Ice sheets as a missing source of silica to the polar oceans

Ice sheets play a more important role in the global silicon cycle than previously appreciated. Input of dissolved and amorphous particulate silica into natural waters stimulates the growth of diatoms. Here we measure dissolved and amorphous silica in Greenland Ice Sheet meltwaters and icebergs, demonstrating the potential for high ice sheet export. Our dissolved and amorphous silica flux is 0.20 (0.06-0.79) Tmol year(-1), ∼50% of the input from Arctic rivers. Amorphous silica comprises >95% of this flux and is highly soluble in sea water, as indicated by a significant increase in dissolved silica across a fjord salinity gradient. Retreating palaeo ice sheets were therefore likely responsible for high dissolved and amorphous silica fluxes into the ocean during the last deglaciation, reaching values of ∼5.5 Tmol year(-1), similar to the estimated export from palaeo rivers. These elevated silica fluxes may explain high diatom productivity observed during the last glacial-interglacial period

Continuity in the photosynthetic production of dissolved organic carbon from eutrophic to oligotrophic waters

International audienceWe studied the photosynthetic production of dissolved organic carbon (DOCp) during summer oligotrophic conditions in the Celtic Sea. The production rate of size-fractionated, particulate organic carbon (POCp) was also determined, as well as the relationship between irradiance and DOCp and the kinetics of DOCp during a 24 h light-dark cycle. Mean (1 SE), euphotic layer-integrated POCp was 510 +/- 61 mg C m(-2) d(-1), and cells in the 15 000 mg C m(-2) d(-1), PER was relatively constant at similar to 20%, and integrated DOCp could be predicted from integrated POCp with the equation: log (DOCp) = 0.96 x log (POCp) - 0.51 (r(2) = 0.90, n = 35, p < 0.001)

Size dependence of coastal phytoplankton photosynthesis under vertical mixing conditions

We have determined the relationship between carbon-specific photosynthesis and phytoplankton cell size in a coastal ecosystem. The normalized size spectra of carbon (C) biomass and photosynthesis allow to determine both biomass and photosynthesis within any size class along the community size spectrum. By dividing the size spectra of photosynthesis and biomass, the size spectrum of C-specific photosynthesis is derived. Our results indicated a high variability in the slope of the C-specific photosynthesis size spectrum. Under favourable conditions for growth, in the upper euphotic layer, the slope was positive, indicating that larger phytoplankton attained higher C-specific photosynthesis rates than the smaller cells. This pattern represents a significant departure from the expected, literature value of -0.25 for the size-scaling of biomass-specific metabolism. We suggest that this change in the slope may be caused by the changes in the taxonomic composition along the community size spectrum. Towards the bottom of the euphotic layer, we observed a decrease in the slope of the C-specific photosynthesis size spectrum, which could be associated with an enhanced package effect in larger cells under light-limited conditions. These results question the applicability of single and overall exponents to describe the size scaling of photosynthesis in natural phytoplankton assemblages

Maximum photosynthetic efficiency of size-fractionated phytoplankton assessed by C-14 uptake and fast repetition rate fluorometry

Under high nutrient concentrations and sufficient light conditions, large phytoplankton may display higher photosynthetic efficiency than smaller cells. This is unexpected since smaller phytoplankton, because of their higher surface to volume ratio, possess a greater ability to take up nutrients and absorb light. In order to investigate the causes of the increased photosynthetic efficiency in larger phytoplankton, we assessed the maximum photosynthetic efficiency of coastal assemblages in three size classes ( 20 mu m) by concurrently conducting C-14- based photosynthesis-irradiance experiments and fast repetition rate fluorescence measurements. The light-saturated, chlorophyll-specific photosynthesis (P-max(b)) and the maximum photosystem II (PSII) photochemical efficiency (F-v/ F-m) of each size class were determined during winter mixing (March 2003) and summer stratification (June 2003). During winter mixing, size-fractionated P-max(b) and F-v/F-m were similar in all size classes. In contrast, during summer stratification, size-fractionated P-max(b) and F-v/F-m were significantly higher in the > 20-mu m size class. In the entire data set, size-fractionated P-max(b) and F-v/F-m were not significantly correlated. However, a significant relationship was found between size-fractionated P-max(b) and F-v/F-m for phytoplankton assemblages acclimated to low light conditions. Under high light, an excess PSII capacity may be responsible for the discrepancy between size-fractionated P-max(b) and F-v/F-m measurements, whereas under low light conditions, photosynthetic electron transport chain and components downstream of PSII become more balanced, which results in a tight covariation between both variables. Higher maximum photosynthetic efficiencies of large-sized phytoplankton might be associated with a higher PSII photochemical efficiency characteristic of certain taxonomic groups such as diatoms

Scaling of phytoplankton photosynthesis and cell size in the ocean

We have determined the scaling relationship between photosynthesis rate and cell size in natural phytoplankton assemblages of contrasting marine environments. We found that phytoplankton photosynthesis in the ocean does not scale as the L-power of cell size, but scales approximately isometrically with cell size, indicating that a single model cannot predict the metabolism–size relationship in all photosynthetic organisms. The scaling relationship between cellular chlorophyll a content and cell size is also isometric. Taxonomical changes along the size spectrum may explain the deviation of phytoplankton photosynthesis from the general allometric rule. The size scaling exponent for photosynthesis is significantly higher (1.14) in coastal productive waters than in the oligotrophic open ocean (0.96), which provides a physiological basis to explain the dominance of larger cells in nutrient-rich environments. The size scaling exponent for phytoplankton abundance is significantly less negative in coastal productive waters (-0.90) than in the oligotrophic open ocean (-1.25). The observed size scaling relationships imply that carbon fixation per unit volume decreases with cell size in oligotrophic waters, whereas the opposite occurs in productive ones. By controlling the metabolism–size scaling relationship, nutrient supply plays a major role in determining community size structure and the energy flow through the pelagic ecosystem

