Planktonic Systems Research Articles

Effects of Experimental Fertilization on the Benthic Macroinvertebrate Community of an Arctic Lake

The benthic macroinvertebrate community of experimentally divided Lake N-2 (67°38′27″N, 149°37′30″W) was studied during six years of experimental fertilization. The dominant snail, Lymnaea elodes, increased on the fertilized side of the lake. Another snail, Valvata lewisi, previously shown to be competitively inferior to Lymnaea, did not increase in the fertilized sector until the fifth year of the study, but then increased dramatically. Chironomids were more abundant and had higher biomass on the reference side of the lake, an initial condition, and did not increase in the fertilized sector. Slimy sculpin, previously shown to control chironomids on bare sediments in nearby Toolik Lake, were the only fish predators on chironomids in Lake N-2; they did not grow faster in the fertilized sector. These results suggest that the dominant benthic invertebrates in N-2, chironomids and snails, were limited by different factors; chironomids were not food limited and appeared to be limited by predators, but snails, free from predation in N-2, appeared food limited. The planktonic model of cascading trophic interactions, if applied to benthic communities, would predict that, at low (zero) piscivore density, sculpin density and/or growth would increase as increased primary productivity was passed up the food web; they did not. The physical structure of the benthic zone is such that edible food can accumulate, whereas this cannot occur over extended time scales in planktonic systems. I suggest that this structural difference leads to functionally different constraints on planktonic and benthic communities.

Ecology of bottom ice algae: II. Dynamics, distributions and productivity

Spring blooms of bottom ice algae are a common feature of landfast congelation ice in polar regions. Because ice algae are usually associated with a substrate, their population dynamics can be followed with considerable confidence. Although ice algal dynamics are closely related to irradiance, their dynamics and distributions are influenced by other abiotic and biotic factors. Ice algal abundance varies horizontally over all scales examined. Factors such as grazing and nutrient availability may contribute to local and geographic differences. Loss terms for most sea ice assemblages are largely unquantified. Ice algal biomass is most concentrated near the ice-water interface in spring. Environmental factors affecting ice algal abundance and productivity are considered here, emphasizing recent results from several well-studied sites. Biomass accumulation, growth rates and productivity have been documented for spring blooms of bottom interstitial and sub-ice assemblages. On an areal basis biomass accumulation in bottom ice assemblages can be comparable with planktonic systems. At low ambient temperatures and irradiances average specific growth rates (≤ 0.25 d −1) and production rates (≤ 1.0 mg C mg Chl −1 h −1) for ice algae are low. Current methods of measuring productivity are compared. Results are consistently low but variable with little systematic difference among them. At present, apparent differences in productivity between bottom ice assemblages in the Arctic and Antarctic, or among different antarctic assemblages, are so confounded by methodological and other sources of variability that no firm differences can be detected.

Rôle du bactérioplancton en milieu lacustre pélagique : prépondérance des plus grosses bactéries

Bacterioplankton abundance (AB), heterotrophic production (PB) (thymidine incorporation) and renewal time (tr = AB/PB) were determined in eight Canadian Shield lakes in two size classes of bacteria (< 1 μm and > 1 μm). Although large bacteria (> 1 μm) were often less than 50% in numbers, they produced most of the new cells in 20 out of 21 trials. Large bacteria had a tr shorter than that of small ones (< 1 μm). Large bacteria not only represent much of the biomass, but seem to be the principal site of carbon and energy flow within the bacterial community. There was a significant positive correlation between PB of the whole bacterial community and primary production (PP), but we found no correlation between PB and chlorophyll a concentration (CHL). Similar results were obtained for the large bacteria. On the opposite, we found no correlation between PB of the small bacteria and PP, but a significant positive correlation between PB and CHL. Only the small bacteria showed a significant correlation between PB and AB, as well as a more constant rate of reproduction. Our results modify the importance of AB values found in the literature, give a good reason for the weak correlation between PB and AB found in other studies, and change the generally accepted scheme of organic and energy flow in planktonic systems. Key words: bacterioplankton, size class, productivity, correlation.

