The importance of silicon for marine production*

A review of silicon, with emphasis on its forms, uptake, dissolution and role in marine primary production, is given. The importance of silicon in marine food webs is discussed, as well as the concentrations of silicon in various areas and the importance of changing N:Si:P ratios. The methodology for measuring silicate transformations has recently been improved by the introduction of the highly enriched ³²Si isotope. Results from uptake experiments using ³²Si in nutrient rich coastal water and in open ocean are presented. The uptake kinetic experiments showed that the silicate uptake usually is unsaturated. We propose that closer attention in the future should be paid to the importance of balanced nutrient composition as well as nutrient supply dynamics for the development of eutrophication versus efficient trophic transfer and fish production in nutrient enriched systems. Close attention should also be paid to the mechanisms that reduce the inputs of silicate to coastal waters.     

22 Nutrient use efficiency and coastal productivity: comparative perspectives on ecosystem structure and mechanisms of control

Net primary production in marine ecosystems ultimately reflects the inputs of nutrients and the efficiency with which nutrients are acquired and used by phytoplankton in growth. In contrast to our understanding of the linkages between nutrient loading and production, the influence of nutrient use efficiency (NUE) on cross-system variations in coastal productivity remains unclear. Nutrient use efficiency at the ecosystem scale is the product of the per capita efficiency of nutrient use in phytoplankton growth and the efficiency with which phytoplankton communities are able to assimilate limiting nutrient(s). We measured the relative dominance of ecosystem N pools by phytoplankton biomass as an index of NUE across 56 inner-shelf sites. These sites were distributed across a strong geographic range of upwelling intensity and productivity along the coasts of Oregon, California and New Zealand. We also compiled an extensive dataset of published NUE values in coastal and oceanic sites in order to assess cross-system patterns and differences in NUE. Our results indicate that exceptional rates of productivity in inner-shelf upwelling systems arise as a consequence of near dominance of ecosystem N pools by phytoplankton biomass. Elevated rates of NUE nevertheless appear to be a transient phenomenon in marine systems. Cross-shelf transects across upwelling fronts off the Oregon coast reveal a temporal pattern of intense phytoplankton blooms and decline that reflects the eventual dominance of ecosystems N pools by detrital and dissolved organic N pools. Our findings suggest that NUE may play a central role in governing the productivity of marine ecosystems.     

Trophic status based on nutrient concentration scales and primary producers community of tropical coastal lagoons influenced by groundwater discharges

Changes in the rate of primary production as an indication of trophic status of aquatic ecosystems have been one of the major indicators of their health. Despite different approaches been devised for evaluating this process, few are useful for comparison and generalization of their results. The coastal zone ecosystems of Yucatan (SE Mexico) exhibit hydrological variability closely associated with the groundwater discharges, which are the only freshwater sources in the coastal ecosystems. In order to learn about the trophic status of karstic tropical coastal lagoons, three of them (Celestun, Chelem and Dzilam) located in Yucatan (SE, Mexico) were monitored for inorganic nutrients, Chl-a, phytoplankton and macrophyte productivity. The nitrate concentrations suggest that the trophic status was influenced by fresh water springs, being meso-eutrophic in Celestun, oligotrophic in Chelem, and mesotrophic in Dzilam. In the case of ammonium ions, the three lagoons were mesotrophic, indicating that processes such as remineralization play an important role in the trophic dynamic of these shallow ecosystems. According to phosphate concentrations, Celestun and Dzilam were mesotrophic, and Chelem was oligotrophic. External inputs of phosphate and bioturbation by waterfowl may be responsible of these differences. Primary productivity at Celestun was greater than at Chelem and Dzilam lagoons and the contribution by seagrasses was significant in all three. It is found that indices based on nutrient concentration and phytoplankton biomass are useful as an indication of trophic status in groundwater influenced coastal lagoons. Moreover, estimations on the total system productivity, and the relative contribution of each primary producer, is a holistic approach useful for understanding trophic dynamic in shallow tropical coastal ecosystems.     

