Recently developed techniques for estimating bacterial biomass and productivity indicate that bacterial biomass in the sea is related to phytoplankton concentration and that bacteria utilise 10 to 50 % of carbon fixed by photosynthesis. Evidence is presented to suggest that numbers of free bacteria are controlled by nanoplankton~c heterotrophic flagellates which are ubiquitous in the marine water column. The flagellates in turn are preyed upon by microzooplankton. Heterotrophic flagellates and microzooplankton cover the same size range as the phytoplankton, thus providing the means for returning some energy from the 'microbial loop' to the conventional planktonic food chain.

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The Ecological Role of Water-Column Microbes in the Sea*

The discovery that viruses may be the most abundant organisms in natural waters, surpassing the number of bacteria by an order of magnitude, has inspired a resurgence of interest in viruses in the aquatic environment. Surprisingly little was known of the interaction of viruses and their hosts in nature. In the decade since the reports of extraordinarily large virus populations were published, enumeration of viruses in aquatic environments has demonstrated that the virioplankton are dynamic components of the plankton, changing dramatically in number with geographical location and season. The evidence to date suggests that virioplankton communities are composed principally of bacteriophages and, to a lesser extent, eukaryotic algal viruses. The influence of viral infection and lysis on bacterial and phytoplankton host communities was measurable after new methods were developed and prior knowledge of bacteriophage biology was incorporated into concepts of parasite and host community interactions. The new methods have yielded data showing that viral infection can have a significant impact on bacteria and unicellular algae populations and supporting the hypothesis that viruses play a significant role in microbial food webs. Besides predation limiting bacteria and phytoplankton populations, the specific nature of virus-host interaction raises the intriguing possibility that viral infection influences the structure and diversity of aquatic microbial communities. Novel applications of molecular genetic techniques have provided good evidence that viral infection can significantly influence the composition and diversity of aquatic microbial communities.

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Virioplankton: viruses in aquatic ecosystems

Publisher Summary The degree of match and mismatch in the time of larval production and production of their food has been put forward as an explanation of part of the variability in recruitment to a stock of fish. The magnitude of recruitment is not completely determined until the year-class finally joins the adult stock, and the processes involved probably begin early in the life-history of the fish when both their growth and mortality rates are high. The match/mismatch hypothesis is given in this chapter to cover the subsequent development through larval life up to metamorphosis, and possibly just beyond. The match/mismatch hypothesis has now been extended to the upwelling areas and oceanic divergences equatorward of 40° latitude on the basis that fish in these regions release batches of eggs more frequently when they are well fed and, more generally, that pelagic fish may modify their reproductive strategies such that they can feed and spawn at the same time. A delay in predation is of great importance, particularly when production peaks in early development. This model illustrates the difficulties that occur when growth and mortality are allowed to interact. On the other hand, there are three consequences of the match/mismatch hypothesis that are presented in this chapter. However, the limited conclusion drawn in this chapter is that, investigations of fish larvae should continue to be a part of the study of population dynamics of fishes.

Plankton Production and Year-class Strength in Fish Populations: an Update of the Match/Mismatch Hypothesis

Kelp forests are phyletically diverse, structurally complex and highly productive components of coldwater rocky marine coastlines. This paper reviews the conditions in which kelp forests develop globally and where, why and at what rate they become deforested. The ecology and long archaeological history of kelp forests are examined through case studies from southern California, the Aleutian Islands and the western North Atlantic, well-studied locations that represent the widest possible range in kelp forest biodiversity. Global distribution of kelp forests is physiologically constrained by light at high latitudes and by nutrients, warm temperatures and other macrophytes at low latitudes. Within mid-latitude belts (roughly 40–60° latitude in both hemispheres) well-developed kelp forests are most threatened by herbivory, usually from sea urchins. Overfishing and extirpation of highly valued vertebrate apex predators often triggered herbivore population increases, leading to widespread kelp deforestation. Such deforestations have the most profound and lasting impacts on species-depauperate systems, such as those in Alaska and the western North Atlantic. Globally urchin-induced deforestation has been increasing over the past 2–3 decades. Continued fishing down of coastal food webs has resulted in shifting harvesting targets from apex predators to their invertebrate prey, including kelp-grazing herbivores. The recent global expansion of sea urchin harvesting has led to the widespread extirpation of this herbivore, and kelp forests have returned in some locations but, for the first time, these forests are devoid of vertebrate apex predators. In the western North Atlantic, large predatory crabs have recently filled this void and they have become the new apex predator in this system. Similar shifts from fish- to crab-dominance may have occurred in coastal zones of the United Kingdom and Japan, where large predatory finfish were extirpated long ago. Three North American case studies of kelp forests were examined to determine their long history with humans and project the status of future kelp forests to the year 2025. Fishing impacts on kelp forest systems have been both profound and much longer in duration than previously thought. Archaeological data suggest that coastal peoples exploited kelp forest organisms for thousands of years, occasionally resulting in localized losses of apex predators, outbreaks of sea urchin populations and probably small-scale deforestation. Over the past two centuries, commercial exploitation for export led to the extirpation of sea urchin predators, such as the sea otter in the North Pacific and predatory fishes like the cod in the North Atlantic. The large-scale removal of predators for export markets increased sea urchin abundances and promoted the decline of kelp forests over vast areas. Despite southern California having one of the longest known associations with coastal kelp forests, widespread deforestation is rare. It is possible that functional redundancies among predators and herbivores make this most diverse system most stable. Such biodiverse kelp forests may also resist invasion from non-native species. In the species-depauperate western North Atlantic, introduced algal competitors carpet the benthos and threaten future kelp dominance. There, other non-native herbivores and predators have become established and dominant components of this system. Climate changes have had measurable impacts on kelp forest ecosystems and efforts to control the emission of greenhouse gasses should be a global priority. However, overfishing appears to be the greatest manageable threat to kelp forest ecosystems over the 2025 time horizon. Management should focus on minimizing fishing impacts and restoring populations of functionally important species in these systems.

