Midsummer heat wave effects on lacustrine plankton: Variation of assemblage structure and fatty acid composition

The effects of the 2003 European heat wave on a freshwater plankton assemblage and its fatty acid (FA) composition were investigated. Composition and FA profiles of four size categories of planktonic organisms collected in 2003 were compared to those of the colder year 2002.     

Compartmentalisation of nitrogenase in a non-heterocystous cyanobacterium: Trichodesmium contortum

Abstract In Trichodesmium contortum , nitrogenase was detected in only a limited number (about 10%) of microscopically distinguishable, consecutively arranged cells in central regions of the trichomes. Cells with nitrogenase also contained the photosystem II associated pigment phycoerythrin. These cells were not distinguishable from other cells on a structural basis, but were clearly visible at low magnification microscopy as all in the zone were more compact and shorter than those on either side. The compartmentalisation of nitrogenase into a chain of cells and in a possibly photosynthetic environment represents a previously undescribed phenomenon. The nitrogenase containing cells apparently perform the O₂ protective function of heterocysts yet are different in several aspects.     

Occurrence of bacterivory in Cryptomonas,a common freshwater phytoplankter

Summary Bacterivory was detected by incorporation of 0.57 m diameter, fluorescent polystyrene beads and fluorescently labeled bacteria (FLB) in two cultured species of Cryptomonas (C. ovata and C. erosa), and a population of Cryptomonas sp in a humic, mesotrophic lake. Rates of ingestion and clearance were very low, and similar for the cultures and the in situ population. The in situ population incorporated 0.7–1.7 bacteria cell^-1 h^-1, thereby ingesting 0.3%–2.0% of the total bacterial numbers present in the water per day, and receiving less than 2% of its carbon content per day through bacterivory. Thus, bacterivory by Cryptomonas was quantitatively important neither as a sink for bacterial biomass, nor as a carbon source for the algal cells. Possibly, it served in the uptake of essential nutrients.     

Planktivory in benthic nototheniid fish in McMurdo Sound,Antarctica

Synopsis Four species of nototheniid fish were sampled from below the sea ice near Cape Armitage, McMurdo Sound:Pagothenia borchgrevinki from just below the ice 1.5 km offshore,Trematomus bernacchii, Trematomus hansoni andTrematomus centronotus from off the bottom in about 20 m of water near the shore. Scale worms and isopods were conspicuous non-planktonic prey, and present in the three benthic fish species. The planktonic pteropod molluscLimacina helicina was numerically common in all four species of fish. The planktonic hyperiid amphipodHyperiella dilatata was also found in all fish species. WhereasP. borchgrevinki is planktivorous in accord with its pelagic habit, theTrematomus spp. clearly also feed on plankton from the water column.T. hansoni is particularly planktivorous, taking smaller copepods thanP. borchgrevinki.     

Selenium in sediment and food webs of the Tapajós River basin (Brazilian Amazon) and its relation to mercury

BackgroundWe investigated Se levels along the Tapajós River basin - which is an important tributary of the Amazon River - and the possible antagonistic effect of Se in Hg availability. This is the first study to investigate Se transfer from abiotic to biotic compartments and along the food chain in aquatic ecosystems of the Amazon basin.MethodsSe concentrations were measured in superficial sediment (n = 29), plankton (n = 28) and fishes (n = 121) along two stretches of the Tapajós River basin (Tap_up/mi and Tap_low), comprising approximately 500 km with different hydrological characteristics.ResultsSe concentrations in sediment were significantly higher in the Tap_low (345−664 μg kg⁻¹) than in the Tap_up/mi (60−424 μg kg⁻¹). The seasonal flooding of the Amazon River probably helps to carry selenium-rich sediment to the Tapajós mouth (Tap_low stretch). We suggest that Se in sediment could decrease the bioavailability of Hg resulting in lower MeHg concentrations in fish, as observed in the Tap_low (45−934 μg kg⁻¹). Sediment and plankton were positively correlated in relation to their Se concentrations (r = 0.62; p = 0.001) suggesting that sediment can possibly be the main source of Se to plankton. Our data indicate Se uptake by primary consumers, as noted in phytoplankton levels. The decrease of Se concentrations along the food chain was also noteworthy.ConclusionThis work elucidates some aspects of Se biogeochemistry in the Amazon basin and shows its importance regarding Hg cycles in aquatic ecosystems.     