Nutrient dynamics and pelagic food web interactions in oligotrophic and eutrophic environments: an overview

The concept of limiting nutrients is a cornerstone of theories concerning the control of production, structure and dynamics of freshwater and marine plankton. The current dogma is that nitrogen is limiting in most marine environments while freshwater ecosystems are mostly phosphorus-limited, although evidence of phytoplankton limitation by either N or P has been found in both environments.However, the same considerations apply to the availability of phosphorus in freshwater as to nitrogen in oceans. In resource-limited environments the plankton dynamics depend mostly on the internal mechanisms which act to recycle the limiting nutrient many times over within the surface waters. As the overall productivity increases, this dependence on nutrient regeneration decreases.The relationship between the stock of limiting nutrient, rates of supply and plankton dynamics must therefore be seen in the light of the processes operating within the entire food chain over quite different time scales. There is strong evidence that process-rates are mostly size-dependent and that food web interactions at the microbial level (picophytoplankton, bacteria, microheterotrophs) strongly effect the production of carbon and the regeneration of nutrients in the pelagic zone.     

Bacterial Growth Efficiency in a Tropical Estuary: Seasonal Variability Subsidized by Allochthonous Carbon

Bacterial growth efficiency (BGE) is a key factor in understanding bacterial influence on carbon flow in aquatic ecosystems. We report intra-annual variability in BGE, and bacteria-mediated carbon flow in the tropical Mandovi and Zuari estuaries (southwest India) and the adjoining coastal waters (Arabian Sea). BGE ranged from 3% to 61% and showed clear temporal variability with significantly (ANOVA, p < 0.01) higher values in the estuaries (mean, 28 ± 14%) than coastal waters (mean, 12 ± 6%). The greater variability of BGE in the estuaries than coastal waters suggest some systematic response to nutrient composition and the variability of dissolved organic matter pools, as BGE was governed by bacterial secondary production (BP). Monsoonal rains and its accompanied changes brought significant variability in BGE and bacterial productivity/primary productivity (BP/PP) ratio when compared to nonmonsoon seasons in the estuaries and coastal waters. High BP/PP ratio (>1) together with high carbon flux through bacteria (>100% of primary productivity) in the estuarine and coastal waters suggests that bacterioplankton consumed dissolved organic carbon in excess of the amount produced in situ by phytoplankton of this region, which led to the mismatch between primary production of carbon and amount of carbon consumed by bacteria. Despite the two systems being subsidized by allochthonous inputs, the low BGE in the coastal waters may be attributable to the nature and time interval in the supply of allochthonous carbon.     

Can export of organic matter from estuaries support zooplankton in nearshore,marine plumes?

Marine and terrestrial ecosystems are connected via transfers of nutrients and organic matter in river discharges. In coastal seas, such freshwater outflows create prominent turbidity plumes. These plumes are areas of high biological activity in the pelagos, of which zooplankton is a key element. Conceptually, the increased biomass of zooplankton consumers in plumes can be supported by two alternative trophic pathways—consumption of fresh marine phytoplankton production stimulated by riverine nutrients, or direct trophic subsidies through the uptake of terrestrial and estuarine organic matter flushed to sea. The relative importance of these two pathways has not been established previously. Isotopic tracing (carbon and nitrogen) was used to measure the extent of incorporation of marine versus terrestrial matter into mesozooplankton consumers in the plumes off a small estuary in eastern Australia. Replicate zooplankton samples were taken during baseflow conditions with minimal freshwater influence to the sea, and during pulsed discharge events that generated turbidity plumes in coastal waters. Food sources utilized by zooplankton differed among locations and with the strength of freshwater flow. Terrestrial and estuarine carbon only made a sizeable contribution (47%) to the carbon demands of zooplankton in the lower estuary during pulsed freshwater flows. By contrast, in plumes that developed in nearshore marine waters, phytoplankton supplied up to 90% of the dietary carbon of zooplankton feeding in the plumes. Overall, it was “fresh” carbon, fixed by marine phytoplankton, the growth of which became stimulated by fluvial nutrient exports, that dominated energy flows in plume regions. The trophic role of terrestrial and estuarine organic exports was comparatively minor. The trophic dynamics of plankton in small coastal plumes is closely linked to variations in freshwater flow, but this coupling operates mainly through the enhancement of in-situ phytoplankton production rather than cross-boundary transfers of organic matter to marine food webs in the pelagos.     

