An examination of photosynthetic production,excretion of photosynthetic products,and heterotrophic utilization of dissolved organic compounds with reference to results from a coastal subtropical sea

The rate of primary production, excretion of photosynthetic products and turnover of glucose and amino acids was measured at a station in a coastal region in the Bahamas. Over the depths 0 to 50 m, total photosynthetic rates varied from 1.7 to 12.7 gC fixed 1^-1day^-1, averaging 4.3. The extent of extracellular photosynthetic products ranged from undetectable to 23%, averaging 6.9%. Neither the field data nor studies with axenic cultures of Dunaliella tertiolecta, Skeletonema costatum, and Monochrysis lutheri showed any evidence of an increase in the percentage excretion at low population densities or low photosynthetic rates. Rates of amino acid turnover varied from 21 to 168% day^-1, and that of glucose from 8.3 to 41% day^-1. Light seems to have little effect on the uptake and respiration of these substrates by the planktonic population. There was a significant relationship between the fraction of the substrate used for respiration and that retained by the cell. On average, 42% of the glucose taken up was respired and 21% of the amino acid mixture. Tentative calculations suggest that the production of dissolved organic material as extracellular photosynthetic products would be insufficient to supply the heterotrophic population, and it was concluded that some other route(s) must be of major importance.     

Rates of nitrogen fixation on coral reefs across the continental shelf of the central Great Barrier Reef

This study was undertaken in 1981 to determine whether there were major variations in potential rates of nitrogen fixation on apparently bare coralline substrate from reefs across the continental shelf of the central Great Barrier Reef. Nitrogen fixation, measured as rates of ethylene production (nmol cm^-2h^-1), was significantly lower on substrata from two inner-shelf reefs, (0.46 and 1.07) than on two middle-shelf reefs (2.10 and 2.97) and on two outer-shelf reefs (3.20 and 3.81). By contrast, algal biomass (mg cm^-2) on experimental substrate was significantly higher on inner-shelf reefs (80.8 and 59.4) than on middleshelf (27.1 and 23.8) and outer-shelf reefs (26.4 and 22.4). The rate of nitrogen fixation was positively correlated with the proportion of bare substratum and significantly higher concentrations of dissolved inorganic nitrogen were found in waters over the reefs than in water flowing onto those reefs. The abundance of algal-grazing fishes was reported previously to be significantly lower on inner-shelf reefs. It is suggested that this cross-shelf variation in the activity of algal-grazing fishes may be a determinant of the observed cross-shelf variations in potential nitrogen fixation.Contribution No. 233 from the Australian Institute of Marine Science     

Resource distributions among habitats determine solitary bee offspring production in a mosaic landscape.

Within mosaic landscapes, many organisms depend on attributes of the environment that operate over scales ranging from a single habitat patch to the entire landscape. One such attribute is resource distribution. Organisms' reliance on resources from within a local patch vs. those found among habitats throughout the landscape will depend on local habitat quality, patch quality, and landscape composition. The ability of individuals to move among complementary habitat types to obtain various resources may be a critical mechanism underlying the dynamics of animal populations and ultimately the level of biodiversity at different spatial scales. We examined the effects that local habitat type and landscape composition had on offspring production and survival of the solitary bee Osmia lignaria in an agri-natural landscape in California (U.S.A.). Female bees were placed on farms that did not use pesticides (organic farms), on farms that did use pesticides (conventional farms), or in seminatural riparian habitats. We identified pollens collected by bees nesting in different habitat types and matched these to pollens of flowering plants from throughout the landscape. These data enabled us to determine the importance of different plant species and habitat types in providing food for offspring, and how this importance changed with landscape and local nesting-site characteristics. We found that increasing isolation from natural habitat significantly decreased offspring production and survival for bees nesting at conventional farms, had weaker effects on bees in patches of seminatural habitat, and had little impact on those at organic farm sites. Pollen sampled from nests showed that females nesting in both farm and seminatural habitats relied on pollen from principally native plant species growing in seminatural habitat. Thus connectivity among habitats was critical for offspring production. Females nesting on organic farms were buffered to isolation effects by switching to floral resources growing at the farm site when seminatural areas were too distant. Overall local habitat conditions (farm management practices) can help bolster pollinators, but maintaining functional connectivity among habitats will likely be critical for persistence of pollinator populations as natural habitats are increasingly fragmented by human activities.     

