Springtime Nutrient and Phytoplankton Dynamics on Georges Bank

The dynamics of phytoplankton and nutrients before, during and after the winter-spring bloom on Georges Bank were studied on 6 monthly survey cruises from January to June 1999. We measured hydrography, phytoplankton cell densities, chlorophyll a, dissolved inorganic nutrients (NO3 + NO2, NH4, Si(OH)(4), PO4), dissolved organic nitrogen (DON) and phosphorus (DOP), particulate organic carbon (POC) and nitrogen (PON) and total particulate phosphorus (TPP). We present evidence that phytoplankton production may be significant year-round, and that the winter-spring bloom may have started in January. From January to April the phytoplankton was comprised almost exclusively of diatoms, reaching cell densities in March and April of ca. 450 cells ml(-1); chlorophyll a concentrations exceeded 10 mug l(-1) in April. Diatoms decreased to relatively low levels in May (< 50 x 10(3) cells l(-1)) and increased again in June (>300 x 10(3) cells l(-1)). Densities of dinoflagellates and nanoflagellates were low (< 10 x 10(3) cells l(-1)) from January to April, and increased in May and June to nearly 300 x 10(3) cells l(-1). Nitrate + nitrite concentrations in January were <3 muM in the shallow, central portion of the bank and decreased steadily each month. Silicate was also <3 muM over an even larger area of the central bank in January and declined to <1.5 muM over most of the Bank in April. The data suggest that silicate depletion, not DIN, contributed to the cessation of the diatom bloom. Regeneration of silicate occurred in May and June, presumably as a result of rising water temperatures in late spring which increased the dissolution rate of diatom frustules from the earlier diatom bloom. Dissolved organic nitrogen may have been utilized at the start of the winter-spring bloom; concentrations were ca, 14 muM in January, dropping to < 6 mug l(-1) in February, after which DON concentrations steadily rose to > 15 mug l(-1) in June. Overall micro-and nanoplankton biomass, measured as POC, PON and TPP, increased over the 6 mo period, as did nutritional quality of that biomass as indicated by declining C:N ratios. Our results suggest there may have been an increase in the heterotrophic component of the plankton in May and June which coincided with a second burst in diatom abundance. We discuss general features of planktonic production and nutrient dynamics with respect to year-round production on the Bank.

Possible Effects of Anthropogenically-Increased CO2 on the Dynamics of Climate: Implications for Ice-Age Cycles

A dynamical model, developed to account for the observed major variations of global ice mass and atmospheric CO2 during the late Cenozoic, is used to provide a quantitative demonstration of the possibility that the anthropogenically-forced increase of atmospheric CO2, if maintained over a long period of time (perhaps by tectonic forcing), could displace the climatic system from an unstable regime of oscillating ice ages into a more stable regime representative of the pre-Pleistocene. This stable regime is characterized by orbitally-forced oscillations that are of much weaker amplitude than prevailed during the Pleistocene.

Influence of Equatorial Diatom Processes on Si Deposition and Atmospheric CO(2) Cycles at Glacial/Interglacial Timescales

The causes of the glacial cycle remain unknown, although the primary driver is changes in atmospheric CO(2), likely controlled by the biological pump and biogeochemical cycles. The two most important regions of the ocean for exchange of CO(2) with the atmosphere are the equatorial Pacific and the Southern Ocean ( SO), the former a net source and the latter a net sink under present conditions. The equatorial Pacific has been shown to be a Si(OH)(4)-limited ecosystem, a consequence of the low source Si(OH)(4) concentrations in upwelled water that has its origin in the SO. This teleconnection for nutrients between the two regions suggests an oscillatory relationship that may influence or control glacial cycles. Opal mass accumulation rate (MAR) data and delta(15)N measurements in equatorial cores are interpreted with predictions from a one- dimensional Si(OH)(4)-limited ecosystem model (CoSINE) for the equatorial Pacific. The results suggest that equatorial Pacific surface CO(2) processes are in opposite phase to that of the global atmosphere, providing a negative feedback to the glacial cycle. This negative feedback is implemented through the effect of the SO on the equatorial Si(OH)(4) supply. An alternative hypothesis, that the whole ocean becomes Si(OH)(4) poor during cooling periods, is suggested by low opal MAR in cores from both equatorial and Antarctic regions, perhaps as a result of low river input. terminations in this scenario would result from blooms of coccolithophorids triggered by low Si(OH)(4) concentrations.

Observing Biogeochemical Cycles at Global Scales With Profiling Floats and Gliders Prospects for a Global Array

Chemical and biological sensor technologies have advanced rapidly in the past five years. Sensors that require low power and operate for multiple years are now available for oxygen, nitrate, and a variety of bio-optical properties that serve as proxies for important components of the carbon cycle (e.g., particulate organic carbon). These sensors have all been deployed successfully for long periods, in some cases more than three years, on platforms such as profiling floats or gliders. Technologies for pH, pCO(2), and particulate inorganic carbon are maturing rapidly as well. These sensors could serve as the enabling technology for a global biogeochemical observing system that might operate on a scale comparable to the current Argo array. Here, we review the scientific motivation and the prospects for a global observing system for ocean biogeochemistry.

Nutrient Uptake by a Deposit-Feeding Enteropneust: Nitrogenous Sources

We measured carbon, nitrogen, protein, bacterial and microalgal abundance, and mineral-specific surface area in sediments from the feeding zone of undisturbed Saccoglossus kowalewskyi, as well as in their fresh egesta. Comparison of results using surficial material 1 mm) and the top 3 mm of sediments indicated ingestion of surficial material by the enteropneusts. Assuming the surficial sediment as a food source results in apparent absorption efficiencies of 15% for TOC, 35% for TON, 60% for protein and 86% for microalgae. The C:N ratio of the apparently absorbed material was 4.2, consistent with an amino acid-rich diet. Protein- nitrogen uptake, however, accounted for only about 28% of total nitrogen absorption, indicating a dominant use of non-protein nitrogen . Bacterial and microalgal contributions to dietary nitrogen uptake were no more than 3% and 4% respectively. Active worms maintain 2 foraging areas with an average total foraging volume of 0.9 cm3 and a volume ingestion rate of 0.06 to 0.12 cm3 ind.-1 h-1. If the preferred feeding zone of these enteropneusts is the nitrogen -enriched surficial layer, we estimate that their feeding activities will deplete the available food resources every 8 to 16 h and they may rely on biological and tidal redistribution of surface material.