Impacts of Nutrient Reduction on Coastal Communities

Eutrophication due to high anthropogenic nutrient loading has greatly impacted ecological processes in marine coastal waters and, therefore, much effort has been put into reducing nitrogen and phosphorus discharges into European and North-American waters. Nutrient enrichment usually resulted in increase of biomass and production of phytoplankton and microphytobenthos, often coinciding with shifts in species composition within the primary producer community. Consequences of increasing eutrophication for higher trophic levels are still being disputed, and even less is known about the consequences of nutrient reduction on coastal food webs. Here, we present 30-year concurrent field observations on phytoplankton, macrozoobenthos and estuarine birds in the Dutch Wadden Sea, which has been subject to decades of nutrient enrichment and subsequent nutrient reduction. We demonstrate that long-term variations in limiting nutrients (phosphate and silicon) were weakly correlated with biomass and more strongly with community structures of phytoplankton, macrozoobenthos and estuarine birds. Although we cannot conclusively determine if, and if so to what extent, nutrient enrichment and subsequent nutrient reduction actually contributed to the concurrent trends in these communities, it appears likely that part of the variance in the studied coastal communities is related to changes in nutrient loads. Our results imply that nutrient reduction measures should not ignore the potential consequences for policies aimed at bird conservation and exploitation of marine living resources. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Using primary productivity as an index of coastal eutrophication: the units of measurement matter

Eutrophication is a serious environmental and economic problemin coastal marine ecosystems worldwide. It has recently beenrecommended that measurements of primary productivity, beinga sensitive and accurate indicator of eutrophication, shouldbe mandatory when monitoring and assessing the ecological statusof coastal waters. The units of primary productivity chosenfor eutrophication assessment will be very important becausenot all measures of primary productivity vary monotonically(or even straightforwardly) with changes in aquatic fertility.Volumetric expressions of primary productivity (rates of carbonfixation per unit volume of seawater) may prove to be the mostsensitive and most reliable measures to use when evaluatingthe eutrophication status of coastal marine ecosystems. Anotherpotential measure of primary productivity, the light-saturatedrate of photosynthesis per unit Chlorophyll a (P:B^Chl) ratio,is unsuitable for the assessment of aquatic ecosystem responsesto nutrient enrichment.     

The influence of coastal upwelling on the functional structure of rocky intertidal communities

Summary Relationships between organisms at all trophic levels are influenced by the primary productivity of the ecosystem, and factors which enhance rates of primary production may modify trophic relationships and community structure. Nutrient enrichment of intertidal and nearshore waters leads to enhanced production by intertidal algae, and it was hypothesized that where rocky shores are washed by nutrient-rich upwelled waters, the intertidal communities should show a characteristic functional structure, based on the effects of enhanced primary production. Study sites were chosen on rocky shores in southern Africa, central Chile and the Canary Islands, in areas with and without coastal upwelling, and mid-shore community structure at these sites was analysed in terms of the abundance of certain functional guilds of organisms.It was found that algal cover and the biomass of herbivorous limpets supported per unit area on rocky shores were significantly greater in regions of coastal upwelling than in regions where upwelling did not occur. Ground cover by sessile filter-feeding organisms was significantly greater on shores in non-upwelled areas. However, correspondence analysis showed no functional aspect of intertidal community structure that was characteristic of coasts washed by upwelled waters. Primary reasons for this are probably the large variations in the nature of nutrient enrichment that accompanies upwelling, and in the nutrient status of non-upwelled areas. Other factors are man's exploitation of intertidal organisms and differences in the genetic origins of the intertidal species involved.     

Salt marshes: biological controls of food webs in a diminishing environment

This essay reviews two important topics in coastal ecology: the work on the relative role of bottom–up and top–down controls in natural communities and the loss of wetlands worldwide. In salt marshes and other coastal wetlands, bottom–up and top–down mechanisms of control on natural communities are pervasive. Bottom–up effects through nutrient supply may propagate to upper trophic levels via better food quality, or indirectly by altering water and sediment quality. Top–down control by consumers alters lower trophic levels through consumption of primary producers, and indirectly by trophic cascades in which higher predators feed on grazers. The combined forcing of bottom–up and top–down controls govern assemblages of species in natural communities, mediated by physical and biogeochemical factors. Although there is much information about biological controls of coastal food webs, more information is needed. Even more important is that large losses of wetland are occurring along coastlines worldwide due to a variety of economic and social activities including filling, wetland reclamation, and sediment interception. Such losses are of concern because these wetlands provide important functions, including export of energy-rich material to deeper waters, nursery and stock habitats, shoreline stabilization, and intercept land-derived nutrients and contaminants. These important functions justify conservation and restoration efforts; barring such efforts, we will find it increasingly difficult to find coastal wetlands where we can continue to gain further understanding of ecology and biogeochemistry and lack the aesthetic pleasure these wetlands provide to so many of us.     

