Trophic Regimes Research Articles

Ctenophora in the Arctic: the abundance, distribution and predatory impact of the cydippid ctenophore Mertensia ovum (Fabricius) in the Barents Sea

The Ctenophora Mertensia ovum and Beroe cucumis, collected using both conventional sampling gear and scuba divers, were studied in the Barents Sea east of Bjørnøya and North Norway in spring 1987 and summer 1988. Among the gelatinous zooplankton, Mertensia ovum was the most consistently abundant copepod predator. Feeding experiments were conducted to evaluate the predation rate of M. ovum in various trophic regimes. This ctenophore can take prey varying in size from small copepods to amphipods and krill, but gut-content analyses from field-collected specimens as well as experimental results showed that the main food source for adults was large-sized copepods (e.g. Calanus finmarchicus, C. glacialis, C. hyperboreus, Metridia longa). The robust tentacle arrray of M. ovum makes this species effective as a predator on large prey. The high potential predation rate of this ctenophore relative to its estimated metabolic cost of only 1.7% of the body energy content d?1 suggests that M. ovum may be able to maintain a positive energy balance even in conditions of low prey abundance. It is suggested that a single exploitation of a zooplankton patch may provide energy for survival for a very long time. The potential impact of M. ovum on Barents Sea copepod populations is estimated on the basis of the minimal observed average daily ration in experiments and from field data on gut contents. Using abundances of copepods for the area, and the actual predator biomass collected, it was estimated that an average of 0.7% of the copepod fauna per day could fall prey to this predator.

La végétation de deux tourbières de la municipalité régionale de comté du Haut-Saint-Laurent (Québec)

Within a multidisciplinary study of terrestrial ecosystems of the Haut-Saint-Laurent Regional County Municipality, the vegetation of "Large Teafield" and "Small Teafield" peatlands is described from 127 stands. Detrended correspondence analysis and correlations between data indicate that the greatest vegetational variation is expressed by variables related to trophic regime (e.g., pH of peat) and moisture regime (e.g., organic content). In addition to gradient analysis, hierarchical clustering combined with reference to contingency tables is used to characterize the typology along gradients. Mapping of the community types partially relates the distribution analysis to a circular spatial pattern of the vegetation. Highly oligotrophic sites are colonized by low shrubs. Chamaedaphne calyculata is found at the centre of the peatlands, where the peat deposit is thick and fire does not play an important role. Low shrubs dominated by Betula populifolia surround the Chamaedaphne calyculata vegetation and are more affected by burning towards the perimeter. Heavily burned sites are dominated by Epilobium angustifolium. The perimeters of the peatlands are colonized by Betula populifolia or Populus tremuloides open forests, while the most stable forests are dominated by Acer rubrum. Some marshes and swamps are present where the peat deposit is thin. Sixteen community types are described for the study area.

Abyssal traces and megafauna: comparison of productivity, diversity and density in the Arctic and Antarctic

The megafauna and associated behavioral traces of two deep-sea benthic environments, the central Arctic and Antarctic, with a surface primary productivity differential of 104 were compared to assess the role of food availability in foraging strategy and community structure. Bottom photographs, analyzed for megafauna and trace density and diversity at comparable depths in the Arctic Canada Basin and the Antarctic Bellingshausen Basin, indicated that trace frequency was inversely proportional to organism density but that trace diversity directly reflected organism diversity. Those traces identified in the fossil record to represent efficient foraging strategies, i.e., the Nereites facies, were conspicuously absent at all depths in the Arctic and present at all depths in the Antarctic, in contradiction of the paradigm of increasing behavioral complexity and sediment exploitation as food availability decreases. Presence or absence of surface-grazing organisms seems to exert a greater influence on trace diversity than depth or nutrient supply. Trace density, however, may reflect episodic sedimentation events which intermittently influence the deep-sea trophic regime.

