Pelagic Phytoplankton Research Articles

Trophic Relationships and Niche Partitioning of Red Drum Sciaenops ocellatus and Common Snook Centropomus undecimalis in Coastal Estuaries of South Florida

Large marine predators occupying the same spatial arena exhibit a variety of temporal and behavioral differences to minimize competition for habitat and prey. Here, we examined two such species in the Florida Everglades, red drum Sciaenops ocellatus and snook Centropomus undecimalis, to evaluate niche separation based on diet and multiple stable isotope (white muscle, δ13C, δ15N, δ34S) analyses. For these two estuarine predators, our results indicated that although dietary niche overlap was broad, different feeding modes (spatial and behavioral) allow niche partitioning. The diet of red drum was dominated by pink shrimp Farfantepenaeus duorarum and other demersal species. For snook, although their diet included significant numbers of pink shrimp, it was largely dominated by pelagic and epibenthic fishes. Mean red drum δ13C signatures (− 10.5 to − 20.8‰) differed significantly between areas and were strongly correlated with both area-specific seagrass concentration and amount of incidental seagrass ingestion. Mean snook δ13C signatures were generally depleted (− 20.9 to − 22.4‰) with the exception of one area (− 14.1‰). Red drum and snook diet and mean δ15N signatures (10.1‰, 10.8‰, respectively) indicated they were both mid-trophic-level consumers. Mean red drum δ34S signatures were significantly depleted (− 0.31‰) in the seagrass-dominated area, but enriched (2.03 to 3.78‰) in the other areas and indicated benthic but no pelagic sources of primary production. Mean snook δ34S signatures varied widely (0 to 20‰) among areas suggesting dependence on benthic (benthic algae and seagrass) and pelagic (phytoplankton) sources of primary production.

Seasonal and spatial variability of sea ice and phytoplankton biomarker flux in the Chukchi sea (western Arctic Ocean)

Sediment traps were deployed at 870 m water-depth from August 2008 to September 2009 at station DM in the Chukchi Sea (western Arctic Ocean) in an area covered by sea ice in winter to determine seasonal fluxes of HBIs and phytoplankton sterols in order to improve our understanding of sea ice proxies. HBI-III fluxes and PIIIIP25 are for the first time documented in the Arctic Ocean to evaluate their significance for paleoclimate reconstructions. Highest mass fluxes were found from mid-July 2009 to September 2009 contrasting with low values during all other months (i.e., December 2008 to early July 2009). Indeed, during the winter months IP25 was not detected but increased by a factor of nine over summer 2009 reflecting sea ice algae and pelagic phytoplankton production at the sea ice edge. High HBIs and low sterol fluxes at the end of summer 2008 are consistent with the complete melting of sea ice and post-bloom conditions. We found that HBI-III was more abundant in the early stage of sea ice retreat that characterizes the marginal ice zone. These sea ice biomarkers were also measured in surface sediments across a wide range of sea ice cover in the western Arctic region. Higher IP25 values were found in the southeastern Chukchi Sea and decreased westwards where sea ice conditions are less severe. Stronger positive linear relationship were found between the sea ice proxy indexes PBIP25 and PIIIIP25 and spring sea ice concentrations than with IP25 in agreement with earlier findings from other Arctic and sub-Arctic regions.