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Kelp forest ecosystems: biodiversity, stability, resilience and future

Heterotrophic bacteria are thought to be important components of aquatic ecosystems in several ways. These bacteria remineralize organic materials and convert some organic material into bacterial biomass. We examined data from 70 studies in which estimates of production of heterotrophic bacterial biomass (bacterial production) were reported for freshand saltwater ecosystems. In sediments, bacterial production was sigdicantly (p <0.001), positively correlated to sediment organic C content. Systems which had hlgh rates of benthic primary production (such as coral reefs) had rates of bacterial production greater than those predicted by sediment organic C content alone. In the photic zone of lakes and the ocean, bacterial production was significantly correlated with planktonic primary production, chlorophyll a, or numbers of planktonic bacteria. For all planktonic systems analysed, bacterial production ranged from 0.4 to 150 pg C 1-' d-' and averaged 20 % (median 16.5 %) of planktonic primary production. On an area1 basis for the entire water column, bacterial production ranged from 118 to 2439 mg m-2 d-' and averaged 30 % (median 27 %) of water column primary production. Heterotrophic bacterial production is, thus, a large component of total secondary production and is roughly twice as large as the production of macrozooplankton for a given level of primary production.

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Bacterial production in fresh and saltwater ecosystems: a cross-system overview

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Vertical distribution and partitioning of organic carbon in mixed frontal and stratified waters of the English Channel

Detritus from the dinoflagellates Scrippsiella (= Peridinurn) trochoidea and Isochrysis galbana and from the diatoms Skeletonema costaturn, Thalassiosira angstii and Chaetoceros tricornuturn incubated at 10 \"C in seawater is colonised by a succession of micro-organisms. Primary microbial decomposers in the incubation experiments were bacterial rods and cocci which reached a peak standing stock carbon of 1.86 f 0.76 % of the carbon supplied to the incubation media by the third day of incubation. The bacteria were subsequently replaced by flagellates which attained a mean peak biomass of 12.5 f 3.58 % of the bacterial biomass by Day 6 before declining. Synchronous measurement of the utilisation of dissolved and particulate components from the incubation media shows that there is a well-defined initial sequence of aggregation of particulate matter to form bacterioparticulate complexes, much as have been recorded for natural waters. During this phase, carbon is mainly utilised from the dissolved component of phytoplankton cell debris, whilst the more refractory components including particulate debris is used more slowly. The dissolved organic component comprises a mean of 34 24 % of the total carbon in the debris and has a 50 % utilisation time of only 1.56 d (37.44 h), whereas the particulate component comprises 65 76 % of the total carbon and has a 50 % utilisation time of as much as 11.56 d (277.4 h). Bacterial carbon conversion efficiency (bacterial carbon/detrital carbon used X 100) during the initial phases of colonisation is 9.8 Sb a value similar to that recorded for bacterial conversion of dissolved components of macrophyte debris. The results suggest that the carbon conversion budget for the decomposition of phytoplankton cell debris is 100 g carbon yielding 4 644 g of bacterial carbon. This value for incorporation of carbon into bacteria from phytoplankton cell debris is much lower than might be anticipated from the absorption efficiency of selected labile components released in small quantities by living phytoplankton. The carbon conversion budget for whole phytoplankton debris thus suggests that as much as 30.8 % of the carbon is mineralised and returned to the environment within 3 d by the bacteria which initially colonise the material, whereas the more refractory 64.4 %, comprising carbon in the particulate components of the cell debris, is mineralised within approximately 11 d by bacteria characteristically associated with the decomposition phase of a phytoplankton bloom.

