Lawrence R. Pomeroy

Temperature regulation of bacterial activity during the spring bloom in Newfoundland Coastal waters

While the spring phytoplankton bloom in Newfoundland coastal waters is in progress during April and May, at water temperatures between -1� and +2�C, bacterial growth and respiratory rates remain low. Microbial community respiration is not measurable at -0.2�C. Particulate materials that would be utilized by microorganisms in 2 to 3 days at 20� to 25�C require 11 days at 4�C and 18 days at -0.2�C. Thus, photosynthesis is active but microbial utilization of the products is suppressed. High secondary production in cold water may result from the low rate of microbial decomposition, enabling herbivores to utilize much of the primary production.

Energetics of microbial food webs

The energetic demand of microorganisms in natural waters and the flux of energy between microorganisms and metazoans has been evaluated by empirical measurements in nature, in microcosms and mesocosms, and by simulation models. Microorganisms in temperate and tropical waters often use half or more of the energy fixed by photosynthesis. Most simulations and some experimental results suggest significant energy transfer to metazoans, but empirical evidence is mixed. Considerations of the range of growth yields of microorganisms and the number of trophic transfers among them indicate major energy losses within microbial food webs. Our ability to verify and quantify these processes is limited by the variability of assimilation efficiency and uncertainty about the structure of microbial food webs. However, even a two-step microbial chain is a major energy sink.As an energetic link to metazoans, the detritus food web is inefficient, and its significance may have been overstated. There is not enough bacterial biomass associated with detritus to support metazoan detritivores. Much detritus is digestible by metazoans directly. Thus, metazoans and bacteria may to a considerable degree compete for a common resource. Microorganisms, together with metazoans, are important to the stability of planktonic communities through their roles as rapid mineralizers of organic matter, releasing inorganic nutrients. The competition for organic matter and the resultant rapid mineralization help maintain stable populations of phytoplankton in the absence of advective nutrient supply.At temperatures near O °C, bacterial metabolism is suppressed more than is the rate of photosynthesis. As a result, the products of the spring phytoplankton bloom in high-temperate latitudes are not utilized rapidly by bacteria. At temperatures below 0°C microbial food webs are neither energy sinks or links: they are suppressed. Because the underlying mechanism of low-temperature inhibition is not known, we cannot yet generalize about this as a control of food web processes.Microorganisms may operate on several trophic levels simultaneously. Therefore, the realism of the trophic level concept and the reality of the use of ecological efficiency calculations in ecosystem models is questionable.

Evidence for an enhanced substrate requirement by marine mesophilic bacterial isolates at minimal growth temperatures

Bacterial isolates from the subtropical southeastern continental shelf were cultured in a matrix of temperature and substrate concentrations encompassing a range of temperature and substrate concentrations equal to and exceeding natural ones. At the annual minimum temperature, marine heterotrophic bacterial isolates required higher concentrations of dissolved substrates for active growth than are usually found in seawater. We show this to result from a nonlinear interaction of the combined effects of temperature and substrate concentration on bacterial growth and respiratory rate. As a result, bacterial and protozoan utilization of phytoplankton production during winter and early spring is low, permitting greater energy flow to zooplankton and benthic animals, while in late spring, summer, and fall, the microbial loop dominates energy flux and organic carbon utilization. Escherichia coli shows a similar nonlinear response to temperature at minimal substrate concentrations, albeit at a higher range of concentrations than were utilized by the marine isolates. Thus, bacteria from subtropical regions are shown to have a differential growth response near the minimum temperature for growth, depending on the concentration of available substrates.

Occurrence and respiration of ultraplankton in the upper 500 meters of the ocean

Abstract By gentle concentration the ultraplankton of the open ocean were prepared for estimation of their rate of respiration and for microscopic examination while still alive. Except in coastal waters the predominant organisms of the upper 500 m in the western North Atlantic and Southeast Pacific were generally μ flagellates (2–10 μ). Most bacteria and many flagellates were associated with flocculent organic aggregates. Below 25–50 m most of the flagellates were non-pigmented. Evidence is presented that much of the energy fixed by photosynthesis is utilized by Protista. Organic aggregates appear to be the locus of much of the metabolic activity in the ocean.

Leaching and microbial utilization of dissolved organic carbon from leaves of Spartina alterniflora

Abstract Dissolved organic carbon (DOC) is released from Spartina alterniflora leaves into estuarine water which rhythmically inundates the coastal marshes. The carbon added to the DOC pool in the water averages at least 61 kg ha−1 year−1. The organic material released by leaching appears to be readily utilized by microbes.