The Nexus between Sea Ice and Polar Emissions of Marine Biogenic Aerosols

AbstractAccurate estimation of the climate sensitivity requires a better understanding of the nexus between polar marine ecosystem responses to warming, changes in sea ice extent, and emissions of marine biogenic aerosol (MBA). Sea ice brine channels contain very high concentrations of MBA precursors that, once ventilated, have the potential to alter cloud microphysical properties, such as cloud droplet number, and the regional radiative energy balance. In contrast to temperate latitudes, where the pelagic phytoplankton are major sources of MBAs, the seasonal sea ice dynamic plays a key role in determining MBA concentration in both the Arctic and Antarctic. We review the current knowledge of MBA sources and the link between ice melt and emissions of aerosol precursors in the polar oceans. We illustrate the processes by examining decadal-scale time series in various satellite-derived parameters such as aerosol optical depth (AOD), sea ice extent, and phytoplankton biomass in the sea ice zones of both hemispheres. The sharpest gradients in aerosol indicators occur during the spring period of ice melt. In sea ice–covered waters, the peak in AOD occurs well before the annual maximum in biomass in both hemispheres. The results provide strong evidence that suggests seasonal changes in sea ice and ocean biology are key drivers of the polar aerosol cycle. The positive trend in annual-mean Antarctic sea ice extent is now almost one-third of the magnitude of the annual-mean decrease in Arctic sea ice, suggesting the potential for different patterns of aerosol emissions in the future.

Microalgal community structure and primary production in Arctic and Antarctic sea ice: A synthesis

Sea ice is one the largest biomes on earth, yet it is poorly described by biogeochemical and climate models. In this paper, published and unpublished data on sympagic (ice-associated) algal biodiversity and productivity have been compiled from more than 300 sea-ice cores and organized into a systematic framework. Significant patterns in microalgal community structure emerged from this framework. Autotrophic flagellates characterize surface communities, interior communities consist of mixed microalgal populations and pennate diatoms dominate bottom communities. There is overlap between landfast and pack-ice communities, which supports the hypothesis that sympagic microalgae originate from the pelagic environment. Distribution in the Arctic is sometimes quite different compared to the Antarctic. This difference may be related to the time of sampling or lack of dedicated studies. Seasonality has a significant impact on species distribution, with a potentially greater role for flagellates and centric diatoms in early spring. The role of sea-ice algae in seeding pelagic blooms remains uncertain. Photosynthesis in sea ice is mainly controlled by environmental factors on a small scale and therefore cannot be linked to specific ice types. Overall, sea-ice communities show a high capacity for photoacclimation but low maximum productivity compared to pelagic phytoplankton. Low carbon assimilation rates probably result from adaptation to extreme conditions of reduced light and temperature in winter. We hypothesize that in the near future, bottom communities will develop earlier in the season and develop more biomass over a shorter period of time as light penetration increases due to the thinning of sea ice. The Arctic is already witnessing changes. The shift forward in time of the algal bloom can result in a mismatch in trophic relations, but the biogeochemical consequences are still hard to predict. With this paper we provide a number of parameters required to improve the reliability of sea-ice biogeochemical models.

Food Composition for Blue Mussels (Mytilus edulis) in the Menai Strait, UK, Based on Physical and Biochemical Analyses

To clarify the composition of food ingested by blue mussels (Mytilus edulis) in the Menai Strait, where the largest mussel bed in the United Kingdom lies, hydrodynamic observations with organic carbon and nitrogen biochemical analysis were conducted. This strait is characterized by a narrow channel and wide tidal flats with large tidal mixing that influences the distribution of particulate organic matter (POM), which is food formussels. The maximum velocities of the tidal currents were estimated to be 0.5–1.0 m s-1 during northeastward flow and 0.6–1.3 m s-1 during southwestward flow. The residual current was southwestward and the water flux was estimated to be 510 m3 s-1. Field observations indicate that the inflow of pelagic phytoplankton into the strait mainly occurred in the northern region of the strait. Resuspended benthic primary production, however, also causes higher chlorophyll-a concentration in the central region of the strait near the mussel bed and its contribution to mussel growth was estimated to be 22%–50%. The concentrations of chlorophyll-a and particulate organic carbon indicated that the POM in the strait mainly originated from detritus. Benthic microalgae is, however, an important factor to understand the total primary production in the strait. The consumption of the primary producers was estimated to be approximately 6.3 t C day-1. In the phytoplankton community, pelagic and thycopelagic (living in both pelagic and benthic environments) species comprised 59.3%–77.2% and 12.8%–40.7% of the cells, respectively. The primary production by resuspended benthic species in the Liverpool Bay region was considerably larger than that in the Menai Strait region. As a conclusion, theMenai Strait is an area with complex hydrodynamic conditions causing horizontal advection and resuspension of both pelagic and benthic phytoplankton, and this complex would cause the most suitable condition for mussel production.

