Total Plankton Biomass Research Articles

Regime shift in a coastal pelagic ecosystem with increasing human-induced nutrient inputs over decades

Human-induced nutrient inputs to global coastal waters are leading to increasing nutrients and escalating eutrophication. However, how aquatic ecosystem functioning responds to these changes remains insufficiently studied. Here we report the long-term changes in the nutrient regime and planktonic ecosystem functioning in the Daya Bay, a typical subtropical semi-enclosed bay experiencing rapid economic and social development for several decades. Time-series (from 1991 to 2018) data with a mostly quarterly resolution were collected to depict long-term changes in dissolved inorganic nutrients and plankton abundances, based on which we constructed simplified abundance size spectra (SASS) and plankton abundance ratios to describe the functioning of the planktonic ecosystem. The results revealed a long-term increase in system productivity but a decrease in integrated energy transfer efficiency of the planktonic ecosystem, with rising concentrations of dissolved inorganic nitrogen (DIN). Shifts in the nutrient regime and planktonic ecosystem functioning were detected at a tipping point or threshold around 2006–2007. The shifts were characterized by abrupt changes in the trends of nutrient (phosphate, ammonia, nitrite) concentrations, nutrient ratios (DIN/phosphate, silicate/phosphate), plankton abundance, and total plankton biomass. Compared to the nutrient regime, the planktonic ecosystem functioning shifted several years later. Overall, this study indicates that the pelagic ecosystem regime can shift significantly in response to long-term increasing input of human-induced nutrients in coastal waters such as the Daya Bay. The regime shifts may have profound implications for fishery production, and ecosystem management in the bay.

Scaling effects of a eutrophic river plume on organic carbon consumption

AbstractThe amount of fluvial input has important impacts on shelf regions. To understand how the magnitude of fluvial discharge affects plume ecosystems, particularly organic carbon consumption, data over 10 consecutive summers (2003–2012) were examined in the Changjiang River plume of the East China Sea. The area of the Changjiang River plume ranged from approximately 4.90 × 103 km2 to 94.83 × 103 km2 and varied in proportion to the freshwater discharge rate. Plankton community respiration was at the medium to high end of the values reported for coastal regions. Total organic carbon consumption over the Changjiang River plume was positively correlated with the Changjiang freshwater discharge. This scaling relationship might be associated with river flows delivering allochthonous organic carbon and dissolved inorganic nutrients into the East China Sea. However, plankton community respiration (per m3 basis) decreased as the area of the Changjiang River plume increased; this rate appeared to have been influenced by particulate organic carbon (POC) levels and/or total plankton biomass. Even though POC was dominated by phytoplankton biomass allometrically, bacteria contributed more to plankton community respiration, possibly signifying that these bacteria were more reliant on plankton‐derived organic matter. Even with high phytoplankton biomass and primary productivity, this plume ecosystem was heterotrophic, albeit close to a state of carbon balance. To support high organic carbon consumption, in addition to allochthonous sources from fluvial runoff, a large amount of organic matter might have been autochthonous and derived from plankton in the Changjiang River plume.

ТРАНСФОРМАЦИЯ ОДНОКЛЕТОЧНОГО ПЛАНКТОНА В СИСТЕМЕ РЕКА–ЗАЛИВ–РАВНИННОЕ ВОДОХРАНИЛИЩЕ В НАЧАЛЬНОЙ ФАЗЕ ЦИАНОБАКТЕРИАЛЬНОГО ЦВЕТЕНИЯ

The structure and spatial distributionof unicellular plankton of the river Usa, Usinsky Bay and the adjacent section of the Kuibyshev reservoir in the initial period of cyanobacterial bloom is discussed. The greatest development of plankton was recorded in the central part of Usinsky Bay. In the river part, the basis of the total plankton biomass was formed by heterotrophic bacteria and diatoms, and in the bay and reservoir –by cyanobacteria, mainly of the genera Aphanizomenon and Anabaena. Among eukaryotes in Usinskiy Bay, chlorophytesand diatoms prevailed, and in the reservoir - diatoms and ciliates. Another feature of plankton in the bay and reservoir was the increased proportion of heterotrophic bacteria and ciliates associated with cyanobacteria. The analysis of the structural transformation of communities made it possible to distinguish two main clusters of communities, "lotic" and "letic", which differ in their structure and quantitative characteristics.