Temporal variations and pond age effect on plankton communities in semi-intensive freshwater marron (Cherax cainii,Austin and Ryan, 2002) earthen aquaculture ponds in Western Australia

The abundance and diversity of the plankton community represents the health of the aquatic ecosystem, and plays an important role in the growth of cultured animals under aquaculture conditions. The temporal variations of plankton abundance, taxonomic composition, diversity, evenness and species richness were studied in three old and three new semi-intensive marron (Cherax cainii, Austin and Ryan, 2002) ponds. Water parameters such as temperature, dissolved oxygen, pH, turbidity, TAN, nitrite, nitrate and reactive phosphate were recorded, and plankton samples were collected every two months, for one year of juvenile production cycle. A total of twenty-six phytoplankton and seven zooplankton genera were recorded. Chlorophyceae was the dominant class of phytoplankton throughout the year, followed by Trebouxiophyceae. Rotifera comprised 49.8% of the total zooplankton community (individuals L^?1), the largest proportion of any group. Temporal variations impacted the plankton abundance and community structure, and plankton abundance were more abundant during summer. The pond age did not influence the phytoplankton abundance, whereas zooplankton abundance was higher in older ponds.     

Modelling the Effects of Current on Prey Acquisition in Planktivorous Fishes

Many planktivorous fishes forage in currents, where they actively maintain position and visually strike at current-entrained zooplankton. In general, the zooplankton are wafted by the foraging fish at a rate equivalent to the current velocity. From a fish's viewpoint the plankton approach either head-on or offset at varied distances from the fish's position. We present a model that describes the relative motion of particles as they approach and pass a foraging fish at different offset distances, and the rate of change in apparent size as they close on a fish. In addition, a series of experiments of fish feeding on plankton in a flume at increasing current velocities revealed that two basic tactics are utilized. At low current velocities (<10-14 cm s m 1), the fish swims toward the prey, whereas at higher current velocities the fish tends to fall back with the current to capture a prey item. The model and experimental results are discussed in terms of the visual problems associated with the detection and tracking of items in motion.     

Excitability of an age-structured plankton ecosystem

 We adapt a simple two-component model of a plankton ecosystem to account for the life spans of individual predatory organisms. We investigate the system’s short-term dynamics, in particular its excitability, and its long-term dynamics, and show how both can be highly sensitive to initial conditions. We discover that this effect is enhanced by imposing age structure on the system. Received: 21 June 1996 / Revised version: 19 October 1998     

Emergent Features Due to Grid-Cell Biology: Synchronisation in Biophysical Models

Modelling studies of upper ocean phenomena, such as that of the spatial and temporal patchiness in plankton distributions, typically employ coupled biophysical models, with biology in each grid-cell represented by a plankton ecosystem model. It has not generally been considered what impact the choice of grid-cell ecosystem model, from the many developed in the literature, might have upon the results of such a study. We use the methods of synchronisation theory, which is concerned with ensembles of interacting oscillators, to address this question, considering the simplest possible case of a chain of identically represented interacting plankton grid-cells. It is shown that the ability of the system to exhibit stably homogeneous (fully synchronised) dynamics depends crucially upon the choice of biological model and number of grid-cells, with dynamics changing dramatically at a threshold strength of mixing between grid-cells. Consequently, for modelling studies of the ocean the resolution chosen, and therefore number of grid-cells used, could drastically alter the emergent features of the model. It is shown that chaotic ecosystem dynamics, in particular, should be used with care.     

Goose-mediated nutrient enrichment and planktonic grazer control in arctic freshwater ponds

A dramatic increase in the breeding population of geese has occurred over the past few decades at Svalbard. This may strongly impact the fragile ecosystems of the Arctic tundra because many of the ultra-oligotrophic freshwater systems experience enrichment from goose feces. We surveyed 21 shallow tundra ponds along a gradient of nutrient enrichment based on exposure to geese. Concentrations of total phosphorus (P) and dissolved inorganic nitrogen (DIN) in the tundra ponds ranged from 2–76 to 2–23 μg l⁻¹ respectively, yet there was no significant increase in phytoplankton biomass (measured as chlorophyll a; range: 0.6–7.3 μg l⁻¹) along the nutrient gradient. This lack of response may be the result of the trophic structure of these ecosystems, which consists of only a two-trophic level food chain with high biomasses of the efficient zooplankton grazer Daphnia in the absence of fish and scarcity of invertebrate predators. Our results indicate that this may cause a highly efficient grazing control of phytoplankton in all ponds, supported by the fact that large fractions of the nutrient pools were bound in zooplankton biomass. The median percentage of Daphnia–N and Daphnia–P content to particulate (sestonic) N and P was 338 and 3009%, respectively, which is extremely high compared to temperate lakes. Our data suggest that Daphnia in shallow arctic ponds is heavily subsidized by major inputs of energy from other food sources (bacteria, benthic biofilm), which may be crucial to the persistence of strong top–down control of pelagic algae by Daphnia.