass. When a multivariate approach incorporating multiple substances within components is substituted for this univariate one, a stoichiometric model is formed. Here we show that the Nitrogen:Phosphorus ratio within zooplankton herbivores varies substantially intraspecifically but not intraspecifically. By using stoichiometric theory and recent measurements of the N:P ratio within different zooplankton taxa, we calculate large differences in ratios of nutrients recycled by different zooplankton species. Finally, we demonstrate that N:P stoichiometry can successfully account for shifts in N- and P-limitation previously observed in whole-lake experiments. Species stoichiometry merges food-web dynamics with biogeochemical cycles to yield new insights.Abbreviations b N:P in zooplankton biomass - f N:P in algal biomass - L maximum accumulation eficiency - N:P ratio of nitrogen to phosphorus (moles:moles) - s N:P supply ratio from grazers - TN Total nitrogen = seston N + dissolved N (µmoles/liter) - TP Total phosphorus = seston P + dissolved P (µmoles/liter)