Rising temperature and marine plankton community dynamics: Is warming bad?

Global warming is a major threat to the natural environment worldwide with potential adverse impact on plankton community. This will ultimately lead to a change in the dynamics of aquatic food webs. In this study we used seasonally forced multi-species version of the classic Rosenzweig–MacArthur predator–prey model to understand the role and stochastic influence of increasing temperature on marine plankton. First, stable coexistence of four phytoplankton and three zooplankton species was created in a system and then the level of temperature changed to achieve our research goal. We found that the stable coexistence of phytoplankton and zooplankton was related to periodic shifts in species biomass, variation in inter-specific competition and niche configuration. Warming significantly reduced total plankton biomass and changed turnover time of a species, with gradual warming breaking the stable coexistence of phytoplankton and zooplankton. In addition, we found that warming make specialist species more vulnerable than generalist species. After adding noise, a significant variation was observed in plankton biomass and amplification of noise was higher for phytoplankton compared to zooplankton. These results suggest that stochastic or unpredictable nature of temperature fluctuations may create a window of opportunity for the emergence of new species. Overall, warming would induce a shift in plankton dynamics and thereby exert pressure on plankton dependent communities such as fish in the long run.

Planktonic Community of a Large Eutrophic Reservoir during a Period of Anomalously High Water Temperature

The rise in temperature to anomalously high values for the Middle Volga (27–29°С) in the Cheboksary Reservoir in the summer of 2010 significantly increased (2–14 times) the biomass of some plankton components (phytoplankton, bacterioplankton, protozoans, and zooplankton) and the community as a whole. The total plankton biomass reached a record value of 3 g C/m3 for deep parts in the Upper and Middle Volga. The cyanobacterial bloom of water was observed in most of the reservoir. Differences in the structure of the plankton community and the bacterioplankton-to-phytoplankton production ratio were found between riverine and lacustrine parts of the reservoir. Autotrophic (in the riverine part) and heterotrophic (in the lower lacustrine part) stages of the plankton community development were observed simultaneously in different parts of the reservoir. The functioning of the plankton community in a water temperature that exceeded usual summer heating by almost 9°C led to the deterioration of water quality in the reservoir.

Do anthropogenic hydrological alterations in shallow lakes affect the dynamics of plankton?

• Plankton was affected differently by natural and anthropogenic water level changes. • Higher total plankton biomass with anthropogenic WL regulation. • Higher zooplankton species richness with anthropogenic WL regulation. • Higher phytoplankton species richness with natural WL changes. • Higher cyanobacterial microcystins concentration with anthropogenic WL regulation. Anthropogenic and natural climate-impacted water level (WL) fluctuations are one of many environmental problems affecting natural ecosystems. The aim of this study was to explain differences in the development of autotrophic and heterotrophic plankton in two shallow, temperate, embanked lakes with different modes of WL regulation. Lake Mytycze has natural WL changes supplied exclusively by precipitation; a decrease in water level was predominantly due to more evaporation than precipitation. In this lake, nutrient concentration, particulate matter, growth of toxigenic cyanobacteria and contamination with microcystins correlated positively with decreases in WL. Lake Tomaszne has anthropogenic WL regulation, elevated values of nutrients and particulate matter, but they were caused by the periodic, abrupt entrance of river waters rich in allochthonous nutrients, organic and inorganic particles. In this lake, a higher biomass of cyanobacteria was observed than in the lake with natural WL changes; the concentrations of intracellular microcystins in water correlated positively with the biomass of toxigenic species. In both lakes, decreases in the biomass of algivorous crustaceans correlated with the occurrence of cyanobacterial biomass. Biomasses of algivorous crustaceans increased abruptly when concentration of microcystins diminished below 2 μg∙dm −3 . These biomass peaks occurred due to rapid development of the cladocerans; Bosmina longirostris was found in the lake with natural WL changes and Daphnia cucullata was found in the lake with anthropogenic WL regulation. The development of predacious crustaceans was not related to the blooms of filamentous cyanobacteria or the biomass of algivorous crustaceans (as potential prey). In both lakes, Thermocyclops crassus and Mesocyclops leuckartii (Copepoda) contributed significantly to the biomass of predacious crustaceans; their abundance correlated positively with the amount of particulate matter, which was probably utilised as an alternative food source. These findings illustrate indirect influence of WL on plankton composition and biomass through alteration of environmental variables. The results indicate that the mode of WL regulation (natural vs anthropogenic) must be taken into consideration when estimating the influence of WL changes on plankton dynamics.