Variable littoral‐pelagic coupling as a food‐web response to seasonal changes in pelagic primary production

Abstract Lakes are among the most seasonally forced ecosystems on Earth. Seasonal variation in temperature and light produce cyclic patterns in water column mixing, nutrient supply and phytoplankton biomass. Diet responses of consumers to these patterns have rarely been quantified. Moreover, pelagic‐littoral coupling of dietary resources by mobile consumers is commonly considered to be static over annual cycles. This study quantifies littoral‐pelagic diet responses of multiple consumers to a strong shift in pelagic phytoplankton abundance over an annual cycle (September 2014 to August 2015) in a large (area 614 km2), oligotrophic, monomictic lake (Lake Taupō, New Zealand). Intra‐annual patterns in pelagic phytoplankton (chlorophyll a) and zooplankton were determined over multiple years. Major resource and consumer δ13C and δ15N were then collected over an annual cycle. Temporal patterns in food‐web structure were examined using convex hulls as a proxy of community trophic niche size. Diet was quantified using mixing models for zooplankton, meso‐predatory zooplanktivorous common smelt (Retropinna retropinna) and benthivorous common bullies (Gobiomorphus cotidianus), as well as the top‐predator rainbow trout (Oncorhynchus mykiss). Trophic structure patterns for smelt, bullies and trout were then independently examined using compound‐specific amino acid δ15N analyses (CSIA‐AA). Lake Taupō demonstrated similar food‐web patterns to other lakes globally. Phytoplankton and zooplankton demonstrated strong seasonal oscillations of abundance driven by both bottom‐up (nutrient supply) and top‐down (stable limit cycle) drivers. The food web demonstrated the typical nested structure. It responded to seasonally low and high pelagic resource availability periods by expansion and contraction, respectively, of trophic niche space. In response to lower pelagic phytoplankton abundance during summer stratification, and phytoplankton accumulation at a deep chlorophyll maximum (DCM), zooplankton abundance reduced and their diet became dominated by phytoplankton from below the thermocline (i.e. the hypolimnion and DCM). This change may have been prompted by the combined drivers of avoidance of predation and depauperate food supply in surface waters. The diet of smelt and bullies switched from predominantly zooplankton to benthic macroinvertebrates, synchronous with the decline in pelagic zooplankton. Trout diet, inferred from comparison of isotopic signatures of tissues with different turnover rates, also increased littoral resource reliance over the stratified period. Smelt, bully and trout CSIA‐AA data confirmed estimates of trophic position and indicated a greater degree of trophic complexity in the littoral than the pelagic food chain. Food webs in large, deep lakes such as Taupō are expected to be primarily pelagic. This study demonstrates the need to re‐examine this expectation due to seasonal variations in productivity. The relatively small littoral areas in large lakes, combined with meso‐predators’ highly seasonally variable littoral resource use, may drive strong seasonal top‐down effects on littoral macroinvertebrate prey. Our study supports the notion that food‐web interactions are highly dynamic and responsive to seasonal forcing. By linking food‐web dynamics to dynamic environmental conditions, this study provides a framework for future studies research on understanding lake food‐web responses to a range of annual/seasonal and global environmental change drivers.