Eutrophication and macroalgal blooms in temperate and tropical coastal waters: nutrient enrichment experiments with Ulva spp.

Receiving coastal waters and estuaries are among the most nutrient‐enriched environments on earth, and one of the symptoms of the resulting eutrophication is the proliferation of opportunistic, fast‐growing marine seaweeds. Here, we used a widespread macroalga often involved in blooms, Ulva spp., to investigate how supply of nitrogen (N) and phosphorus (P), the two main potential growth‐limiting nutrients, influence macroalgal growth in temperate and tropical coastal waters ranging from low‐ to high‐nutrient supplies. We carried out N and P enrichment field experiments on Ulva spp. in seven coastal systems, with one of these systems represented by three different subestuaries, for a total of nine sites. We showed that rate of growth of Ulva spp. was directly correlated to annual dissolved inorganic nitrogen (DIN) concentrations, where growth increased with increasing DIN concentration. Internal N pools of macroalgal fronds were also linked to increased DIN supply, and algal growth rates were tightly coupled to these internal N pools. The increases in DIN appeared to be related to greater inputs of wastewater to these coastal waters as indicated by high δ¹⁵N signatures of the algae as DIN increased. N and P enrichment experiments showed that rate of macroalgal growth was controlled by supply of DIN where ambient DIN concentrations were low, and by P where DIN concentrations were higher, regardless of latitude or geographic setting. These results suggest that understanding the basis for macroalgal blooms, and management of these harmful phenomena, will require information as to nutrient sources, and actions to reduce supply of N and P in coastal waters concerned.     

THE EFFECT OF NITROGEN SOURCE ON THE GROWTH AND TOXICITY OF THREE POTENTIALLY HARMFUL DINOFLAGELLATES

Increases in population and agriculture in coastal areas can result in increased nutrient inputs and alterations in the ratios of organic to inorganic nutrients in coastal waters. Such changes in coastal nutrient regimes can affect phytoplankton community structure by creating conditions favorable for growth and dominance of algae that were not dominant before. The effect that changes in ratios and concentrations of nutrients have on toxicity of harmful algal species is not well known. There seems to be a relationship; however, between nutrient stress and toxin production among harmful phytoplankton producing low‐N toxins, e.g. Diarrhetic Shellfish Poisoning (DSP) toxins. Even less is known about the relationship between organic nutrient uptake and toxin production. Benthic species and species in coastal areas are probably exposed to greater fluxes of dissolved organic nitrogen (DON). In this study, benthic and planktonic species of Prorocentrum were grown on L1 media with the sole N‐source varying among treatments as nitrate, ammonium, urea, L‐glutamic acid, and high molecular weight natural DON. An ELISA specific to the DSP toxins, okadaic acid and 35‐methylokadaic acid, was used to determine toxin production by each species when grown on the different N sources. Preliminary results indicate that some organic forms of N support growth as well as inorganic forms for Prorocentrum minimum, P. mexicanum, and P. hoffmannianum.     

A decline in primary production in the North Sea over 25 years,associated with reductions in zooplankton abundance and fish stock recruitment

Phytoplankton primary production is at the base of the marine food web; changes in primary production have direct or indirect effects on higher trophic levels, from zooplankton organisms to marine mammals and seabirds. Here, we present a new time‐series on gross primary production in the North Sea, from 1988 to 2013, estimated using in situ measurements of chlorophyll and underwater light. This shows that recent decades have seen a significant decline in primary production in the North Sea. Moreover, primary production differs in magnitude between six hydrodynamic regions within the North Sea. Sea surface warming and reduced riverine nutrient inputs are found to be likely contributors to the declining levels of primary production. In turn, significant correlations are found between observed changes in primary production and the dynamics of higher trophic levels including (small) copepods and a standardized index of fish recruitment, averaged over seven stocks of high commercial significance in the North Sea. Given positive (bottom‐up) associations between primary production, zooplankton abundance and fish stock recruitment, this study provides strong evidence that if the decline in primary production continues, knock‐on effects upon the productivity of fisheries are to be expected unless these fisheries are managed effectively and cautiously.     