Herbicide effects on planktonic systems of different complexity

Bioassays of different complexity were compared with respect to their capability to predict the environmental impact of the herbicide atrazine in aquatic systems. Acute toxicity tests with Daphnia did not yield meaningful results. Sublethal tests with Daphnia (feeding inhibition, reduction of growth and reproduction) were more sensitive, but effective concentrations of atrazine were still rather high (2 mg/L). A relatively complicated ‘artificial food chain’ system that incorporated direct and indirect effects on Daphnia yielded significant reduction of daphnid population growth at 0.1 mg/L. Enclosure experiments with natural communities were by far the most sensitive tools. Community responses could be measured at concentrations as low as 1 µg/L and 0.1 µg atrazine/L. At the lowest concentration, however, communities recovered after three weeks. We conclude that in complex systems indirect effects can be more important than direct effects, so that, contrary to the conditions in simple tests, non-target organisms may be the better indicators of herbicide stress to natural communities.

Effects of Fertilization and Planktivorous Fish on Epilimnetic Phosphorus and Phosphorus Sedimentation in Large Enclosures

Enclosure experiments were conducted during 1986 and 1987 to test the hypothesis that predation-induced shifts in plankton size-structure affect epilimnetic total phosphorus (TP) through changes in sedimentation rates, both with and without external nutrient loading. Fertilization and/or planktivorous fish were applied to eight enclosures in a 2 × 2 factorial design. Epilimnetic TP was higher with fish in the unfertilized enclosures and lower with fish in the fertilized enclosures; that is, fertilization increased epilimnetic TP only in enclosures without fish, and much less than expected based on the amounts added. The fraction of TP sedimenting per unit time, and the spring to summer decline in TP, was consistently lower with fish. Sedimentation rates were poorly correlated with epilimnetic TP, but were more strongly correlated with TP decline rates and particulate phosphorus larger than 20 μm. Presence or absence of planktivorous fish can alter the trophic status of planktonic systems, and the impact of external nutrient loading by changing the size-distribution and biomass of plankton.

MARINE PLANKTON FOOD CHAINS

The view on the ecology of marine plankton has changed significantly within the last decade. In some respects the is sufficiently dramatic to make some authors speak about a change in paradigm (84). The new picture of plankton dynamics includes the recognition that phototrophic and heterotrophic microorganisms play a substantial and sometimes dominating role in the cycling of matter in the sea, that plankton food chains include a higher number of trophic levels than hitherto believed, and that a large fraction of the primary production is not consumed directly by herbivorous consumers but is channelled through a pool of dead organic matter before it becomes available-via bacterial production-to phagotrophic organisms. In this latter respect the structure of planktonic systems resembles benthic and terrestrial systems. This changing view is remarkable for more than one reason. Among different types of ecosystems, that of the open sea was long considered the simplest and best understood. Many ecology textbooks used marine plankton to introduce the concept of food chains. A graphical presentation might be in the form of a pyramid with its base representing primary producers; above them two hori-zontal layers represented zooplankton and fish, respectively; and at the top a fisherman (or a whale) represented the highest trophic level. While this essentially remains correct, it has certainly proven to be an incomplete picture of pelagic food chains. Modem biological oceanography began at the turn of the century. During the following decades it developed into an established discipline which successfully explained fundamental aspects of plankton dynamics. Organic

Zooplankton‐mediated transitions between N‐ and P‐limited algal growth1

Limitation of algal growth by nitrogen and phosphorus was assessed in three north‐temperate lakes with physiological bioassays and nutrient enrichment experiments. In addition, mesocosm experiments were performed in the three lakes to examine the effects of nutrient enrichment and zooplankton biomass and size on algal nutrient status. In situ indicators of N and P availability were inversely related in magnitude and transitions between N and P limitation were abrupt. Physiological bioassay results did not indicate simultaneous limitation by N and P. However, limited responses to single‐nutrient enrichment and pronounced responses to simultaneous N and P addition in enrichment experiments suggested that potential limitation by the secondary nutrient was usually in close proximity to limitation by the primary nutrient. Transitions between N and P limitation closely accompanied major shifts in the zooplankton community. The importance of the zooplankton community in regulating the relative degree of N or P limitation was confirmed by the mesocosm experiments, which demonstrated that transitions between algal N or P limitation could be induced by manipulations of zooplankton biomass or size. This result supports a hierarchical view of the function of planktonic systems, in which biotic interactions structure the response of the algal community to a given nutrient load.