Diatoms dominate the eukaryotic metatranscriptome during spring in coastal ‘dead zone’ sediments

An important characteristic of marine sediments is the oxygen concentration that affects many central metabolic processes. There has been a widespread increase in hypoxia in coastal systems (referred to as 'dead zones') mainly caused by eutrophication. Hence, it is central to understand the metabolism and ecology of eukaryotic life in sediments during changing oxygen conditions. Therefore, we sampled coastal 'dead zone' Baltic Sea sediment during autumn and spring, and analysed the eukaryotic metatranscriptome from field samples and after incubation in the dark under oxic or anoxic conditions. Bacillariophyta (diatoms) dominated the eukaryotic metatranscriptome in spring and were also abundant during autumn. A large fraction of the diatom RNA reads was associated with the photosystems suggesting a constitutive expression in darkness. Microscope observation showed intact diatom cells and these would, if hatched, represent a significant part of the pelagic phytoplankton biomass. Oxygenation did not significantly change the relative proportion of diatoms nor resulted in any major shifts in metabolic 'signatures'. By contrast, diatoms rapidly responded when exposed to light suggesting that light is limiting diatom development in hypoxic sediments. Hence, it is suggested that diatoms in hypoxic sediments are on 'standby' to exploit the environment if they reach suitable habitats.

Two new needle-shaped Nitzschia taxa from a deep East African crater lake

Two new diatoms, Nitzschia fabiennejansseniana Cocquyt & Ryken sp. nov. and Nitzschia pseudoaequalis Cocquyt sp. nov., are described from Lake Challa, a deep crater lake in East Africa, based on light and electron microscopical observations. These Nitzschia species are morphologically similar to other needle-shaped, planktonic Nitzschia present in freshwater lakes of tropical Africa. The differences between the newly described species and related Nitzschia taxa are a combination of the shape of the poles, the number of striae and fibula in 10 µm, and the presence of a central gap between the fibulae. Nitzschia fabiennejansseniana is common in the plankton of Lake Challa towards the end of the main dry and windy season during the northern hemisphere summer, when deep water-column mixing causes upwelling of nutrient-rich hypolimnetic water. Together with Afrocymbella barkeri, this taxon is the dominant diatom in the pelagic phytoplankton of Lake Challa today, and has been continuously over at least the last several hundreds of years, as recorded in its sediments. The temporal dynamics of both taxa in Lake Challa play a major role in the interpretation of past climate change in equatorial East Africa. The other new taxon, N. pseudoaequalis, was common in some epilithic samples taken from the littoral zone of Lake Challa. It was also sporadically observed in the pelagic phytoplankton near the end of the mixing season.

Importance of ice algae and pelagic phytoplankton as food sources revealed by fatty acid trophic markers in a keystone species (Mytilus trossulus) from the High Arctic

Importance of ice algae and pelagic phytoplankton as food sources revealed by fatty acid trophic markers in a keystone species (Mytilus trossulus) from the High Arctic

Understanding Abiotic and Biotic Conditions in Post-Mining Pit Lakes for Efficient Management: A Case Study (Poland)

This study was aimed at determining whether the origin, morphometry, and hydrology of post-mining lakes affect their hydrochemical and hydrobiological parameters (i.e. water quality). The investigated post-mining lakes were very young compared to glacial lakes and represent early stages of ecosystem succession. Despite their different ages and morphometries, they are all mesotrophic and have good water quality. They have not been supplied with phosphorus and nitrogen, which can cause excessive development of pelagic phytoplankton; as a result, they share low chlorophyll a (Chl a) content, low phytoplankton biomass, and relatively high water transparency. Low abundance and species richness of zooplankton indicate low trophic levels in all of the lakes. Chl a in Lakes Przykona and Bogdałów were within the range typical of mesotrophic lakes, while Lake Janiszew had very low Chl a, typical of an oligotrophic water body. The low N:P ratios (4–6), especially in summer, indicates nitrogen limitation of primary production. There is a risk that such a proportion of the major biogenic elements could lead to harmful cyanobacterial blooms. The lake basins were formed using quaternary deposits (sand, clay) at their bottoms; as a result, the lakes had a slightly alkaline pH (>8), which favors the development of aquatic organisms. Optimum depth helps establish lake stratification and ensures ecological stability. This applies to post-mining lakes as well; an optimum depth should be determined to ensure the development of thermal stratification, which affects lake processes.

