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• W4236692272 abstract "Plankton comprises unicellular plants — phytoplankton — and generally small (millimetres or less) animals — zooplankton — that are adrift on the currents. Phytoplankton are responsible for about 45% of global annual primary production and are grazed by zooplankton, which in turn are suitably sized food items for predators including commercially important fish and great whales. Plankton are vital components of marine and freshwater water-column ecosystems. They also make major contributions to global biogeochemical cycling, and ameliorate atmospheric accumulation of carbon dioxide by ‘pumping’ carbon to the deep sea. The integrity of these roles is under threat from climate-related physiological impacts on individual organisms and on the wide-scale distribution of plankton communities. Plankton comprises unicellular plants — phytoplankton — and generally small (millimetres or less) animals — zooplankton — that are adrift on the currents. Phytoplankton are responsible for about 45% of global annual primary production and are grazed by zooplankton, which in turn are suitably sized food items for predators including commercially important fish and great whales. Plankton are vital components of marine and freshwater water-column ecosystems. They also make major contributions to global biogeochemical cycling, and ameliorate atmospheric accumulation of carbon dioxide by ‘pumping’ carbon to the deep sea. The integrity of these roles is under threat from climate-related physiological impacts on individual organisms and on the wide-scale distribution of plankton communities. Plankton — named from the Greek planao, meaning ‘to wander’ — are aquatic organisms that inhabit the water column (that is, they live away from the seabed, in the pelagic realm) and which by dint of their immobility or weak swimming capability are moved horizontally at the mercy of currents (ocean current velocities are highly variable, but are in the order of about 0.1 to 1 metres per second; millimetre-scale crustacean zooplankton swim at only tens of metres per hour). Ocean currents are not simple, linear features, and rotating eddies at various scales (for example, Figure 1) have major impacts on water movement: passive particles — including plankton — originating as neighbours just a few metres apart can, within months, become separated by hundreds of kilometres. Differences in thermal, nutrient, food, competition and predation conditions along divergent trajectories can have major biological impacts on individual plankton: the obligate wandering existence of plankton has consequences for their growth, reproduction and evolution, as well as their distribution. Plankton are taxonomically diverse, comprised of plants, animals, bacteria and viruses. Globally the biomasses of phytoplankton and zooplankton are about equal, although the doubling time of zooplankton (typically weeks to months) is considerably longer than for phytoplankton (hours to days). The majority of plankton are sub-millimetre to millimetre size, but collectively their dimensions span several orders of magnitude, and zooplankton include jellyfish with tentacles trailing many metres. The attention of the ocean-facing public is fixed principally upon large and charismatic marine animals such as whales, albatrosses and polar bears, but it is the plankton and bacteria that collectively fuel the engine room of the ocean. In addition to single-celled plants, including diatoms and coccolithophores (Figure 1), the phytoplankton also includes photosynthetic cyanobacteria and flagellate protists (dinoflagellates). In oceans, seas and lakes at temperate latitudes, the onset of spring brings a rapid increase in phytoplankton activity. Increasing sunlight and rising temperatures promote photosynthesis in illuminated surface waters (the euphotic zone) which, following vertical mixing by wind and waves during storms of the previous winter, are also rich in essential nutrients like nitrate, phosphate and silicate. Chlorophyll in the proliferating phytoplankton reduces in-water visibility and is measurable from space. The rapid greening of surface waters in spring is called the ‘spring bloom’. About 45% of Earth’s primary production occurs in the marine environment (approximately 50 x 1015 g carbon per year). Planktic diatoms are responsible for fixing at least a quarter of the inorganic carbon in the ocean each year. Photosynthesis by phytoplankton draws carbon dioxide from the atmosphere (Figure 2). About 40% of all anthropogenic carbon dioxide has entered the oceans, resulting in the atmospheric carbon dioxide concentration being about 200 ppm less than would otherwise be the case. Photosynthesis by phytoplankton draws carbon dioxide from the atmosphere (Figure 2). Planktic diatoms are responsible for fixing at least a quarter of the total inorganic carbon in the ocean each year. Seasonal and spatial