Impact of water column stability dynamics on the succession of plankton food web types in the offshore area of the Adriatic Sea

Vertical mixing and stratification are among the most important physical processes controlling nutrient dynamics, the dominant category of primary producers and consequently the dominant types of food web, and are therefore important for the assessment of the marine ecosystem's response to global climate change. This study showed consistent short-term cyclic successions of the plankton food web types, governed by the dynamics of water column stability changes, occurring in generally oligotrophic, phosphate deficient surface waters of the open middle Adriatic Sea. The biogeochemical nitrogen cycle appeared as a key driving force responsible for the food web structure changes. The ‘herbivorous food web’ dominated during the nitrate-rich mixed water column period (winter) and gradually changed to ‘multivorous food web’ where large phytoplankton still constitute a significant fraction of phytoplankton. This intermediate type of food web lasted for a short time and quickly changed to the typical ‘microbial food web’, which then dominated during the stratified water column period (summer) and was characterised by a large proportion of picoplankton size-fraction organisms in total plankton biomass and production. Furthermore, at the very end of summer, the high bacterial carbon flux through the ‘microbial loop’ was established. The succession of food web types affects the mechanisms of bacterial control in a way that ‘bottom up’ control dominated during the mixed water column period and ‘top-down’ control prevailed during the stratified period. Since the ongoing global warming is expected to change water column stability dynamics and thereby significantly affect the supply of nutrients in surface waters, this study helps to understand the possible direction of changes in the plankton food webs of the Adriatic Sea, and consequent changes in marine nitrogen and carbon biogeochemical cycles.

Plankton community interactions in an Amazonian floodplain lake, from bacteria to zooplankton

The simple view of the classical phytoplankton–zooplankton–fish food chain (CFC) has been replaced by a more complex framework, integrating microbial compartments (microbial food web, MFW). Few studies considered all components of the pelagic MFW in freshwaters and mostly are from temperate regions. We investigated carbon partitioning in the CFC and the MFW in an Amazonian floodplain system and analyzed the strength of interactions among components through structure equation modeling. We hypothesized that (i) MFW contributes highly to total plankton biomass throughout the year; and (ii) all plankton communities increase in biomass during low water, increasing the role of trophic interactions. We collected 30 subsurface samples (nutrients and plankton communities). MFW predominated over CFC in carbon biomass, and plankton components and their interactions changed according to the contrasting water level. Because phosphorus can be a potentially limiting resource for strict primary producers, higher biomass and a more complex MFW occurred during low water. We concluded that hydrology is a key factor shaping biotic interactions during low-water periods, and that MFW plays a key role in floodplain lakes, being potential mixotrophy an important strategy for phytoplankton.

The effects of hydraulic works and wetlands function in the Salado-River basin (Buenos Aires, Argentina).