Pelagic phytoplankton community change‐points across nutrient gradients and in response to invasive mussels

Summary Phytoplankton communities can experience nonlinear responses to changing nutrient concentrations, but the nature of species shifts within phytoplankton is not well understood and few studies have explored responses of pelagic assemblages in large lakes. Using pelagic phytoplankton data from the Great Lakes, we assessed phytoplankton assemblage change‐point responses to nutrients and invasive Dreissena, characterising community responses in a multi‐stressor environment and determine whether species responses to in situ nutrients can be approximated from nutrient loading. We demonstrate assemblage shifts in phytoplankton communities along major stressor gradients, particularly prominent in spring assemblages, providing insight into community thresholds at the lower end of the phosphorus gradient and species‐stressor responses in a multi‐stressor environment. We show that responses to water nutrient concentrations could not be estimated from large‐scale nutrient loading data likely due to lake‐specific retention time and long‐term accumulation of nutrients. These findings highlight the potential for significant accumulation of nitrates in ultra‐oligotrophic systems, nonlinear responses of phytoplankton at nutrient concentrations relevant to current water quality standards and system‐specific (e.g. lake or ecozone) differences in phytoplankton responses likely due to differences in nutrient co‐limitation and effects of dreissenids.

Trophic structure of a coastal fish community determined with diet and stable isotope analyses.

A combination of dietary guild analysis and nitrogen (δ(15) N) and carbon (δ(13) C) stable-isotope analysis was used to assess the trophic structure of the fish community in Rhode Island and Block Island Sounds, an area off southern New England identified for offshore wind energy development. In the autumn of 2009, 2010 and 2011, stomach and tissue samples were taken from 20 fish and invertebrate species for analysis of diet composition and δ(15) N and δ(13) C signatures. The food chain in Rhode Island and Block Island Sounds comprises approximately four trophic levels within which the fish community is divided into distinct dietary guilds, including planktivores, benthivores, crustacivores and piscivores. Within these guilds, inter-species isotopic and dietary overlap is high, suggesting that resource partitioning or competitive interactions play a major role in structuring the fish community. Carbon isotopes indicate that most fishes are supported by pelagic phytoplankton, although there is evidence that benthic production also plays a role, particularly for obligate benthivores such as skates Leucoraja spp. This type of analysis is useful for developing an ecosystem-based approach to management, as it identifies species that act as direct links to basal resources as well as species groups that share trophic roles.

Winter severity determines functional trait composition of phytoplankton in seasonally ice-covered lakes.

How climate change will affect the community dynamics and functionality of lake ecosystems during winter is still little understood. This is also true for phytoplankton in seasonally ice-covered temperate lakes which are particularly vulnerable to the presence or absence of ice. We examined changes in pelagic phytoplankton winter community structure in a north temperate lake (Müggelsee, Germany), covering 18 winters between 1995 and 2013. We tested how phytoplankton taxa composition varied along a winter-severity gradient and to what extent winter severity shaped the functional trait composition of overwintering phytoplankton communities using multivariate statistical analyses and a functional trait-based approach. We hypothesized that overwintering phytoplankton communities are dominated by taxa with trait combinations corresponding to the prevailing winter water column conditions, using ice thickness measurements as a winter-severity indicator. Winter severity had little effect on univariate diversity indicators (taxon richness and evenness), but a strong relationship was found between the phytoplankton community structure and winter severity when taxon trait identity was taken into account. Species responses to winter severity were mediated by the key functional traits: motility, nutritional mode, and the ability to form resting stages. Accordingly, one or the other of two functional groups dominated the phytoplankton biomass during mild winters (i.e., thin or no ice cover; phototrophic taxa) or severe winters (i.e., thick ice cover; exclusively motile taxa). Based on predicted milder winters for temperate regions and a reduction in ice-cover durations, phytoplankton communities during winter can be expected to comprise taxa that have a relative advantage when the water column is well mixed (i.e., need not be motile) and light is less limiting (i.e., need not be mixotrophic). A potential implication of this result is that winter severity promotes different communities at the vernal equinox, which may have different nutritional quality for the next trophic level and ecosystem-scale effects.