variability in light intensity, temperature and nutrient availability has major impacts on the abundance, size structure and taxonomic composition of phytoplankton communities, and hence on biogeochemical cycling and pelagic food webs (Figure 2). Global ocean-surface biogeography can be demarked by patterns of phytoplankton production and community composition. Phytoplankton diversity can be extraordinarily high, with dozens or more species per cubic metre of water. In homogenous environments, competitive exclusion usually results in domination by a few species that outcompete others. The observation of many species in the apparently homogenous environment of the relentless sea led to the so-called ‘Paradox of the Plankton’ in the 1960s. There is in fact no paradox because the pelagic environment can be extremely heterogeneous with multiple discrete microlayers, sometimes just a few centimetres thick, each containing distinct communities. This now-evident fine structure could not be resolved with instrumentation available in the 1960s. In temperate zones, rapid phytoplankton growth in spring consumes nutrients in the surface waters. Combined with the stratification of the water column that develops as surface waters warm and storm-driven mixing decreases, this growth leads to the establishment of a vertically separated two-component pelagic system. Warmer, illuminated, nutrient-depleted waters overlay deeper, darker, and cooler nutrient-rich waters. The boundary between these is marked by rapid changes over narrow depth of water density and temperature — the pycnocline and thermocline, respectively — occurring at depths of tens of metres in tidally mixed shelf seas and deeper in tropical open oceans. A deep maximum in chlorophyll concentration can develop just above these boundaries: phytoplankton use a variety of mechanisms to regulate their depth (diatoms can control their buoyancy and alter their shape, and flagellates are motile), and can trade off a reduction in available light energy with depth against improved access to nutrients leaking upwards across the boundary. Nutrient limitation in the euphotic zone causes a succession, as spring progresses, from relatively large, fast-growing phytoplankton species to smaller ones. Smaller cells have larger surface area to volume ratios that are more effective for nutrient acquisition, and so can outcompete larger cells when nutrient concentrations are low. In the North Atlantic, there is a seasonal succession from diatoms (hundreds of μm in diameter), including the simple pill-box Thalassiosira spp., which have a high demand for silica for their armoured cell walls (frustules), to smaller-celled primnisiophytes such as Phaeocystis globosa, individuals of which are about 10 μm in diameter. Whereas diatoms produce frustules of silica, coccolithophores (Figure 1)produce a calcareous outer shell. Carbonate chemistry is susceptible to changing pH, and there are signs that some coccolithophores are already suffering with ocean acidification (caused by the increasing quantities of atmospheric carbon dioxide dissolving in the ocean) because their ability to precipitate their carbonate coccoliths (Figure 1) is compromised. The coccoliths reflect light, enabling coccolithophores such as Emiliania huxleyi to be detected by satellite (Figure 1). Also, satellite imagery reveals spatial patchiness at many scales, and patchiness is a prominent feature of planktic ecosystems: it brings challenges to grazers and predators hunting for food, but enables higher levels of overall production than would be achieved in homogenously distributed communities. In the tropics, nutrient concentrations are typically low because of permanent stratification, limited riverine input and only occasional pulses of wind-borne particulates; in this environment, production is seasonally invariable and low. Small-sized phytoplankton such as nanoflagellates, dinoflagellates and diatoms, including the genus Chaetoceros (less than 20 μm diameter), dominate. Even against the background of low production, there is evidence from satellite data that net primary production has decreased significantly in the tropics since 1999 because of strengthening stratification. However, there is strong disagreement as to whether a decline of 1% per year since 1899 is evident in measurements of chlorophyll concentration and water clarity made at sea. In temperate locations the onset of autumn typically brings storms that break down the thermocline and cause mixing and a replenishment of surface nutrients. In those circumstances, a second phytoplankton bloom can occur before sunlight and temperature dwindle going in to winter. The autumn bloom is generally less intense and shorter-lived than the spring bloom. In polar locations, nutrient availability is less of a constraint, but there is usually only a single open-water bloom in summer because of low temperatures and limited light at other times. Light