Man-made activities exert great influences on fluvial ecosystems, with lowland rivers being substantially modified through agricultural land use and populations. The recent construction of drainage canals in the upper stretch of the Salado-River basin caused the mobilization of huge amounts of salts formerly stored in the groundwater. The main aim of this work was to analyze the effect of the discharges of those canals into the Salado-River water, under different hydrologic conditions, and the role of the wetlands and shallow lakes placed along the canals' system. Physicochemical variables were measured and water samples were taken during times of high water, mean flows, drought, and extreme drought. The environmental variables and the plankton development were related to the hydrologic regime and reached minimum values during floods because of low temperatures and dilution. Local effects on the water's ionic composition became pronounced during droughts because of groundwater input. Nutrient concentrations were mainly associated with point wastewater discharges. Conductivity, ion concentrations, total plankton biomass, and species richness increased in the Salado-River downstream site, after the canals' discharges. The artificial-drainage system definitely promotes the incorporation of salts into the Salado-River basin. In this scenario, a careful hydraulic management is needed to take into account this issue of secondary salinization that threatens the economic exploitation of the region. The wetlands present in this study acted as service environments not only helping to reduce salt, nutrient, and suspended-solid concentrations downstream but also contributing a plethora of species and plankton biomass into the Salado-River main course.

Abundance, biomass, and production of heterotrophic bacteria in a large plain reservoir during the ice-covered period

The abundance, biomass, production, and size and morphological structure of heterotrophic bacteria have been determined and their contribution to the total plankton biomass has been assessed in the Rybinsk Reservoir (the Upper Volga) during the ice-covered period. Structural and functional characteristics of bacteria, except for their cell sizes in winter, are lower than those in the growing season. However, heterotrophic bacteria have made the main contribution (89.3%) to the total biomass of plankton community in winter. During the growing season, phytoplankton is a major component of plankton (48.8%), while the bacteria average 36.5%. The total plankton biomass under the ice averages 148 mg C/m3, which is 2.6 times lower than in the period of open water. During the ice-covered period, the number of protists is small and they consume an insignificant part of the bacterial production; viral lysis is the main reason for bacterial mortality.

Content of total free nucleotides in the plankton of the Rybinsk reservoir (Upper Volga)

The content of total free nucleotides (TFNs) in the plankton of the Rybinsk reservoir and in specific size fractions of the plankton has been determined by the spectrophotometric method modified by the authors. The content of TFNs amounts to 58.1 ± 5.4, 75.2 ± 4.6, and 64.1 ± 7.3 μg/L in spring, summer, and autumn, respectively, and exhibits a close correlation with the total plankton biomass (r = 0.67), whereas the TFN/Chl parameter is closely correlated to the ratio of autotroph and heterotroph organism biomass (r = 0.96). The contribution of the fraction of more than 3 μm in size (microplankton and nanoplankton) to the TFN pool is 77 ± 4% in spring, 74 ± 3% in summer, and 47 ± 3% in autumn, whereas the contribution of the fraction of 0.45–3 μm in size (ultraplankton) during these seasons is 23 ± 4, 26 ± 3, and 53 ± 3%, respectively. Regression equations that can be used to infer an estimate of plankton biomass from TFN content have been proposed.

Structure and functioning of the microbial loop in a boreal reservoir

The abundance and biomass of the main components of the microbial plankton food web (“microbial loop”)—heterotrophic bacteria, phototrophic picoplankton and nanoplankton, heterotrophic nanoflagellates, ciliates and viruses, production of phytoplankton and bacterioplankton, bacterivory of nanoflagellates, bacterial lysis by viruses, and the species composition of protists—have been determined in summer time in the Sheksna Reservoir (the Upper Volga basin). A total of 34 species of heterotrophic nanoflagellates from 15 taxa and 15 species of ciliates from 4 classes are identified. In different parts of the reservoir, the biomass of the microbial community varies from 26.2 to 64.3% (on average 45.5%) of the total plankton biomass. Heterotrophic bacteria are the main component of the microbial community, averaging 63.9% of the total microbial biomass. They are the second (after the phytoplankton) component of the plankton and contribute on average 28.6% to the plankton biomass. The high ratio of the production of heterotrophic bacteria to the production of phytoplankton indicates the important role of bacteria, which transfer carbon of allochthonous dissolved organic substances to a food web of the reservoir.