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Rate of Degradation and Efficiency of Conversion of Phytoplankton Debris by Marine Micro-Organisms

Estimates of the numbers and biomass of bacteria as a function of depth in coastal and upwelling waters off the western approaches to the Engllsh Channel and in the southern Benguela upwelling region off the Cape Peninsula, South Africa, show that the numbers of bacteria are correlated with the standing stocks of phytoplankton as assessed by chlorophyll a concentration. Standing stocks of heterotrophic microflagellates in the size range 3 to 10 pm, amount to some 16.9 % on average, of bacterial standing stocks (mg C m-3) estimated by direct microscopy. Calculations of carbon flow through the microheterotrophic consumer community suggest that approximately 20 to 60 % of primary production, posslbly representing the dissolved components leaching out of, and lost from phytoplankton cells during zooplankton grazing, enters the microbial food chain. Much of this appears to be dissipated by bacteria, with some 5.2 to 8.1 % of the photoassimilated carbon being incorporated into bacterial carbon production. At least 66% of this is exploited by the heterotrophic n~icroflagellates leaving a maximum of 34 % of bacterial production for the larger bactivorous suspension feeders

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Quantitative relationships between phytoplankton, bacteria and heterotrophic microflagellates in shelf waters

The micro-organisms which colonise seawater incubated with mucilage from the kelps Ecklonia maxima or Laminaria pallida show a clear succession. The media are first colonised by bacterial cocci followed by rods which are subsequently replaced by flagellates and ciliates whose combined biomass reaches some 6-10 % of that of the bacteria. The maximal biomass of bacteria is achieved in 7-10 d incubation at 10 \"C but is dependent both on the time of appearance and biomass of the flagellate and ciliate populations. Estimates for the rate of consumption of bacteria by flagellates of only 10 pm3 body volume suggest that mineralisation of bacteria by marine microflagellates may considerably exceed that in larger organisms at higher trophic levels.

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Heterotrophic Utilisation of Mucilage Released During Fragmentation of Kelp (Ecklonia maxima and Laminana pallida). I. Development of Microbial Communities Associated with the Degradation of Kelp Mucilage

There is a well-defined succession of micro-organisms which colonize powdered leaf debris from Spartina alterniflora and Juncus roemerianus, and aged natural detrital material when these were incubated in estuarine water at temperatures near to those recorded in the habitat at the time of collection. The natural assemblage of free-living bacteria in estuarine water rapidly enters logarithmic growth, subsequently declining with the increase in numbers of bactivorous microflagellates. These are then replaced by a mixed population of ciliates, choanoflagellates, amoeboid forms and attached bacteria which form part of a complex microbial community associated with particulate debris. The rate of increase of bacterial cells (μ), in both spring and summer experiments ranged from 0·010–0·108 h−1 whilst estimates of bacterial carbon production ranged from 1·5 to 10·1 μg C 1−1 h−1, values which conform well with estimates obtained from natural assemblages of marine bacteria in coastal and estuarine waters elsewhere. Although both the ease of degradation of the detrital substrate and incubation temperature are of importance, enrichment of both powdered Spartina leaf debris and aged natural detritus with inorganic nutrients evidently enhances bacterial production under experimental conditions. In addition, the amount of carbon utilized to sustain bacterial carbon production shows a significant reduction following enrichment with NH4, NO3 or combinations of NO3 + PO4. The bacterial carbon conversion efficiency (μg C incorporated into bacterial production per μg C consumed) × 100, based on powdered Spartina leaves, and aged natural detritus, is thus increased from 9–14%, to as much as 38% in nutrient enriched media. Since NH4, NO3 and PO4 values are generally low in the water column, it seems likely that bacteria achieve a carbon conversion of only 9–14% on natural suspended detrital material, with the possibility of an enhanced conversion of up to 38% occurring at the sediment-water interface where ammonia regeration occurs. This suggests that suspended bacteria which characterize estuarine waters of salt marsh areas may be responsible for the oxidation of 86–91% of the carbon which enters water column microheterotroph food chains, and are probably implicated in the large CO2 fluxes recently recorded from coastal wetland habitats.

E.A.S. Linley

Papers

Vertical distribution and partitioning of organic carbon in mixed frontal and stratified waters of the English Channel

Rate of Degradation and Efficiency of Conversion of Phytoplankton Debris by Marine Micro-Organisms

Quantitative relationships between phytoplankton, bacteria and heterotrophic microflagellates in shelf waters

Heterotrophic Utilisation of Mucilage Released During Fragmentation of Kelp (Ecklonia maxima and Laminana pallida). I. Development of Microbial Communities Associated with the Degradation of Kelp Mucilage

Bacterial production and carbon conversion based on saltmarsh plant debris