Christiane Lancelot

Controlling Eutrophication: Nitrogen and Phosphorus

Improvements in the water quality of many freshwater and most coastal marine ecosystems requires reductions in both nitrogen and phosphorus inputs.

Synthesis of iron fertilization experiments: From the Iron Age in the Age of Enlightenment

Comparison of eight iron experiments shows that maximum Chl a, the maximum DIC removal, and the overall DIC/Fe efficiency all scale inversely with depth of the wind mixed layer (WML) defining the light environment. Moreover, lateral patch dilution, sea surface irradiance, temperature, and grazing play additional roles. The Southern Ocean experiments were most influenced by very deep WMLs. In contrast, light conditions were most favorable during SEEDS and SERIES as well as during IronEx-2. The two extreme experiments, EisenEx and SEEDS, can be linked via EisenEx bottle incubations with shallower simulated WML depth. Large diatoms always benefit the most from Fe addition, where a remarkably small group of thriving diatom species is dominated by universal response of Pseudo-nitzschia spp. Significant response of these moderate (10–30 μm), medium (30–60 μm), and large (>60 μm) diatoms is consistent with growth physiology determined for single species in natural seawater. The minimum level of “dissolved” Fe (filtrate < 0.2 μm) maintained during an experiment determines the dominant diatom size class. However, this is further complicated by continuous transfer of original truly dissolved reduced Fe(II) into the colloidal pool, which may constitute some 75% of the “dissolved” pool. Depth integration of carbon inventory changes partly compensates the adverse effects of a deep WML due to its greater integration depths, decreasing the differences in responses between the eight experiments. About half of depth-integrated overall primary productivity is reflected in a decrease of DIC. The overall C/Fe efficiency of DIC uptake is DIC/Fe ∼ 5600 for all eight experiments. The increase of particulate organic carbon is about a quarter of the primary production, suggesting food web losses for the other three quarters. Replenishment of DIC by air/sea exchange tends to be a minor few percent of primary CO2 fixation but will continue well after observations have stopped. Export of carbon into deeper waters is difficult to assess and is until now firmly proven and quite modest in only two experiments.

Anthropogenic perturbations of the silicon cycle at the global scale: Key role of the land‐ocean transition

Silicon (Si), in the form of dissolved silicate (DSi), is a key nutrient in marine and continental ecosystems. DSi is taken up by organisms to produce structural elements (e.g., shells and phytoliths) composed of amorphous biogenic silica (bSiO(2)). A global mass balance model of the biologically active part of the modern Si cycle is derived on the basis of a systematic review of existing data regarding terrestrial and oceanic production fluxes, reservoir sizes, and residence times for DSi and bSiO(2). The model demonstrates the high sensitivity of biogeochemical Si cycling in the coastal zone to anthropogenic pressures, such as river damming and global temperature rise. As a result, further significant changes in the production and recycling of bSiO(2) in the coastal zone are to be expected over the course of this century.

Natural isotopic composition of nitrogen as a tracer of origin for suspended organic matter in the Scheldt estuary

Abstract The natural isotopic composition of suspended particulate organic nitrogen was determined in the Southern Bight of the North Sea and in the Scheldt estuary. These data show that δ 15 N constitutes a convenient tracer of the origin of the suspended matter. In the winter, in the absence of intensive primary production, the suspended organic matter of the Scheldt estuary is a mixture of two components: a continental detrital component characterized by a low δ value of 1.5%. and a marine component with a mean δ value of 8%.. During the phytoplankton flowering period, lasting from early May to October, intensive primary production occurs throughout the estuary giving rise to a third source of organic matter. This material is characterized by high δ values reflecting the isotopic composition of ammonia, the nitrogenous nutrient assimilated by phytoplankton in the estuary. The nitrification process occuring in the mixing area of the Scheldt estuary leads to higher downstream δ values of ammonia (>20%.) which permits the distinction between estuarine from fresh-water phytoplankton. Simple isotopic budget calculations show that, both in the upstream part and in the downstream part, autochthonous phytoplanktonic material contributes a major part of the total suspended matter in the Scheldt estuary during summer.

ECOLOGY Controlling Eutrophication: Nitrogen and Phosphorus

Improvements in the water quality of many freshwater and most coastal marine ecosystems requires reductions in both nitrogen and phosphorus inputs.