Microcommunities and microgradients: Linking nutrient regeneration,microbial mutualism,and high sustained aquatic primary production

Nutrient regeneration is essential to sustained primary production in the aquatic environment because of coupled physical and metabolic gradients. The commonly evaluated ecosystem perspective of nutrient regeneration, as is illustrated among planktonic paradigms of lake ecosystems, functions only at macrotemporal and spatial scales. Most inland waters are small and shallow. Consequently, most organic matter of these waters is derived from photosynthesis of emergent, floating-leaved, and submersed higher plants and microflora associated with living substrata and detritus, including sediments, as well as terrestrial sources. The dominant primary productivity of inland aquatic ecosystems is not planktonic, but rather is associated with surfaces. The high sustained rates of primary production among sessile communities are possible because of the intensive internal recycling of nutrients, including carbon. Steep gradients exist within these attached microbial communities that (a) require rapid, intensive recycling of carbon, phosphorus, nitrogen, and other nutrients between producers, particulate and dissolved detritus, and bacteria and protists: (b) augment internal community recycling and losses with small external inputs of carbon and nutrients from the overlying water or from the supporting substrata; and (c) encourage maximal conservation of nutrients. Examples of microenvironmental recycling of carbon, phosphorus, and oxygen among epiphytic, epipelic, and epilithic communities are explained. Recalcitrant dissolved organic compounds from decomposition can serve both as carbon and energy substrates as well as be selectively inhibitory to microbial metabolism and nutrient recycling. Rapid recycling of nutrient and organic carbon within micro-environments operates at all levels, planktonic as well as attached, and is mandatory for high sustained productivity.     

Shifting sources of productivity in the coastal marine tropics during the Cenozoic era

Changes in the rates and sources of marine primary production over time are difficult to document owing to the absence of direct estimates of past productivity. Here, I use the maximum body sizes of the largest species in each of 23 tropical shallow-water marine molluscan guilds (groups of species with similar habits and trophic roles) to trace the relative importance of planktonic and benthic primary productivity from the Eocene (55 Ma) onwards. The largest members of guilds are least constrained in exploiting resources and therefore reflect the availability and accessibility of those resources most accurately. Maximum sizes of suspension-feeders and predators increased by a factor of 2.3 and 4.0, respectively, whereas those in four out of five herbivorous guilds declined. I interpret these patterns, which are discernible throughout the coastal tropics, to mean that primary production in the Eocene marine tropics was concentrated on the seafloor, as is the case today on offshore reefs and islands, and that the Miocene to the recent interval witnessed a dramatic increase in planktonic productivity along continental margins. The rise in planktonic fertility is best explained by an increase in nutrient supply from the land associated with intense global tectonic activity and more vigorous ocean mixing owing to cooling.     

Spatial and temporal variability of nutrient retention in river basins: A global inventory

Nutrient export by rivers may cause coastal eutrophication. Some river basins, however, export more nutrients than others. We model the Basin-Wide Nutrient Export (BWNE) Index, defined as nutrient export by rivers as percentage of external nutrient inputs in the basins. We present results for rivers worldwide for the period 1970–2050. The results indicate that nutrient retentions differ largely among basins. They indicate that BWNE increases with nutrient inputs to the land, indicating that the percentage of, for instance, fertilizers exported to sea increases with fertilization rate. We argue that a better understanding of the BWNE Index might help to identify where measures and technologies to reduce nutrient inputs to coastal waters are most effective.     

Upwelling-like bottom intrusion enhances the pelagic–benthic coupling by a fish predator in a coastal food web

Upwelling regions where nutrients are transported from deep to surface waters are among the most productive in the oceans. Although it is well known that the upwelling affects fishery production through bottom-up trophic cascading, it remains unexplored how temporal variation in its intensity alters overall trophic energy flows within a focal food web. In the present study, we demonstrate that inter-annual variation in the intensity of upwelling-like bottom intrusion alters food web properties in coastal waters of the Uwa Sea by focusing on the levels of δ¹³C and δ¹⁵N for a demersal fish predator, Acropoma japonicum. This approach integrates information on prey–predator interactions. In the season following a stratification period when pelagic productivity is limited by nutrient availability, A. japonicum showed lower levels of δ¹³C in years with high bottom intrusion intensity than in those with low intensity. One possible cause for this isotopic depletion is that the bottom intrusion-induced nutrient supply enhances pelagic productivity and consequently facilitates a foraging shift by A. japonicum from ordinary benthic prey to supplementary pelagic prey with a lower δ¹³C. In conclusion, the increased intensity of bottom intrusion results in coupling of two major trophic energy flows, pelagic and benthic food chains, through the demersal predator’s foraging shift.     