is in part limited by ice cover, but ice-melt can seed the water column with phytoplankton that had been entrained in the ice, and freshwater (low density) input from melting ice stabilises the upper water column, promoting production. Thus, there may be an early season ice-edge bloom before a later open-water bloom, and the larvae of some zooplankton species graze on the former and the adults on the latter. Despite the high nutrient concentrations that prevail in much of the Southern Ocean, low phytoplankton abundance and hence low chlorophyll concentrations occur because of limited supplies of iron (iron is an essential component of enzymes involved in nitrogen fixation). Some diatoms in low-iron environments have altered photosynthetic architecture, with much-reduced photosystem I and cytochrome b6f complex concentrations. These substantially decrease cellular iron requirements but not photosynthetic rates. A downside of this modification is a loss of ability to acclimate to rapid fluctuations in light intensity, but this is not usually a problem in the clear, open ocean. The realization that photosynthesis was iron-limited in vast swathes of the ocean led to the suggestion that iron fertilization (geoengineering) could increase biological production — all the way up the food chain to fish — and reduce atmospheric carbon dioxide concentration. Increased phytoplankton production has been achieved in some fertilization experiments, but bloom compositions and other outcomes have not always been as expected. Most of the biomass produced by small phytoplankton is, for example, recycled rapidly in the euphotic zone, whereas it is the dead and sinking cells of large phytoplankton that serve to transport carbon to the ocean interior (Figure 2). The sometimes-unintended blooms seen in iron-fertilization experiments are symptomatic of the complexities of food webs, with which human interference often does not end well. Although phytoplankton are the essential base of most pelagic ecosystems, they also have negative impacts. Proliferation of some dinoflagellates (such as Karenia spp.) and diatoms (for example, Pseudo-nitzschia spp.) cause harmful algal blooms — sometimes called ‘red tides’ because of their vivid colours. These adversely affect water quality and can be directly toxic to human bathers, or lead to paralytic shellfish poisoning. They can also have ill effects, sometimes terminal, on wild and farmed fish. Furthermore, proliferation of phytoplankton under conditions of elevated nutrient availability (eutrophication) can produce large quantities of biomass, the bacterial degradation of which consumes oxygen and leads to the development of ‘dead zones’ such as those that periodically occur in the Benguela. Oxygen depletion also occurs in the North Sea: there has been a transition there from the traditional two-peaked spring and autumn phytoplankton blooms to a single, continuous bloom, stimulated in part by nitrates and phosphates in detergents and agricultural fertilizers brought in on rivers. Whereas phytoplankton are the main primary producers in freshwater and marine pelagic ecosystems, zooplankton are the main primary consumers. The marine zooplankton includes members of all animal phyla, but this taxonomic diversity is sometimes collapsed for functional-group analyses to a simple dichotomy of gelatinous or non-gelatinous on the basis of body-carbon percentage. Gelatinous plankton are generally less well known than non-gelatinous because conventional sampling techniques — be they net-based, optical or acoustic — are inefficient for delicate, soft-bodied organisms that have densities very similar to water and which can be almost transparent. Some zooplankton — the holoplankton — live out their entire lives in the plankton, whereas meroplankton spend only some stages of their lifecycles there, escaping either by growth (to larger, more mobile micronekton or nekton) or metamorphosis (to bottom-dwelling benthic stages). The planktic stages of benthic organisms facilitate dispersal, but face increased risk of predation. Crustaceans are particularly well represented in the mesozooplankton (defined as zooplankton in the range of 0.2–20 millimetres), both as meroplankton (such as barnacle and crab larvae) and holoplankton (for example copepods, possibly the most numerous metazoans on earth; Figure 3). However, whereas these are readily apparent in samples from commonly used 200 μm-mesh plankton nets, it is the action of much smaller microzooplankton — heterotrophic nanoflagellates and bacteria — that drive the ‘microbial loop’. Recycling of dissolved organic matter and carbon by the microzooplankton greatly improves the productivity of pelagic systems: the small body sizes of members of this group belie their major importance. Copepods are a particularly well-studied component of the zooplankton. They span a range of sizes from, for example, the cyclopoids (such as Oithona, possibly the “most important copepod in the world’s oceans” according to Gallienne and Robins, 2001Gallienne C.P. Robins D.B. Is Oithona the most important copepod in the world’s oceans?.J. Plankton Res. 2001; 23: 1421-1432Crossref Scopus (311) Google Scholar) to the calanoids (for example, Calanus finmarchicus, Figure 3), and include broadcast spawners and egg incubators. In keeping with the generally size-structured foodwebs in pelagic systems (larger things eat smaller things), smaller zooplankton are herbivorous and larger zooplankton are omnivorous or carnivorous. However, diet also depends on the trophic state of the system: ciliates make up a larger proportion of copepod diet in oligotrophic environments where phytoplankton concentration is low or dominated by small cells, whereas copepods ingest larger amounts of phytoplankton under eutrophic conditions where diatoms dominate. Both herbivorous and carnivorous zooplankton face the challenge of finding food in dynamic and patchy environments. It is generally the case that both are subject to strong bottom-up control (that is, they are food-limited), as opposed to being top-down controlled by predators. In the terrestrial world, plant biomass accumulates because grazer numbers are kept in check by predators, leading to a ‘green world’. The alternative explanation, that plants repel grazers with toxins, has been rejected for terrestrial systems, but there has been a somewhat controversial suggestion that diatoms may have an insidious effect on copepod reproduction because some diatom aldehydes adversely affect copepod embryonic development and hatching success. Although phytoplankton abundance limits zooplankton production, zooplankton are only able to graze phytoplankton cells of a size that can be accommodated by their mouth dimensions. So, although individual Phaeocystis cells, for example (10 μm in diameter), can be consumed, colonial Phaeocystis aggregations are several millimetres in diameter and cannot: sometimes an apparent glut of food is not a glut of available food. Trophic cascades can occur in planktic systems, usually associated with alien invasions, and these can reduce grazing pressure on phytoplankton, leading to particularly strong blooms. Introductions of the predatory ctenophore Mnemiopsis leydii, sometimes in ships’ ballast water, have driven cascades reducing their copepod prey five-fold and leading in turn to a doubling in phytoplankton biomass as grazing pressure was reduced. There has been an increase in reports of jellyfish blooms in the past 15 years or so. These may be climatically driven, or may occur as a consequence of collapse following overfishing of finfish stocks. But whatever the cause, the consequences of outbreaks of these gelatinous plankton can be dramatic. For example, they can block cooling water intakes at nuclear power stations and have caused some to be temporarily shut down. At higher latitudes, zooplankton with annual lifecycles face a race against time, and a grazing challenge, to obtain sufficient food at a fast enough rate to fuel growth, metamorphosis through multiple stages from larvae to adults, and to reproduce in a single season. The energetic demands upon females of egg production are particularly high. Zooplankton that live for more than one year may spawn in advance of the spring bloom such that their emerging larvae have maximum opportunity to graze on the bloom. Some such species, including C. finmarchicus in the North Atlantic and Calanoides acutus in the Southern Ocean, overwinter at depth, where the risk of predation is less than in the illuminated surface waters, in a state of much-reduced activity known as diapause. They emerge from diapause and ascend — sometimes from more than 1,000 metres depth — to spawn. The cues for the end of diapause (which seem unlikely to be light at high latitudes and great depth) are the subject of ongoing study, and may involve hormonal signalling triggered by dwindling lipid reserves, or clocks under genetic control. The overwintering diapause of C. finmarchicus is fuelled by lipid reserves accumulated during the previous summer. An effect of lipid-rich zooplankton descending to depth is referred to as the ‘lipid pump’. This moves carbon into the deep sea at a rate almost equivalent to sinking detritus (such as faecal pellets and the ‘marine snow’ of dead phytoplankton falling at the end of the bloom). Zooplankton also contribute to the ‘biological pump’ that transports surface-fixed carbon downward to the ocean interior because they feed near the surface and excrete at depth. They do this because of their daily, or diel, vertical migration as follows. Although the horizontal swimming prowess of zooplankton is limited in the context of ocean currents, vertical migrations can be extensive and rapid (hundreds of metres in a few hours). Zooplankton adjust their depth in tune with the day–night cycle, feeding in the productive near-surface under cover of darkness