Phytoplankton of the North Sea and its dynamics: A review

Abstract Phytoplankton is the major contributor to algal biomass and primary production of the North Sea, although crops of macroalgae can locally be up to 2000 g C.m−2 along the coast of the U.K. and Norway, and microphytobenthos dominates production in the shallow tidal flat areas bordering the coasts of England, the Netherlands, Germany and Denmark. Data collected since 1932 during the Continuous Plankton Recorder Survey show consistent patterns of geographical, seasonal and annual variation in the distribution of phytoplankton and its major taxonomic components. There is a trend of increased colouration in Recorder silks in the southern North Sea until approximately 1975 since when Colour levels (assumed to be indicative of algal biomass) have declined. In the eutrophic Dutch Wadden Sea the algal crop continued to increase; in Dutch coastal North Sea waters a trend of biomass increase reversed since 1984, apparently due to a reduction in Rhine river outflow. Long-term observations made at Helgoland since the 60s also show trends of increasing nutrients and phytoplankton biomass only to 1984. Adverse effects such as deoxygenation, foam formation and toxin production have been linked to mass concentrations of algae known as blooms. There is no evidence from existing reports for an increase in their frequency, although some years stand out with larger numbers. Occurrence of blooms can partly be explained by hydrographic conditions. More than 30 taxa are recognised as occurring in bloom proportions in the North Sea, approximately one third of which can be toxic. The crop of Bacillariophyceae (diatoms) is not likely to increase with eutrophication due to silicate limitation. An extensive subsurface maximum of armoured dinoflagellates, its abundance gouverned by hydrographic conditions, is the most characteristic feature of the central and northern North Sea in the summer months. Abundance, sometimes dominance, of picoplankton and of species that are not readily detected by microscopic observations has been documented by measurements of taxon-specific pigments such as chlorophyll b (green algae), alloxanthin (Cryptophyceae) and 19′ - hexanolyloxyfucoxanthin (Prymnesiophyceae or Haptophyceae). Analysis of time series of satellite images is a promising way to assess in a quantitative and, more important, synoptic way the patchy distribution of phytoplankton over large regions. Growth processes of the phytoplankton respond according to variables amenable to such satellite remote sensing. Empirical and theoretical relationships that can be established between chlorophyll a, 14C uptake, turbidity, stratification, suspended sediment type, irradiance and temperature in some well-investigated areas make remote sensing a potential tool to obtain reliable estimates of primary production in the whole North Sea. The 14C method for estimates of the rate of algal growth processes appears to agree reasonably well with other methods, both involving incubation of samples and in situ measurements of temporal changes of oxygen and pH. The level of net primary production is 250 g C.m−2.a−1 in the central North Sea, 150 to 200 g C.m−2.a−1 in the northern North Sea, and 200 g in the South. The main metabolic processes involved in phytoplankton growth have been modelled mathematically in terms of the most important controlling environmental parameters. Such parameters comprise not only those of a chemical signature (micro- and macronutrients, both inorganic and organic) but also physical effects of vertical mixing and sinking, and biological effects including allelopathic interactions, antibiotic excretions, vertical migration, and mortality due to grazing and parasitism. The balance between primary production and consumption of organic matter appears to vary both geographically and seasonally. The process of regeneration of primary products both in the water column and in and near the bottom seems to be of major importance. Future research should center around a study of growth-controlling parameters in laboratory culture experiments. The studies should include uptake of dissolved organic compounds by all taxonomic groups, including pico- and nanophytoplankton, and all aspects of ecosystem structure and function involving the relation between algae and microheterotrophs making up the small food web. There is a need to synthesize existing information on phytoplankton in the North Sea and the factors gouverning its growth, such as nutrients, river input and stratification intensity. Complicated inter-relationships and successional patterns between individual species which are limited by varying physiological requirements and adaptation to differing hydrographic regimes reemphasizes the importance of species identification in phytoplankton studies. Many future problems in phytoplankton research will not be resolved without accurate identification of algal species. Taxonomic expertise takes many years to acquire; there is at present a shortage of skills in this area and more resources should be turned towards training and long-term support.

Calculating carbon biomass of Phaeocystis sp. from microscopic observations

Conversion factors for calculating carbon biomass ofPhaeocystis sp. colonies and free-living cells were determined from microscopic observations and chemical analysis conducted on cultured and naturalPhaeocystis sp. populations originating from the Southern Bight of the North Sea in 1986 and 1987. They allow calculation, in terms of carbon biomass, of the different forms ofPhaeocystis sp. that succeed each other when the population is growing, on the basis of microscopic observations. The latter include enumerations of free-living cells (flagellated and non-motile) and colonies, as well as colonial biovolume measurement. Specific application to natural populations from Dutch coastal waters during spring 1986 shows that more than 90% ofPhaeocystis sp. carbon biomass is under colonial form, most of it exceeding the grazing characteristics of current zooplankton at this period of the year. Detailed analysis of seasonal changes shows in addition that the size of the colonies greatly increases during the course ofPhaeocystis sp. flowering, reaching sizes as high as 1 mm diameter at the top of the bloom when nutrients are depleted. Physiologically this corresponds to an enhanced synthesis of mucilaginous substances, with the decrease of available nutrients leading to an increasing contribution of the matrix to the total colonial carbon during the course of the bloom. Carbon content ofPhaeocystis sp. colonies therefore greatly varies with their size, ranging from 0.3 to 1430 ngC colony−1.