Response of a stoichiometrically imbalanced ecosystem to manipulation of nutrient supplies and ratios.

Cuatro Ciénegas Basin (CCB) is a desert ecosystem that hosts a large diversity of water bodies. Many surface waters in this basin have imbalanced nitrogen (N) to phosphorus (P) stoichiometry (total N:P > 100 by atoms), where P is likely to be a limiting nutrient. To investigate the effects of nutrient stoichiometry on planktonic and sediment ecosystem components and processes, we conducted a replicated in situ mesocosm experiment in Lagunita, a shallow pond located in the southwest region of the basin. Inorganic N and P were periodically added to mesocosms under three different N:P regimes (P only, N:P = 16 and N:P = 75) while the control mesocosms were left unamended. After three weeks of fertilization, more than two thirds of the applied P was immobilized into seston or sediment. The rapid uptake of P significantly decreased biomass C:P and N:P ratios, supporting the hypothesis that Lagunita is P-limited. Meanwhile, simultaneous N and P enrichment significantly enhanced planktonic growth, increasing total planktonic biomass by more than 2-fold compared to the unenriched control. With up to 76% of added N sequestered into the seston, it is suspected that the Lagunita microbial community also experienced strong N-limitation. However, when N and P were applied at N:P = 75, the microbes remained in a P-limitation state as in the untreated control. Two weeks after the last fertilizer application, seston C:P and N:P ratios returned to initial levels but chlorophyll a and seston C concentrations remained elevated. Additionally, no P release from the sediment was observed in the fertilized mesocosms. Overall, this study provides evidence that Lagunita is highly sensitive to nutrient perturbation because the biota is primarily P-limited and experiences a secondary N-limitation despite its high TN:TP ratio. This study serves as a strong basis to justify the need for protection of CCB ecosystems and other low-nutrient microbe-dominated systems from anthropogenic inputs of both N and P.

Water level and fish-mediated cascading effects on the microbial community in eutrophic warm shallow lakes: a mesocosm experiment

Information on the effects of water level changes on microbial planktonic communities in lakes is limited but vital for understanding ecosystem dynamics in Mediterranean lakes subjected to major intra- and inter-annual variations in water level. We performed an in situ mesocosm experiment in an eutrophic Turkish lake at two different depths crossed with presence/absence of fish in order to explore the effects of water level variations and the role of top-down regulation at contrasting depths. Strong effects of fish were found on zooplankton, weakening through the food chain to ciliates, HNF and bacterioplankton, whereas the effect of water level variations was overall modest. Presence of fish resulted in lower biomass of zooplankton and higher biomasses of phytoplankton, ciliates and total plankton. The cascading effects of fish were strongest in the shallow mesocosms as evidenced by a lower zooplankton contribution to total plankton biomass and lower zooplankton:ciliate and HNF:bacteria biomass ratios. Our results suggest that a lowering of the water level in warm shallow lakes will enhance the contribution of bacteria, HNF and ciliates to the plankton biomass, likely due to increased density of submerged macrophytes (less phytoplankton); this effect will, however, be less pronounced in the presence of fish.

The relevance of deep chlorophyll maximum in the open Mediterranean Sea evaluated through 3D hydrodynamic-biogeochemical coupled simulations

Decades of oceanographic observations on the Mediterranean Sea have consistently indicated that its open-sea region presents oligotrophic characteristics during long periods of the year (from late spring to autumn) with very low and uniform surface chlorophyll values interrupted by the presence of several productivity hot-spots associated to mesoscale features. Another commonly observed phenomenon in the basin is the widespread and consistent presence of subsurface or deep phytoplankton biomass accumulations. However, given the difficulty of observing those deep structures with the adequate spatial and temporal resolution, a comprehensive description of their spatial and temporal variability is yet to be done. Here we use a 3D hydrodynamic-biogeochemical coupled model of the entire Mediterranean Sea to explore the contribution of the deep chlorophyll maximum (DCM) to total plankton biomass and primary productivity in the open-sea regions of the basin. We found that, on annual average, DCM are present in 73.5% of total open-sea regions with a clear annual cycle, being more frequent (up to 98% of the area) in summer months and less common (∼9%) during winter. Our model also indicates that, on average, nearly 53% of total phytoplankton biomass and ∼62% of total primary production in open-sea regions of the Mediterranean takes place within such structures. They must, then, be duly considered for a correct assessment of the biological productivity in the open sea regions of the Mediterranean basin.