Dynamic responses of the benthic bacterial community at the Western English Channel observatory site L4 are driven by deposition of fresh phytodetritus

The impact of the seasonal deposition of phytoplankton and phytodetritus on surface sediment bacterial abundance and community composition was investigated at the Western English Channel site L4. Sediment and water samples were collected from January to September in 2012, increasing in frequency during periods of high water column phytoplankton abundance. Compared to the past two decades, the spring bloom in 2012 was both unusually long in duration and contained higher than average biomass. Within spring months, the phytoplankton bloom was well mixed through the water column and showed accumulations near the sea bed, as evidenced by flow cytometry measurements of nanoeukaryotes, water column chlorophyll a and the appearance of pelagic phytoplankton at the sediment. Measurements of chlorophyll and chlorophyll degradation products indicated phytoplankton material was heavily degraded after it reached the sediment surface: the nature of the chlorophyll degradation products (predominantly pheophorbide, pyropheophorbide and hydroxychlorophyllone) was indicative of grazing activity. The abundance of bacterial 16S rRNA genesg−1 sediment (used as a proxy for bacterial biomass) increased markedly with the onset of the phytoplankton bloom, and correlated with measurements of chlorophyll at the surface sediment. Together, this suggests that bacteria may have responded to nutrients released via grazing activity. In depth sequencing of the 16S rRNA genes indicated that the composition of the bacterial community shifted rapidly through-out the prolonged spring bloom period. This was primarily due to an increase in the relative sequence abundance of Flavobacteria.

Physiological characteristics of open ocean and coastal phytoplankton communities of Western Antarctic Peninsula and Drake Passage waters

Photophysiological processes as well as uptake characteristics of iron and inorganic carbon were studied in inshore phytoplankton assemblages of the Western Antarctic Peninsula (WAP) and offshore assemblages of the Drake Passage. Chlorophyll a concentrations and primary productivity decreased from in- to offshore waters. The inverse relationship between low maximum quantum yields of photochemistry in PSII (Fv/Fm) and large sizes of functional absorption cross sections (σPSII) in offshore communities indicated iron-limitation. Congruently, the negative correlation between Fv/Fm values and iron uptake rates across our sampling locations suggest an overall better iron uptake capacity in iron-limited pelagic phytoplankton communities. Highest iron uptake capacities could be related to relative abundances of the haptophyte Phaeocystis antarctica. As chlorophyll a-specific concentrations of humic-like substances were similarly high in offshore and inshore stations, we suggest humic-like substances may play an important role in iron chemistry in both coastal and pelagic phytoplankton assemblages. Regarding inorganic carbon uptake kinetics, the measured maximum short-term uptake rates (Vmax(CO2)) and apparent half-saturation constants (K1/2(CO2)) did not differ between offshore and inshore phytoplankton. Moreover, Vmax(CO2) and K1/2(CO2) did not exhibit any CO2-dependent trend over the natural pCO2 range from 237 to 507µatm. K1/2(CO2) strongly varied among the sampled phytoplankton communities, ranging between 3.5 and 35.3µmolL−1CO2. While in many of the sampled phytoplankton communities, the operation of carbon-concentrating mechanisms (CCMs) was indicated by low K1/2(CO2) values relative to ambient CO2 concentrations, some coastal sites exhibited higher values, suggesting down-regulated CCMs. Overall, our results demonstrate a complex interplay between photophysiological processes, iron and carbon uptake of phytoplankton communities of the WAP and the Drake Passage.

Large, omega-3 rich, pelagic diatoms under Arctic sea ice: sources and implications for food webs.