Phytoplankton size structure and primary production in a highly dynamic. coastal ecosystem (Ria de Vigo, NW-Spain): Seasonal and short-time scale variability

Size-fractionated phytoplankton biomass and primary production, together with net community metabolism, were measured in a coastal ecosystem (Ria de Vigo, NW-Spain) during a full annual cycle (July 2001-July 2002). On a seasonal scale, this ecosystem was characterized by two distinct oceanographic conditions, namely upwelling and downwelling favourable seasons. During the upwelling season, total chlorophyll a (Chl a) and particulate organic carbon production rates (POC-pr) were in the range 36-129 mg Chl a m(-2) and 89-834 mg C m(-2) h(-1), respectively, and were mainly accounted for (> 80%) by the microphytoplankton size fraction (> 20 pm). During the downwelling season, total Chl a and POC-pr were much lower (< 27 mg Chl a m(-2) and < 97 mg C m(-2) h(-1), respectively), and the pico- (< 2 mu m) and nano-(2-20 mu m) phytoplankton size fractions significantly increased their contribution to total Chl a (46-87%) and POC-pr (30-86%). The seasonal and short-time scale variability in the hydrographic conditions, in particular upwelling intermittency, provides a feasible explanation for the continuous dominance of large-sized phytoplankton during the upwelling period. Shelf water intrusions, continuous vertical mixing and the size-dependent limitation in light acquisition (package effect), suffered in a higher degree by larger phytoplankton, were likely to account for the shift in phytoplankton size structure during the downwelling period. During the upwelling season, community respiration represented a minor fraction of gross primary production (15-30%), which highlights the large export potential of organic matter by this ecosystem. On the contrary, community respiration accounted for a major fraction of primary production (85%) during the downwelling period, which suggests that most of the photosynthesised organic matter was remineralised within the ecosystem. Although the microbial plankton community of the Rfa de Vigo exhibits a net autotrophic functioning throughout the year, the magnitude of the carbon flows and budgets seems to be dependent on phytoplankton size structure. (c) 2005 Elsevier Ltd. All rights reserved

Quantifying energy intake in Pacific bluefin tuna (Thunnus orientalis) using the heat increment of feeding

SUMMARYUsing implanted archival tags, we examined the effects of meal caloric value, food type (sardine or squid) and ambient temperature on the magnitude and duration of the heat increment of feeding in three captive juvenile Pacific bluefin tuna. The objective of our study was to develop a model that can be used to estimate energy intake in wild fish of similar body mass. Both the magnitude and duration of the heat increment of feeding (measured by visceral warming) showed a strong positive correlation with the caloric value of the ingested meal. Controlling for meal caloric value, the extent of visceral warming was significantly greater at lower ambient temperature. The extent of visceral warming was also significantly higher for squid meals compared with sardine meals. By using a hierarchical Bayesian model to analyze our data and treating individuals as random effects, we demonstrate how increases in visceral temperature can be used to estimate the energy intake of wild Pacific bluefin tuna of similar body mass to the individuals used in our study.</jats:p

Resource levels allometric scaling of population abundance and marine phytoplankton diversity

7 páginas, 4 figuras, 1 tabla.We analyzed the relationship between population abundance and cell size in phytoplankton assemblages from coastal, shelf, and open-ocean environments. Our results show that across the entire size spectrum considered, population abundance increases over two orders of magnitude from subtropical to coastal regions. We find a highly significant linear relationship between nutrient concentration and the intercept of the log-log relationship between population abundance and cell size. In contrast to overall patterns reported mainly for vascular plants and animals, marine phytoplankton diversity does not show any consistent trend along either latitudinal or productivity gradients. These results imply that large-scale (biogeographic) variations in phytoplankton standing stocks are controlled by changes in population abundances rather than by systematic variations in species richness. These findings provide a mechanistic connection among nutrient availability, population dynamics, and phytoplankton diversity over macroecological scales.The research of R.P.H. is a contribution to the Plymouth Marine Laboratory Core Strategic Research Programme. Atlantic Meridional Transect (AMT) data collection was supported by the UK Natural Environmental Research Council through the Atlantic Meridional Transect consortium (NER/O/S/2001/ 00680). P.C. was supported by a Fulbright Postdoctoral Research Fellowship from the Spanish Ministry of Education and Science. This is contribution 157 of the AMT programme.Peer reviewe

Dimensions of marine phytoplankton diversity

Biodiversity of phytoplankton is important for ecosystem stability and marine biogeochemistry. However, the large-scale patterns of diversity are not well understood and are often poorly characterized in terms of statistical relationships with factors such as latitude, temperature and productivity. Here we use ecological theory and a global trait-based ecosystem model to provide mechanistic understanding of patterns of phytoplankton diversity. Our study suggests that phytoplankton diversity across three dimensions of trait space (size, biogeochemical function and thermal tolerance) is controlled by disparate combinations of drivers: the supply rate of the limiting resource, the imbalance in different resource supplies relative to competing phytoplankton demands, size-selective grazing and transport by the moving ocean. Using sensitivity studies we show that each dimension of diversity is controlled by different drivers. Models including only one (or two) of the trait dimensions will have different patterns of diversity than one which incorporates another trait dimension. We use the results of our model exploration to infer the controls on the diversity patterns derived from field observations along meridional transects in the Atlantic and to explain why different taxa and size classes have differing patterns.NASA (Grant NNX16AR47G