Nutrient-enhanced productivity in the northern Gulf of Mexico: past,present and future

Nutrient over-enrichment in many areas around the world is having pervasive ecological effects on coastal ecosystems. These effects include reduced dissolved oxygen in aquatic systems and subsequent impacts on living resources. The largest zone of oxygen-depleted coastal waters in the United States, and the entire western Atlantic Ocean, is found in the northern Gulf of Mexico on the Louisiana/Texas continental shelf influenced by the freshwater discharge and nutrient load of the Mississippi River system. The mid-summer bottom areal extent of hypoxic waters (<2 mg l^–1 O₂) in 1985–1992 averaged 8000 to 9000 km² but increased to up to 16000 to 20700 km² in 1993–2001. The Mississippi River system is the dominant source of fresh water and nutrients to the northern Gulf of Mexico. Mississippi River nutrient concentrations and loading to the adjacent continental shelf have changed in the last half of the 20th century. The average annual nitrate concentration doubled, and the mean silicate concentration was reduced by 50%. There is no doubt that the average concentration and flux of nitrogen (per unit volume discharge) increased from the 1950s to 1980s, especially in the spring. There is considerable evidence that nutrient-enhanced primary production in the northern Gulf of Mexico is causally related to the oxygen depletion in the lower water column. Evidence from long-term data sets and the sedimentary record demonstrate that historic increases in riverine dissolved inorganic nitrogen concentration and loads over the last 50 years are highly correlated with indicators of increased productivity in the overlying water column, i.e. eutrophication of the continental shelf waters, and subsequent worsening of oxygen stress in the bottom waters. Evidence associates increased coastal ocean productivity and worsening oxygen depletion with changes in landscape use and nutrient management that resulted in nutrient enrichment of receiving waters. A steady-state model, calibrated to different observed summer conditions, was used to assess the response of the system to reductions in nutrient inputs. A reduction in surface layer chlorophyll and an increase in lower layer dissolved oxygen resulted from a reduction of either nitrogen or phosphorus loading, with the response being greater for nitrogen reductions.     

La Posidonia oceanica: problemi di ecologia,di funzionamento e di biologia

Abstract

The functional roles of seagrasses in the economy of coastal waters are to be fully realised and more clearly delineated. The biology, ecology and energetics of seagrasses meadows of the Mediterranean Sea, and in particular of Posidonia oceanica inclusing the stabilization of coastal sediments, this role as a base for major detrital food chains, nutrient resources contributory to general productivity of the open sea and the support of dense epiphytic communities, are discussed.     

Dissolved primary production and the strength of phytoplankton- bacterioplankton coupling in contrasting marine regions

We analyzed the strength of phytoplankton–bacterioplankton coupling by comparing the rate of particulate (PPP) and dissolved primary production (DPP) with bacterial carbon demand (BCD) in four contrasting marine regions: offshore and coastal waters of the Southern Ocean, a coastal area of the NE Atlantic, and a coastal–offshore transect in the NW Mediterranean. We measured bacterial heterotrophic production (BHP) and estimated BCD from a literature model. Average phytoplanktonic percent extracellular release [PER = DPP/(DPP + PPP)] was 18–20% in the Antarctic (offshore and coastal, respectively), 16% in the NW Mediterranean, and 7% in the NE Atlantic. A significant inverse relationship was found between PER and total system productivity with pooled data. On average BHP amounted to <5% of total primary production in all regions. However, the strength of phytoplankton–bacterioplankton coupling, estimated as the potential importance of DPP in meeting BCD, differed greatly in the four regions. DPP was highly correlated to BCD in offshore Antarctic waters and was sufficient to meet BCD. In contrast, BCD exceeded DPP and bore no significant relationship in the remaining regions. The data suggest that a strong dependence of bacteria on algal extracellular production is only expected in open-ocean environments isolated from coastal inputs of DOC.