Nitrogen fixation (acetylene reduction) associated with the living coral Acropora variabilis

To estimate N₂-fixation, acetylene reduction assays were carried out on portions of the branches of the coral Acropora variabilis from the west coast of Malaysia. In some experiments, a sub-surface incubation apparatus was employed that was designed to keep the coral fragments near to their natural depth of occurrence. Other shipboard experiments used metabolic inhibitors to investigate the class of organism reducing acetylene. Stumps of coral gave the highest rates of activity, probably attributable to loosely associated cyanophytes. Coral tips also reduced acetylene at relatively high rates; reduction was enhanced in light by increased CO₂ concentration and decreased O₂ tensions indicative of photosynthetic bacteria. Algal material was not obvious on the tip surfaces and so the active organism was probably more integral to the coral structure than it was in the stumps. Maximum rates of acetylene reduction measured translated to 2.5 mg N₂ fixed per outcrop per day.     

Amylase,cellulase and protease activities in surface and gut tissues ofDendraster excentricus,Pisaster ochraceus andStrongylocentrotus droebachiensis (Echinodermata)

In general, variance in tissue hydrolase (amylase, cellulase and protease) activity between individuals of a particular species is as great or greater than variance between tissue types or between species, e.g. although the specific activity of amylase in the gut tissue ofDendraster excentricus is significantly lower than that of aboral surface tissue, it is not significantly different than that of the oral surface tissue, or of the aboral surface of eitherPisaster ochraceus orStrongylocentrotus droebachiensis. Cellulase activity of the gut tissue ofS. droebachiensis was significantly higher than that of the gut tissue ofP. ochraceus andD. excentricus, reflecting the omnivorous feeding habit of the sea urchin. Cellulase activity of the gut tissue ofD. excentricus was not significantly higher than inP. ochraceus, an unexpected result, given the highly carnivorous behavior of the latter. Protease activity of the gut tissue ofD. excentricus is significantly higher than that of the oral or aboral surface tissues, but not significantly higher than that of the gut tissues ofP. ochraceus orS. droebachiensis. Amylase and cellulase activities in the guts of detritus- and shrimp-fedD. excentricus showed significant increases over that of field individuals; protease activity displayed no significant increase. Some significant surface (oral and aboral) changes were noted with respect to hydrolase activity, but the direction and lack of consistency of the changes argues that these are incidental to the process of digestion. The lack of significant differences in the surface enzyme activity ofD. excentricus, P. ochraceus andS. droebachiensis suggests that macronutrient digestive events in surface tissues are not uniquely important inD. excentricus.     

Grazing effects on nitrogen fixation in coral reef algal turfs

This study addressed whether grazing by the sea urchin Diadema antillarum influenced rates of nitrogen fixation by algal turf communities on Caribbean coral reefs. Because the turfs were nitrogen-limited, we also assessed whether newly-fixed nitrogen was important for supporting net primary productivity by the turfs. We measured acetylene reduction in turfs grown in treatments excluding or including D. antillarum in the presence of other herbivores at 3 m water depth on Tague Bay forereef, St. Croix, U.S. Virgin Islands. These were the first measurements of acetylene reduction on coral reefs under quasi-natural conditions of high water-flow and photosynthetic oxygen generation. Rates of acetylene reduction under these conditions were as high as any measured previously in coral reef communities (mean 7.6 nmol C₂H₄ cm⁻² h⁻¹). Algal turfs grazed by D. antillarum and other herbivores had chlorophyll-specific acetylene reduction rates up to three times higher than when D. antillarum was excluded. High rates of nitrogen fixation by the turfs were sufficient to meet <2% of the nitrogen required to support net chlorophyll-specific primary productivity over 24 h. Grazer-mediated increases in nitrogen fixation do not appear responsible for a parallel enhancement of net primary productivity. Algal turfs at this site must be dependent primarily on external sources of nitrogen. Received: 1 July 1997 / Accepted: 5 September 1997     