Pelagic primary production in Arctic seas has traditionally been viewed as biologically insignificant until after the ice breakup. There is growing evidence however, that under-ice blooms of pelagic phytoplankton may be a recurrent occurrence. During the springs of 2011 and 2012, we found substantial numbers (201–5713 cells m−3) of the large centric diatom (diameter >250 µm) Coscinodiscus centralis under the sea ice in the Canadian Arctic Archipelago near Resolute Bay, Nunavut. The highest numbers of these pelagic diatoms were observed in Barrow Strait. Spatial patterns of fatty acid profiles and stable isotopes indicated two source populations for C. centralis: a western origin with low light conditions and high nutrients, and a northern origin with lower nutrient levels and higher irradiances. Fatty acid analysis revealed that pelagic diatoms had significantly higher levels of polyunsaturated fatty acids (mean ± SD: 50.3±8.9%) compared to ice-associated producers (30.6±10.3%) in our study area. In particular, C. centralis had significantly greater proportions of the long chain omega-3 fatty acid, eicosapentaenoic acid (EPA), than ice algae (24.4±5.1% versus 13.7±5.1%, respectively). Thus, C. centralis represented a significantly higher quality food source for local herbivores than ice algae, although feeding experiments did not show clear evidence of copepod grazing on C. centralis. Our results suggest that C. centralis are able to initiate growth under pack ice in this area and provide further evidence that biological productivity in ice-covered seas may be substantially higher than previously recognized.

Primary production under hypertrophic conditions and its relationship with bacterial production

In shallow hypertrophic lakes where light availability restricts the growth of macrophytes and benthic phytoplankton, pelagic phytoplankton modulates importantly ecosystem production and the energy transfer to heterotrophic bacteria. Diel and seasonal variations in primary production (PP) were studied in the hypertrophic Albufera de Valencia (Spain). Additionally, the relationship between PP and heterotrophic bacterial production (BP) was assessed. PP was extremely high, exceeding most values reported for hypertrophic lakes to date. PP displayed marked diurnal variations defined by the solar radiation curve. Likewise, PP changed importantly across seasons. Minimum PP coincided with maximum water transparency and short water residence times in winter, whereas maximum PP was observed in late spring associated with high chlorophyll a. The spring PP maximum contrasted with the summer maximum often observed in hypertrophic lakes. When compared to spring PP values, summer PP values were lower as a result of strong nitrogen limitation. In contrast to PP, BP remained fairly constant across seasons. Nonetheless, there was a joint diminution during increased water transparency followed by an increase in early spring. Phytoplankton was always the most relevant input to particulate carbon production, but the BP/PP ratio showed clear seasonal variations. The BP/PP ratio was minimum in spring, low in summer and highest in winter. The extracellular dissolved organic carbon released by phytoplankton was sufficient to meet bacterial carbon demand in all experimental dates, suggesting that allochthonous carbon sources play a minor role in sustaining BP, though they cannot be excluded. However, we hypothesize that high availability of dissolved organic carbon might explain the lack of coupling observed between BP and PP.

Importance of ice algal production for top predators: new insights using sea-ice biomarkers

Antarctic seals and seabirds are strongly dependent on sea-ice cover to complete their life history. In polar ecosystems, sea ice provides a habitat for ice-associated diatoms that ensures a substantial production of organic matter. Recent studies have presented the potential of highly branched isoprenoids (HBIs) for tracing carbon flows from ice algae to higher-trophic-level organisms. However, to our knowledge, this new method has never been applied to sub-Antarctic species and Antarctic seals. Moreover, seasonal variations in HBI levels have never been investigated in Antarctic predators, despite a likely shift in food source from ice-derived to pelagic organic matter after sea-ice retreat. In the present study, we described HBI levels in a community of seabirds and seals breeding in Adelie Land, Antarctica. We then validated that sub-Antarctic seabirds had lower levels of diene, a HBI of sea-ice diatom origin, and higher levels of triene, a HBI of phytoplanktonic origin, compared with Antarctic seabirds. Finally, we explored temporal changes in HBI levels after the ice break up in summer. The level of diene relative to triene in Adelie penguin chicks increased and then declined during the breeding season, which was consistent with the short and intense proliferation of sea-ice algae in spring, followed by the pelagic phytoplankton bloom in summer. HBI biomarkers in Antarctic seabirds and seals thus indicate a shift from ice-algal derived organic matter to a pelagic carbon source during the summer breeding season.