Primary production in the Arctic Ocean estimated from dissolved oxygen

Abstract The deep, central basins of the Arctic Ocean have been thought to support little biological production. However, summer dissolved oxygen data from the upper mixed layer of the ice-covered central Arctic Ocean yield estimates of primary production which are high enough to account for oxygen utilization in the halocline. Thus, it may not be necessary to postulate either that all significant primary production is on or near the continental shelves, or that organic matter is transported along isopycnals over decades of time to support respiration in the halocline over the deep basins. Because dissolved oxygen data are available for many parts of the Arctic Basin, it may be possible to begin to look for regional differences in productivity. It remains true that more primary production is occurring on the extensive continental shelves than in the basins, and some dissolved organic matter produced on continental shelves must be entering the basins via the halocline. Some of that dissolved organic matter may also contribute to secondary production and to the observed oxygen utilization. However, the evidence from dissolved oxygen measurements, as well as from the observations on consumers, from bacteria to bears, suggests the presence of a complete, locally supported food web in the permanently ice-covered regions of the Arctic Ocean.

Secondary Production Mechanisms of Continental Shelf Communities

A compartmental model of energy flux through a continental shelf ecosystem is presented which examines the potential for significant energy flow through detritus microorganisms, and dissolved material to terminal consumers. A number of assumptions are examined which have bearing on the outcome of the modeling exercise. Evidence is reviewed that primary production may be substantially higher than the usual 14C method indicates. If this were the case there would be great latitude possible for the other assumptions inherent in the models but for the present it is assumed that current estimates of photosynthesis in the sea are approximately correct. Evidence is presented that the input of primary detritus (new plant material) is a significant one, although relatively little of it is present in samples of particulate matter. Secondary detritus, principally fecal material, takes many forms and is involved in both benthic and pelagic food webs. The observations that bacteria mediate the utilization of detritus by metazoans, but that there are very few bacteria on detritus particles, are not necessarily in contradiction. However, little is known about the rate of growth of bacteria on particles and the frequency with which particles pass through the guts of detritivores. Similar information is also lacking about the population of free-living bacteria in the water.

Microbial distribution and abundance in response to physical and biological processes on the continental shelf of southeastern U.S.A.

The distribution and abundance of bacteria and phytoplankton on the continental shelf of the southeastern United States were observed in relation to physical processes. Phytoplankton production was influenced by inputs of water of reduced salinity from the estuaries and by inputs of high salinity, low-temperature water from the west front of the Gulf Stream. The distribution of chlorophyll suggests that in each case production is influenced both by inputs of nutrients and by the enhanced vertical stability associated with the stratification of waters of different densities. The standing stock of bacteria on the inner shelf, 106 ml−1, is little changed by the influx of water of reduced salinity. On the outer shelf, where the usual standing stock of bacteria is 105 ml−1, the numbers increase to 106 ml−1 in and above intrusions of Gulf Stream water in which phytoplankton blooms have developed, suggesting that the bacteria respond to products of both phytoplankton and zooplankton production. Adenylate energy charge values in the waters of the southeastern shelf are variable and volatile. At times values of 0.7 to 0.8 are widespread over most of the shelf, while at other times values <0.6 are common, with localized patches of high values. Both autotroph-dominated and heterotroph-dominated microbial communities show these variations.

Total plankton respiration

Abstract Total plankton was gently concentrated from sea water, using a membrane filter, and its rate of respiration was quickly measured. Comparable measurements of respiration were made on plankton concentrated with a No. 2 net in a 12-in Clarke-Bumpus sampler. Organisms too small to be retained in the net accounted for 94–99% of the total respiration. In the Gulf Stream and Sargasso Sea flagellates usually appeared to be the most important metabolic component of the plankton.

The Ecosystem Perspective

Although Tansley (1935) is credited with the name, and often with the concept, of ecosystems, the concept of a higher level of organization had been forcing itself to the attention of biologists for many years (Mobius 1877; Forbes 1887; Lotka 1925). Another manifestation was the concept of the superorganism, which was given various interpretations, proponents applying it to anything from a termite colony to the landscape. What survives is the idea of a hierarchy of levels of integration. The term, superorganism, turned out to be an unfortunate one, as it implied specific kinds of organization that are not present beyond the species population. Controls that involve population genetics and natural selection end at the species level. This spills over into community organization, in a sense, through coevolution, but the genetics are still those of interbreeding populations of the individual species. We observe significant homeostasis at the ecosystem level, but it must be understood in terms of the processes of the component populations in their physical and chemical environment. A major thrust of ecosystem studies at this time is to understand in an integrated way the response of communities to their environment, and how this results in the resilience shown by most ecosystems.