The Vertical Distribution of Micro- and Small Mesozooplankton in the Central Red Sea

AbstractThe abundance and taxonomic composition of plankton sampled with nets of 0.1 mm mesh size were determined at the Atlantis II Deep, central Red Sea, in October and November 1980, from the surface to a depth of 1650 m. The fine mesh nets yielded three times as much total plankton biomass and thirty times the number of metazoan organisms compared to what was collected during earlier investigations using nets with a larger, 0.3 mm mesh size. The patterns of vertical distribution, determined by using the two different mesh sizes, were similar with respect to the gradients of plankton abundance but different with regard to the relative proportions of species and higher taxa. When the smaller metazoans were included by the use of fine mesh nets, cyclopoid copepodids became more numerous than the calanoid copepodids in all depth layers sampled, and their relative importance increased with depth. Between 200 m and 1050 m depth, over 50% of all copepodids belonged to the cyclopoid genus Oncaea. A review of ...

Trophic efficiency of plankton food webs: Observations from the Gulf of Mannar and the Palk Bay, Southeast Coast of India

This paper introduces the structure and trophic efficiency of plankton food webs in the Gulf of Mannar (GoM) and the Palk Bay (PB) — two least studied marine environments located between India and Sri Lanka. The study is based on the results obtained from a field sampling exercise carried out in the GoM and the PB in March 2010 (Spring Intermonsoon — SIM), September 2010 (Southwest Monsoon — SWM) and January 2011 (Northeast Monsoon — NEM). Based on multivariate analysis of major environmental parameters during different seasons, it was possible to clearly segregate the GoM and the PB into separate clusters, except during the SWM. This segregation of the GoM and the PB was closely linked with the seasonally reversing ocean currents in the region, as evident from the MIKE 21 flow model results. During the period of relatively low phytoplankton biomass (<23mgCm−3), the organic carbon contribution of the microbial loop was significantly high — both in the GoM and the PB. During the SIM, the carbon biomass available in the plankton food web was significantly higher in the PB (av. 122.8±47.60mgCm−3) than in the GoM (av. 81.89±35.50mgCm−3). This was due to a strong microbial loop in the former region. In the GoM, phytoplankton contributed a considerable portion (>50%) of the carbon biomass during the SWM and the NEM, whereas, microbial loop contributed significantly (80%) during the SIM. The microbial loop was predominant in the PB throughout the study period, being as high as 83% of the total plankton biomass during the SIM. As compared to the PB, the mesozooplankton biomass was higher in the GoM during the SWM and the NEM and lower during the SIM. The relatively high mesozooplankton stock in the PB during the SIM was closely linked with a strong microbial loop, which contributed the major share (av. 101.6±24.3mgCm−3) of the total organic carbon available in the food web (av. 126.6±24.3mgCm−3). However, when microbial loop contributed >65% of the total organic carbon available in the food web, the trophic efficiency was found to be low (~3%), which can be attributed to the wide dispersal of organic carbon in the microbial loop. Importantly, during the NEM, when the copepod Paracalanus parvus was predominant in the PB, the trophic efficiency of the microbial loop dominant food web increased by more than a fold (7.2%). The study provides evidences for the first time from the field that exceptionally high abundance of efficient microzooplankton-consuming zooplankton can significantly increase the trophic efficiency of the microbial loop dominant plankton food web.