Carbon from littoral producers is the major source of energy for bottom-feeding fish in a tropical floodplain

Bottom-feeding fish show great variation in trophic morphology, resulting in a wide array of feeding habits exploiting from periphyton in littoral habitats to ingesting larger amounts of litterfall from terrestrial habitats. Nevertheless, it has been proposed that, in temperate aquatic systems, the energetic demands from bottom-feeding fish are supplied by primary production from phytoplankton in pelagic habitats. Thus, we aimed to determine which of several energy sources support bottom-feeding fish in tropical aquatic systems, where primary producers are diverse. We performed samplings in the Parana River floodplain from primary producers in littoral (periphyton), pelagic (phytoplankton) and terrestrial habitats (riparian vegetation). Using simultaneous signatures of C and N isotope ratios, we analysed the possible origin of available energy in muscles of nine different bottom-feeding fish species. We verified that seven of the nine fish species had the highest contribution of carbon from a littoral source compared to other energy sources, independent of trophic category. The importance of periphyton may be related to the high abundance of substrates (such as macrophytes) that are available in the littoral zone of tropical aquatic systems. Finally, we suggest that the actual carbon dynamics among bottom-feeding fish and primary producers is species-specific, varying within the same trophic guild, and therefore, it is possible to refute the assumption that each trophic guild is specialised for a distinct carbon source.

Trace metal distribution in the Atlantis II Deep (Red Sea) sediments

The Atlantis II Deep is one of the only locations on the modern seafloor where active formation of a brine pool-type stratiform ore deposit can be studied. The presence of the brine pool causes retention of the hydrothermally released metals within the brine covered area, resulting in the accumulation of 90Mt of low-grade metalliferous sediment (2.06% Zn, 0.46% Cu, 41g/t Ag, and 0.5g/t Au; Guney et al., 1988). Almost all metals are derived from hydrothermal input, but some are also derived from seawater (e.g., Mo), pelagic phytoplankton (Ni) and detrital input (Cr). The hydrothermal fluid that is vented into the pool is rich in metals but relatively low in reduced sulfur compared to open ocean black smokers. Metals are deposited as sulfides from the cooling hydrothermal fluid but also by adsorption onto non-sulfidic “surface-active” particles (Si–Fe-OOH) in the brine pool. An unexpected increase in the Cu/Zn ratio of the sediments with distance from the vent source(s) may reflect pulses of higher-temperature venting and increased Cu fluxes to the brine pool, which are recorded as higher Cu/Zn ratios in the distal sediments or, alternatively, more efficient adsorption of Cu to Fe-OOH particles in the distal brine.During early diagenesis (a few thousand years) metals that are loosely bound to surface-active particles in the sediment apparently react with H2S to form sulfides. Proximal to the inferred vents, the ambient pore water is highly concentrated in trace metals such as Cd, Ag and Hg that are incorporated in diagenetic sulfides, including chalcopyrite and sphalerite. At greater distance from the vents, trace metals such as Mo, As, and Ga are taken up by framboidal pyrite. High concentrations of Au (up to 3ppm) are found in both proximal and distal metalliferous sediments, indicating that both primary deposition with sulfides and adsorption by diagenetic pyrite are important depositional processes. Some of the inferred pathways for metal precipitation in the Atlantis II Deep sediments, especially adsorption onto surface-active particles and subsequent incorporation in sulfides during diagenesis, may have been important unrecognized processes for metal accumulation in ancient stratiform ore deposits thought to have formed in brine pools.