Research on the Physiological Basis of Population Dynamics in Relation to Ecotoxicology

One of the main obstacles that hampers ecotoxlcology is the poor insight into the relationship between physiological and population dynamics. The role of laboratory experiments, modeling, mathematical analysis and computer simulation studies is discussed in research aiming at this relation. Energy and nutrient budgets of organisms are found to be of vital importance. This paper evaluates the progress that has been made in concrete efforts to work out energy and nutrient budgets for simple freshwater plankton systems stressed by toxic chemicals with different modes of action.

Thymidine Incorporation by the Microbial Community of Standing Dead Spartina alterniflora

Thymidine incorporation by the microbial community on standing dead leaves of Spartina alterniflora did not obey many of the assumptions inherent in the use of the technique in planktonic systems. Incorporation rates of [methly-H]thymidine were nonsaturable over a wide concentration range (10 to 10 nM). Owing to metabolism by both fungi and bacteria, a major fraction of the radiolabel (mean, 48%) appeared in protein. Extraction of the radiolabeled macromolecules was inefficient, averaging 8.8%. Based on an empirically derived conversion factor, 4 x 10 cells . mol of thymidine, doubling times ranged from 4 to 69 h for the epiphytic bacterial assemblage.

Evolutionary Interpretations of Allelochemical Interactions in Phytoplankton Algae

Although allelochemistry among phytoplankton species has been studied relatively little under natural conditions, it is widely thought to influence the increase and decrease of individual species in the course of a growing season in plankton systems. Some direct evidence supports this view. Allelochemistry involving growth suppression has often been viewed as beneficial to the releaser of allelochemicals, and thus as being maintained by selection pressures deriving from competition. This interpretation is indefensible, however, insofar as individual phytoplankton cells are unable, because of the movement of both the cells and the allelochemicals, to capture selectively the benefit of suppressing other individuals by means of allelochemicals; any such benefits would be shared by many other organisms. Other explanations must therefore be sought. Allelopathy can be considered passive from the viewpoint of the releaser but still capable of having detrimental effects on receptor organisms. This explanation also seems untenable in that it requires receptor organisms to be unable, even over many thousands of generations of exposure, to develop defenses against specific, passively released organic substances. An alternative explanation that avoids these difficulties, while allowing for the possibility that allelochemicals are important in natural systems, is based on the idea that the significance of allelochemicals lies strictly in the receptor and not the releaser. According to this explanation, allelochemicals are significant to the receptor organisms as cues that trigger physiological responses to an improving or deteriorating environment. This interpretation, unlike the traditional one, is consistent with observational and experimental evidence of allelochemical interactions and with the basic principles of natural selection.

Temporal Variation, Spatial Heterogeneity, and Competition for Resources in Plankton Systems: A Theoretical Model

We present a model of two species competing for two resources in a spatially nonuniform (i.e., spatially disaggregated) environment. The formulation is based upon our experience with phytoplankton competing for nutrients in lakes and the ocean. The organisms may diffuse, be carried by currents, grow (controlled by the resources), sink, and die; the resources may diffuse, be carried by currents, be taken up by growing organisms, and be regenerated from dying organisms. External sources (or sinks) of resources are allowed. For reasonable choices of parameters, our model shows much more variable behavior in space and time than the usual smooth approach to coexistence (or exclusion of one of the species) seen in previous resource competition models. For example, damped oscillations and "limit-cycle-like" behavior are exhibited. These are not the result of time delays or varying environmental factors, but are inherent in the theory in which sinking plays a large role. Diffusion in the spatially heterogeneous environment can allow coexistence of two species for situations in which dominance of one is predicted by other models. Conversely, through Turing instability, diffusion can destabilize an otherwise stable, uniform coexistence situation. Turing-type behavior may occur when organisms and nutrients diffuse at different rates and when perturbations in species densities or nutrient concentrations occur at intermediate spatial scales. The upper and lower limits of the intermediate scale region are set by the interaction of physical and biological processes; such regions (and thus Turing behavior) may be common in lake and ocean environments. The results of other models of resource-based competition depend upon behavior found near equilibrium. The conclusions of these equilibrium models may not apply in environments with realistic temporal and spatial variation.