Genetic variation in deep-sea invertebrate populations: The bathyal gastropod Bathybembix bairdii

Five populations of the bathyal trochid gastropod Bathybembix bairdii (Dall) taken from depths of 579 to 1156 m in the Southern California Continental Borderland were surveyed for levels of genetic variability at 18 presumptive gene loci, using techniques of starch gel electrophoresis. All the populations were highly similar genetically; none of the populations possessed unique alleles. Four of 5 polymorphic loci scored in all the populations displayed statistically significant heterogeneity of allelic frequencies among stations. Only the diallelic fumarase (FUM) locus displayed a trend with depth. The geographic and depth patterns of the genetic variability in these populations do not support the hypothesis that the high levels of genetic variability observed in bathyal populations are an artifact of gene flow between populations differentiated with depth. The 16.2% of the loci heterozygous per individual of B. bairdii is of the same order as the average heterozygosities reported for other deep-sea invertebrate taxa. These reports of high genetic variability in a physically constant and seasonless environment are reviewed and discussed in the context of the trophic-stability hypothesis of Ayala and Valentine and the time, size and divergence hypothesis of Soule. Evidence is presented that despite the observed correlation of average heterozygosities in benthic marine invertebrates with seasonality of the trophic regime, this is not apparent for an enzyme system (leucine amino peptidase) which might be expected to display this trend if it reflected a genetic strategy to cope with trophic seasonality. The time, size and divergence hypothesis, focusing on population size, accounts for the general features of levels of genetic variability in deep-sea invertebrates.

LARVAL GROWTH IN BUMBLE BEES (HYMENOPTERA: APIDAE)

AbstractLarval growth in B. terricola Kirby was measured from daily photographs of the combs of colonies. The growth rate of well-fed larvae was exponential until the commencement of cocoon spinning. No difference between the instantaneous growth rates of physiologically determined workers and queens developing in the same larval clump could be detected. Queen morphogenesis consisted principally of an extended stadial duration as compared with workers. Larvae which were starved regained their former growth rates once they were again fed plentifully. As a general feature of larval growth in Bombus it is suggested that the capacity to resume growth at an invariant maximal rate is retained under widely varying trophic regimes. Thus larval growth in bumble bees is interpreted as a succession of bursts of maximum proportional increase separated by periods of temporary starvation induced both by irregularities in colony food intake and the inherent stochasticity of worker feeding behaviour.

Genetic variability in the cosmopolitan deep-water ophiuran Ophiomusium lymani

A population of the cosmopolitan deep-sea ophiuran Ophiomusium lymani was studied by gel electrophoresis, and proved to be highly variable genetically; about 53% of the 15 loci studied are polymorphic, and the average individual is heterozygous at about 17% of the loci. This is approximately the same genetic variability displayed by other species, belonging to other phyla or classes, from the same deep-sea trawl. A similarly high level of genetic variation occurs in deep-sea organisms in general, and in a shallow-water tropical species. Both the deep-sea and the tropics are trophically stable environments. On the other hand, low genetic variabilities have been found in marine species from trophically unstable environments. These data suggest that any phylogenetic effects on genetic variability are secondary, and that the trophic regime may be of major importance in determining genetic strategies of adaptation.

Adaptive Strategies and Structure and Stability of Ecosystem: ABSTRACT

Ecosystem structures and stabilities are based upon the modal adaptive strategies employed by their component populations. These population strategies are, in turn, a reflection of natural selection of individuals for adaptation to particular environmental regimes. The selective pressures that are of primary significance are those related to the trophic resource regime. Fluctuating regimes require flexible responses, broad tolerances, and high reproductive potentials. Ecosystems in such regimes therefore contain few species but may have complex trophic webs; they vary temporally in composition and relative population proportions. Stable regimes permit specialized responses, narrow tolerances, and small populations. Ecosystems in such regimes may contain many species but m y have simple trophic nets and vary but little through time. At present the pattern of ecosystem structure correlates well with the pattern of trophic resource regimes; latitudinally the pattern is chiefly due to variation in solar radiation; longitudinally, to variation in nutrient supply. In the past, variations in resource regimes on a global scale accounted for fluctuations in both diversity and quality of the biota as ecosystem structures adjusted to the changes. To preserve our present marine diversity, nutrient effluents should be engineered so as to stabilize the trophic regimes. End_of_Article - Last_Page 368------------