Modeling phytoplankton blooms and carbon export production in the Southern Ocean: dominant controls by light and iron in the Atlantic sector in Austral spring 1992

The high nutrient low chlorophyll (HNLC) conditions of the Southern Ocean were explored with an ecological model (SWAMCO) describing the cycling of C, N, P, Si and Fe through different, aggregated, chemical and biological compartments of the plankton ecosystem. The structure of the model was chosen to take explicitly into account biological processes of importance in the formation and mineralization of carbon biomass in surface waters and in carbon export production. State variables include major inorganic nutrients (NO3, NH4, PO4, Si(OH)4), dissolved Fe, two groups of phytoplankton (diatoms and nanoflagellates), bacteria, heterotrophic nanoflagellates, microzooplankton, labile DOC and two classes of dissolved and particulate organic polymers with specific biodegradability. The model is closed by export production of particulate organic matter out of the surface layer and, when relevant, by metazooplanton, the grazing pressure of which is described as a forcing function. Parameterization was derived from the current knowledge on the kinetics of biological processes in the Southern Ocean and in other `HNLC’ areas. For its application in the Atlantic sector in spring 1992, the SWAMCO model was coupled `off-line’ to a 1D physical model forced by in situ meteorological and sea-ice conditions. The predictions of the model were successfully compared with chemical and biological observations recorded in the Antarctic circumpolar current (ACC) during the 1992 cruise ANTX/6 of RV Polarstern. In particular, the model simulates quite well the diatom bloom and carbon export event observed in the iron-enriched Polar Frontal region and the lack of ice-edge phytoplankton blooms in the marginal zone (MIZ) of the ACC area. Model analysis shows that sufficient light and iron concentrations above 0.5 μmol m−3 are the necessary conditions for enhancing diatom blooms and particulate carbon export production in the Southern Ocean. Low iron availability prevents diatom growth but is still adequate for nanophytoplankton, the biomass of which is, however, kept to Chl a levels less than 1 mg m−3 due to the loss by the ubiquitous micrograzers. Little carbon export is predicted under iron-limitation conditions. Sensitivity tests conducted on the parameters describing iron and silicon uptake by diatoms reveal the complex nature of Fe and Si limitation in regulating the magnitude and extent of diatom blooms and carbon and opal export production in the Southern Ocean.

Factors controlling phytoplankton ice-edge blooms in the marginal ice-zone of the northwestern Weddell Sea during sea ice retreat 1988 : field observations and mathematical modelling

The factors controlling phytoplankton bloom development in the marginal ice zone of the northwestern Weddell Sea were investigated during the EPOS (Leg 2) expedition (1988). Measurements were made of physical and chemical processes and biological activities associated with the process of ice-melting and their controlling variables particularly light limitation mediated by vertical stability and ice-cover, trace metal deficiency and grazing pressure. The combined observations and process studies show that the initiation of the phytoplankton bloom, dominated by nanoplanktonic species, was determined by the physical processes operating in the marginal ice zone at the time of ice melting. The additional effects of grazing pressure by protozoa and deep mixing appeared responsible for a rather moderate phytoplankton biomass (4 mg Chla m−3) with a relatively narrow geographical extent (100–150 km). The rôle of trace constituents, in particular iron, was minor. The importance of each factor during the seasonal development of the ice-edge phytoplankton bloom was studied through modelling of reasonable scenarios of meteorological and biological forcing, making use of a one-dimensional coupled physicalbiological model. The analysis of simulations clearly shows that wind mixing events — their duration, strength and frequency — determines both the distance from the iceedge of the sea ice associated phytoplankton bloom and the occurrence in the ice-free area of secondary phytoplankton blooms during the summer period. The magnitude and extent of the ice-edge bloom is determined by the combined action of meteorological conditions and grazing pressure. In the absence of grazers, a maximum ice-edge bloom of 7.5 mg Chla m−3 is predicted under averaged wind conditions of 8 m s−1. Extreme constant wind scenarios (4–14 m s−1) combined with realistic grazing pressure predict maximum ice-edge phytoplankton concentrations varying from 11.5 to 2 mg Chla m−3. Persistent violent wind conditions (≥ 14 m s−1) are shown to prevent blooms from developing even during the brightest period of the year.

The mucilage phenomenon in the continental coastal waters of the North Sea

The basic mechanisms behind the mucilaginous phenomenon in the eutrophicated continental coastal waters of the North Sea are analysed in relation to observed short-term and predicted long-term quantitative and qualitative changes in riverine nutrient delivery to this coastal area. It is shown that foam accumulation observed every spring at sea surface and on the beaches, under windy conditions, results from food chain disruption due to the proliferation of one single phytoplanktonic species, the colony-forming Phaeocystis, peculiar physiology and biochemistry of which make them largely unpalatable for mesozooplankton and refractory to microbial degradation. Based on the knowledge of the carbon and nutrient metabolism of Phaeocystis colonies, making them more competitive than other phytoplankters to utilize nitrate as nitrogen source in dim coastal waters, it is concluded that the massive development of Phaeocystis colonies should have been enhanced by new sources of riverine nutrients, severely deficient in silicate and phosphate with respect to nitrogen and for which nitrate is the dominant form. Accordingly, historical and present-day records of bulky Phaeocystis colony blooms are closely related to those changes in land use and /or hydraulic and waste water managements that have led, over the last 100 years, to increased delivery of nitrates to the coastal area