and retreating to depth in daytime to hide from visually hunting predators. Diel vertical migration is a ubiquitous feature of pelagic ecosystems, and a wave of ascent and descent propagates around the planet in time with dawn and dusk. The amplitude and wavelength depend upon latitude and time of year, but even in the high Arctic in the polar night, some daily vertical migrations continue, some driven by moonlight. Aside from food availability, the other major control of zooplankton growth is temperature. In keeping with metabolic ecology, temperature influences growth as a function of body size. Temperature also has a major impact on community development. In the Bering Sea, the timing of the melt of sea ice and the subsequent water temperature at which the plankton bloom occurs have marked impacts on zooplankton community development. In years when sea ice melts late the phytoplankton bloom develops almost immediately in the cold water at the ice edge and fuels only a small copepod biomass. Conversely, if ice melts early, wind-driven mixing of the then-uncapped ocean prevents development of the phytoplankton bloom until late in the spring, when it eventually occurs in ‘warm’ water away from the ice edge, and leads to a large copepod biomass. This variability, which influences the size structure as well as the abundance of the zooplankton, has become known as ‘oscillating control’, and explains some of the variability in survival and recruitment of the hugely commercially important Walleye pollock. It has long been known that recruitment of some commercially important fish species is influenced strongly by variability in the timing of zooplankton production, and hence in its availability as food. The ‘match–mismatch’ hypothesis describes fish larvae surfing a wave of zooplankton production; fish recruitment is maximised when peaks in availability of zooplankton prey coincide with peaks of demand by predatory larvae. Phenologic coincidence, or not, has major ecological implications, and climate-related disruption of producer–grazer interactions in the plankton have consequences for higher trophic levels, including fish, and hence for human fishers. In the North Sea, rising water temperature may have decoupled the timing of phytoplankton production — which is controlled principally by light — from zooplankton demand — which is principally a function of temperature. It is not just food quantity that is important, but also quality. The end of the ‘gadoid outburst’ in the North Sea — a period when cod catches were particularly good — coincided with a reduction in the abundance of euphausiid zooplankton which were rich in lipids. As outlined above, temperature plays an important role in zooplankton physiology and growth. It is also a key habitat characteristic, and warming ocean temperatures are bringing wholesale shifts in zooplankton distributions. There is a tendency to consider monitoring as a pedestrian branch of science — an unglamorous cousin of hypothesis-driven research. However, extended time-series observations of zooplankton communities have yielded a great deal of information about our changing oceans. For example, the Continuous Plankton Recorder database contains records of zooplankton abundance along shipping routes throughout the North Atlantic, accumulated almost continually since the 1940s. Analysis of these data revealed a climate-related northward shift of temperate zooplankton by more than 10 degrees latitude between 1960 and 1999 and a poleward squeeze of cold water species. Zooplankton will increasingly be harvested for protein-rich feed for aquaculture, for biomarine products including omega-3 oils, or even for human food. Alister Hardy, the instigator of the Continuous Plankton Recorder surveys, was involved in plans to harvest zooplankton from Scottish sea lochs in the 1940s, when war in Europe had impacted food supply. Commercial harvest of copepods began in Norway — where dense slicks of copepods at the surface are called ‘red feed’ — in the 1950s. By the 1970s, annual catches of more than 50 metric tons were being made. Interest increased in the use of trawl-caught zooplankton for aquaculture, but in 2006 harvesting was banned for precautionary reasons. It has now resumed, and a quota is set. There is a growing recognition, however, that extracting plankton at the base of the food chain may have far reaching trophic consequences. Indeed, there is growing societal acknowledgment of the need to manage ecosystems. At a European level this is enshrined in the Marine Strategy Framework Directive that aims to achieve Good Environmental Status by 2020. However, at the same time as European political unity shows signs of fracture, changes in the plankton such as the redistribution of communities beyond unbounded boundaries may be a force majeure preventing Good Environmental Status in pelagic communities." @default.
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