Interactions of bactivorous grazers and heterotrophic bacteria with dissolved organic matter

Ciliated Protozoa (Euplotes sp, and Uronema sp.) and natural microzooplankton assem- blages (10 to 200 pm) - collected from Los Angeles Harbor, a coastal lagoon, and San Pedro Channel - were fed bacteria previously labeled with 14C(U)-glucose. Dissolved organic carbon-l4 (D014C) released during grazing was collected and subsequently fractionated by AMICON ultrafiltration into 5 nominal molecular weight (NMW) classes: > 105, 104 to 105, 103 to 104, 5 X 102 to 103, and < 5 X 102 daltons. Release of labeled organic compounds with low (< 5 X 102 daltons) and intermediate NMW (103 to 104 daltons) was most rapid in experimental and control samples and was enhanced in the presence of grazers. Unlabeled bacterioplankton assemblages were incubated 24 h with either fractionated or unfractionated DO14C which was released during grazing experiments. Bacterioplank- ton utilized 196 % d-I + 100 more of the DO14C released in presence of cultured ciliates or microzoo- plankton than D014C released in their absence. Low (< 5 X 102 daltons) and intermediate NMW (103 to 104 daltons) labeled fractions were incorporated and respired more rapidly by bacteria than the other 3 NMW labeled fractions. Fractionation of D014C after bacterial incubation revealed a decrease in all NMW classes and a shift toward a dominance of low NMW labeled fractions in the D0I4C released in the presence of grazers, but not in the controls. Bactivorous ciliates and microzooplankton contribute quantitatively and qualitatively to the dissolved organic matter pool in nearshore planktonic systems. Dissolved organic matter released in the presence of bactivores is a readily available carbon source for bacterioplankton and may thereby influence bacterioplankton metabolism and growth activity.

Interactions Between Bacteria and Algae in Aquatic Ecosystems

Bacteria and algae are the numerically dominant organisms in the plankton of lakes and the ocean and their metabolism largely controls pelagic energy flow and nutrient cycling. The purpose of this review is to examine the means by which algae and heterotrophic bacteria interact in planktonic systems. In terrestrial systems, bacterial-plant interactions have been well studied in the rhizosphere, the zone in the soil where microorganisms are influenced by plant roots (108). The concept of the was introduced as a pelagic analogy to the rhizosphere (7). The phycosphere would be the zone surrounding an algal cell within which microorganisms are influenced by algal products. In considering bacterial-algal interactions, then, we should ask ourselves whether a phycosphere exists. I cite specific evidence from the literature for various types of interactions. In doing this I rely heavily on laboratory studies because relatively little field oriented work is available. I include neither the photosynthetic bacteria (e.g. Chlorobiaceae, Rhodospirillaceae) nor the large macroalgae (seaweeds) in this review. The cyanobacteria have both algal and bacterial characteristics, being prokaryotes and capable of oxygenic photosynthesis. As planktonic, aerobic photo-autotrophs the cyanobacteria clearly belong with the phytoplankton in terms of this review.

Comparison of the Feeding Behaviour of Calanus and Pseudocalanus in two Experimentally Manipulated Enclosed Ecosystems

INTRODUCTIONImplicit in some recent models of the dynamics of planktonic systems, for example that of Steele &amp; Frost (1977) is the concept that filter-feeding copepods may be scaled by size when considering feeding in relation to the size composition of their paniculate food. For a particular organism ingestion of phytoplankton has been considered to be a function of its own weight and the size composition of the phytoplankton available to it. Such scaling has been assumed to occur both in an intra- and interspecific sense, with model development involving a ‘large’ copepod {Calanus) able to feed and grow more efficiently on ‘large’ cells (diatoms) and a ‘small’ copepod {Pseudocalanus) better adapted to feed on ‘small’ cells (flagellates). However, although the general relationship between growth and metabolic rate and body size is widely accepted (Banse, 1976; Fenchel, 1974) this relationship between dietary particle-size composition and size of organism has remained a working hypothesis with little experimental support..