Long‐term effects of warming and nutrients on microbes and other plankton in mesocosms

Summary1. We followed microbial and other planktonic communities during a 4‐month period (February–May) in 12 outdoor flow‐through mesocosms designed to elucidate the effect of global warming and nutrient enrichment. The mesocosms were established in 2003.2. Warming had a smaller effect than nutrients on the biomass of the microbial and planktonic communities, and warming and nutrients together exhibited complex interactions.3. We did not find direct effects of warming on the biomass of bacterioplankton or ciliates; however, warming significantly added to the positive effect of nutrients on these organisms and on heterotrophic nanoflagellates (HNF). No warming effects on any of the other planktonic groups analysed were detected.4. The zooplankton: phytoplankton biomass ratio was lowest, and the HNF: bacteria and rotifer: bacteria biomass ratios highest in the heated, nutrient‐rich mesocosms. We attribute this to higher fish predation on large‐bodied zooplankton, releasing the predation on HNF and competition for rotifers.5. The proportion of phytoplankton to the total plankton biomass increased with nutrients, but decreased with warming. The opposite pattern was observed for the proportion of phytoplankton to the total microbial biomass.6. As climate warming may lead to eutrophication, major changes may occur in the pelagic food web and the microbial community due to changes in trophic state and in combination with warming.

Biomass and trophic structure of the plankton community in subtropical and temperate waters of the northwestern Pacific Ocean

This study examined the biomass structure of autotrophic and heterotrophic plankton along a trophic gradient in the northwestern Pacific Ocean in an attempt to understand planktonic food web structure. Autotrophic biomass exceeded that of heterotrophic organisms in all sampling regions, but with lesser contribution to total planktonic biomass at stations of higher phytoplankton biomass, including the northern East China Sea, compared to the regions of lower phytoplankton biomass. The proportion of the biomass of heterotrophic bacteria, nanoflagellates (HNF), and dinoflagellates (HDF) relative to that of phytoplankton was all inversely related to phytoplankton biomass, but positive relationships were observed for both ciliates and mesozooplankton. Mesozooplankton biomass inclined greater than phytoplankton along the gradient of phytoplankton biomass, with biomass rise being most closely associated with ciliate and HDF biomass and, to a lesser degree, with large phytoplankton (>3 μm). Both bacteria and picophytoplankton were significantly and positively related to the biomass ratio of mesozooplankton to the sum of HDF and ciliates (i.e., proxy of mesozooplankton predation on protozoans), but no positive relationship was apparent either for HNF or for large phytoplankton. Such relationships may result from predation relief on lower food webs associated with mesozooplankton feeding on protistan plankton.

Is there a seamount effect on microbial community structure and biomass? The case study of Seine and Sedlo seamounts (northeast Atlantic).

Seamounts are considered to be “hotspots” of marine life but, their role in oceans primary productivity is still under discussion. We have studied the microbial community structure and biomass of the epipelagic zone (0–150 m) at two northeast Atlantic seamounts (Seine and Sedlo) and compared those with the surrounding ocean. Results from two cruises to Sedlo and three to Seine are presented. Main results show large temporal and spatial microbial community variability on both seamounts. Both Seine and Sedlo heterotrophic community (abundance and biomass) dominate during winter and summer months, representing 75% (Sedlo, July) to 86% (Seine, November) of the total plankton biomass. In Seine, during springtime the contribution to total plankton biomass is similar (47% autotrophic and 53% heterotrophic). Both seamounts present an autotrophic community structure dominated by small cells (nano and picophytoplankton). It is also during spring that a relatively important contribution (26%) of large cells to total autotrophic biomass is found. In some cases, a “seamount effect” is observed on Seine and Sedlo microbial community structure and biomass. In Seine this is only observed during spring through enhancement of large autotrophic cells at the summit and seamount stations. In Sedlo, and despite the observed low biomasses, some clear peaks of picoplankton at the summit or at stations within the seamount area are also observed during summer. Our results suggest that the dominance of heterotrophs is presumably related to the trapping effect of organic matter by seamounts. Nevertheless, the complex circulation around both seamounts with the presence of different sources of mesoscale variability (e.g. presence of meddies, intrusion of African upwelling water) may have contributed to the different patterns of distribution, abundances and also changes observed in the microbial community.