Demands of the herbivore community on phytoplankton production in the Celtic Sea in August

The apodous detritivore holothurian Leptosynapta galliennei (Herapath, 1865) was collected from the intertidal zone (Brittany, France) in spring 1983, and the effect of short-term starvation on the biochemical composition of different body parts was studied. Variations in the levels of the main organic components of the somatic tissues did not correspond to the observed ponderal growth of the gonad in either males or females. Part of the increase in the energetic value of the testis was due to carbohydrates. Accumulation of carbohydrates, proteins, and especially lipids occurred in the ovary. Glycogen, often considered as the most readily mobilized energy source, varied little in concentration or in quantity in any body part. Lipids were not mobilized. No organ of this thin-skinned holothurian functions as a site of rapidly usable food reserves. The organic nitrogen and carbon necessary to compensate for metabolic loss and to permit gonadal growth during shortterm starvation may be provided by dissolved organic matter. An efficient transintegumentary uptake of dissolved organic matter, perhaps mediated by associated subcuticular bacteria, and/or intestinal uptake are hypothesized.     

Vertical distribution,and seasonal and diurnal migration of Calanus helgolandicus in the Celtic Sea

The vertical distribution and migration (seasonal, diel and ontogenetic) of Calanus helgolandicus are described from the shallow (100 m) shelf-seas to the south-west of the British Isles. In 1978 and 1979, the overwintering population of C. helgolandicus consisted primarily of Stage V copepodites and adults. By late winter/early spring the copepodites had moulted to adult females (>90%), which matured and bred the first cohorts of the year, prior to onset of the spring phytoplankton bloom in April/May. C. helgolandicus reached a peak of numerical abundance in August of 20x10³ copepodites m^-2 (over the depth range sampled -0 to 70 m), which was 200 times the population in winter. The seasonal peak of abundance occurred 4 mo after the peak of the bloom of phytoplankton in spring. The yearly development of the copepod was not always out of phase with the diatom bloom, as seen when the data from 1978 was placed in the context of a longer time-series collected at 10 m over 22 yr (1960–1981, inclusive). Large vertical migrations were observed in the younger copepodites (CI and II) in May from below to above the thermocline. In the remainder of the year, the CI and CII stages behaved differently and were located above the thermocline within the euphotic zone. The largest vertical displacements of biomass were seen in the summer months due to the migrations of the CV stages and adults, which had developed from the spring cohorts. It was contended that the seasonal and vertical migrations of C. helgolandicus are part of a more complex pattern of inherent behavior than has been reported previously and that, however difficult this is to discern in the natural populations, it always expresses itself.     

Threatened Status, Rarity, and Diversity as Alternative Selection Measures for Protected Areas: A Test Using Afrotropical Antelopes

A major aim of conservation today is the maintenance of biodiversity. Practically, this pursuit might involve protecting a representative sample of the current biotic diversity (where diversity can have a variety of different meanings as in Vane-Wright et al. 1991), safeguarding species with traits that may be correlated with susceptibility to extinction (see International Council for Bird Preservation 1992), or protecting those species that are currently categorized as under short-term threat of extinction. Priority areas for conservation may vary, however, depending on which of these three approaches is taken. We investigated the designation of priority areas using these different approaches for Afrotropical antelope. Sites were selected on the basis of (1) biotic diversity—simple species richness and taxonomic diversity; (2) uniqueness of the fauna relative to other sites—how geographically restricted the component species were; and (3) degree of endangerment of the fauna. When insufficient sites to represent all the species could be selected, there was little agreement between the priority sites selected using the different methods. Sites selected by each approach were also generally poor at representing the diversity components ranked highly by other approaches. Also, many of the species were represented in only one site in the selected network, which on its own probably does not represent a viable population for the species. Therefore, it is important that the precise aims and consequences of any selection procedure be understood. A combination of different approaches, emphasizing different aspects of biodiversity and implemented sequentially, may be the best compromise for preserving a full range of biotic diversity.