Temporal and Spatial Scales in Phytoplankton Ecology. Mechanisms, Methods, Models, and Management

This article is essentially a review of the temporal and spatial scales of variability in both marine and freshwater planktonic environments and the algal responses to those scales. I assert that there are problems with our present understanding of these scales and the use of inappropriate assumptions concerning the occurrence of steady-state conditions. In a nonsteady-state environment the concepts of limiting nutrients must be changed, and the extrapolation from culture to field conditions is fraught with problems. In this paper I review the evidence for the existence and importance of small-scale, high frequency and large-scale, low frequency variation in the planktonic environment and show that such variation fundamentally affects our understanding of existing processes. Methodology and models must also reflect the true scales of variability which exist. I show that there are, at present, problems with our understanding of planktonic processes which greatly affect our ability to manage water quality. New concepts and models are urgently needed. Finally I propose a new model of community structure and process in variable environments which accounts for the correct 'algal' scales of perturbation and response and allows certain predictions to be made. It is possible to reconcile certain problems and controversies in the literature by the use of such a model. An enhanced ability to manage planktonic systems should result from an improved understanding of the true scales of variability which exist.Key words: lakes, oceans, phytoplankton, communities, nutrients, models, management, eutrophication, fluctuations, scales

Proposal for an Abridged Nitrogen Turnover Cycle in Certain Marine Planktonic Systems Involving Hypoxanthine-Guanine Excretion by Ciliates and their Reutilization by Phytoplankton

Proposal for an Abridged Nitrogen Turnover Cycle in Certain Marine Planktonic Systems Involving Hypoxanthine-Guanine Excretion by Ciliates and their Reutilization by Phytoplankton

The Equilibrium and Stability of Simple Marine Biological Systems. III. Fluctuations and Survival

In even simple planktonic systems, consisting of a constant nutrient supply to primary nutrient users being cropped by herbivores, the possible equilibrium of the system is unstable. The number densities and nutrient concentration in such a system will oscillate widely. An important ecological question is whether or not the system will survive-if one of the number densities becomes very small, under what conditions will it ultimately recover or become extinct? If the population necessarily recovers, the system as a whole can be said to be ecologically stable, even if the equilibrium point is unstable. Two simple systems are studied from this point of view. The first is a linear chain, in which a single nutrient influences the growth of one phytoplankton species, which is cropped by one species of herbivore. If by some chance or another the number density of phytoplankton becomes small, it is shown unconditionally that the number density will, within a finite time, recover. If the herbivore number density becomes very small, condition (17) or (19) is derived for recovery and survival of the system. An interpretation of this condition suggests strongly that in nature it is always satisfied, so that the three-level linear chain is indeed ecologically stable. A second example is considered in which an additional species of phytoplankton is present (such as blue green algae) but is not cropped by the herbivore. It is found unconditionally that the system will recover from events in which the number densities, of either the herbivores or the primary nutrient users being cropped, are very small. Whether the same is true when the numbers of blue green algae become very small is not proved, but there are grounds for conjecturing that it is. In a sense, the presence of blue green algae buffers the rest of the system, decreasing more rapidly under conditions of nutrient depletion and increasing rapidly when nutrients are enriched.

Cybernetic mechanisms in lake plankton systems: how to control undersirable algae

TOXIC algal blooms represent a serious nuisance in lakes and ponds. Not only do they form floating scums and impart odours and tastes to the water, but they are also associated with the occurrence of dermatitis in our own species and fatal poisoning in livestock. To reduce algal levels, man resorts to various practices that range from chemical ‘control’ to artificial mixing of the lake1. Most of these have deleterious, if not immediately toxic, effects on the other compartments of the system as well. In this report safer alternatives are presented.

Development of north sea coastal plankton communities in separate plastic bags under identical conditions

In two experiments lasting 4 to 6 weeks, communities of North Sea coastal plankton kept in separate plastic bags (of about 1400 l) and exposed to the same environmental conditions showed very similar patterns of growth and decline. This result means that the method is suitable for the evaluation of toxic effects of environmental pollutants at low concentrations on complex plankton systems. The phytoplankton in the bags produced a succession of blooms, which were probably limited by shortage of nutrients. The dominant zooplankton organisms were various species of copepods which can develop in the bags from egg to adult. Strong indications were found that mineralization of organic matter occurs in the bags. Chemical parameters and phytoplankton biomass were found not to be stratified, indicating that the contents of the bags were well mixed.

Primary Production Measurements on a Natural Plankton Bloom

A comprehensive series of observations made throughout a natural phytoplankton bloom suggested that culture experiments are satisfactory analogues of natural plankton systems, at least under bloom conditions. Calorific values suggested that there was no tendency for the phytoplankton to store fat in the senescent phase of the bloom. No single quantity was found to be a suitable indicator of the physiological vigour of the population.