Miles J. Furnas

NUTRIENT-PHYTOPLANKTON RELATIONSHIPS IN NARRAGANSETT BAY DURING THE 1974 SUMMER BLOOM

ABSTRACT Nutrient and 14C uptake by natural phytoplankton populations was measured. Concentrated populations were incubated with uptake saturating concentrations of nitrate, silicate and phosphate, and nutrient uptake was then monitored at 30 min intervals over an eight hour period to yield Vmax, V, ρN and ρsi. Changes in the amount of particulate matter and absolute rates of uptake were related to in situ levels of phytoplankton biomass and dissolved nutrients, and replenishment (generation) times (R) estimated. Independent estimates of R were derived from calculations of particulate nitrogen (Np) and silica production (Sip) based on 14C uptake and appropriate elemental ratios. Forty-two percent of the annual carbon production of 308 g m−2 occurred during July and August when in situ nutrient levels were very low. The hourly uptake rates for nitrate (ρN) ranged from 0.118 to 0.136; ρsi was 0.007 to 0.150 μM. The daily N supply for the water column would have to be replenished one to 12 times daily to support Np and 2.5 to 17 days for Sip. It would take from 2 to 5 days for the nitrogen excretion rates of the zooplankton (> 153 μM.) and benthos to supply the daily phytoplankton nitrogen needs.

Growth rates of summer nanoplankton (<10 μm) populations in lower Narragansett Bay, Rhode Island, USA

Growth rates of summer (June–September) phytoplankton assemblages and constituent species were measured in 30 diffusion culture experiments. Size-fractionated (<10 μm) phytoplankton assemblages were incubated in situ or under simulated in-situ conditions in outdoor tanks connected to a running seawater system. Doubling rates of important species and groups (such as microflagellates) were compared to community biomass doubling rates estimated from 14C uptake and changes in chlorophyll a concentrations. Division rates of dominant diatom species generally equalled or exceeded community biomass doubling rates, while those of flagellates and non-motile ultraplankters were slower. Maximum division rates of sixteen common diatom species exceeded 2.1 divisions d-1, while nine had maximum division rates in excess of 3 d-1. Mean division rates of 12 diatom species exceeded 1 d-1. Maximum division rates of flagellated species, uncharacterized microflagellates and non-motile ultraplankton assemblages were 2.1, 1.5 and 1.4 d-1, respectively. Microflagellate and non-motile ultraplankton assemblage doubling rates were less than 0.5 d-1 in over half of all growth experiments.

Diel changes in nitrite concentration in the chlorophyll maximum in the Gulf of Mexico

Abstract A diel study of the deep chlorophyll maximum and its associated nitrite maximum showed that the nitrite maximum strengthens during the day and diminishes at night. A one-dimensional vertical model was used to differentiate physical from biological processes affecting the nitrite distribution. The rates of net production (or uptake) of nitrite, nitrate, ammonia, phosphate, and silicate were estimated at local dawn, noon, dusk, and midnight. The nitrite maximum at the base of the euphotic zone is produced mainly by phytoplankton during the day by reduction of nitrate. Daytime release of nitrite by phytoplankton occurs at a rate higher than the capacity of nitrifying microorganisms to oxidize it, and this leads to the nitrite maximum, which diminishes at night by nitrification. Nitrification rates were 0.2 to 0.5 μg-at. N1 −1 h −1 . The rates are one to three orders of magnitude higher than those based on the long-term (24-h) incubations of samples usually used to estimate nitrification rates.

Influence of temperature and cell size on the division rate and chemical content of the diatom Chaetoceros curvisetum Cleve

Abstract Cell size and temperature influenced the division rate and chemical content of the diatom Chaetoceros curvisetum Cleve when grown at 15, 20 and 25°C in nutrient replete media. Cell-size dependent trends of division rate in individual clones changed with temperature in a complex fashion. Considerable interclonal variability in division rate within a restricted range of cell sizes was also found. Cellular levels of carbon, nitrogen, protein, chlorophyll a , and silicon were linearly related to cell size. Cellular levels of carbon and chlorophyll per unit volume and silicon per unit surface area changed with temperature. No temperature effect on cellular levels of nitrogen and protein was found.

An evaluation of two diffusion culture techniques for estimating phytoplankton growth rates In situ

In comparative tests, acrylic diffusion chambers (voltume=42 ml) with polycarbonate filter membranes (1 μm pore diameter) consistently supported higher cell yields and faster growth rates of summer phytoplankton populations and species from Narragansett Bay than did dialysis bags (volume=50 ml, 0.24–0.48 nm pore diameter) or bottle cultures (with or without added nutrients). Stirring of diffusion chambers or dialysis bags had less effect on growth responses than did the choice of the containment membrane. Exchange measurements showed the polycarbonate filters (0.49 ml cm-2 h-1) to be five times more permeable than dialysis membranes (0.09 ml cm-2 h-1) per unit area. The mean of measured half-life times for water in diffusion chambers was less than one hour while comparable half-life times for dialysis bags were approximately 3 h. Diatoms from the <10 μm size fraction had higher growth rates than assemblages of microflagellates and non-motile ultraplankton. Stirring of diffusion chambers did not adversly effect the growth of microflagellates or non-motile ultraplankton. Growth responses in diffusion chambers moored in situ were generally similar to those measured in diffusion chambers incubated in outdoor tanks connected to a running seawater system.

Inputs of subthermocline waters and nitrate onto the Campeche Bank

Abstract Geostrophic tilting of isopycnal surfaces in the Yucatan Current raises the thermocline and nitrate nutricline (approximately the 22°C surface) above the level of the shelf break along the eastern margin of the Campeche Bank. Along the northern and western margins of the bank, these features lie 20–30 m below the shelf break. Cool nitrate-rich water can move laterally from the Yucatan Current along isopycnal surfaces beneath the thermocline onto the Campeche Bank. Drogues launched on the bank moved westward, corroborating transport away from the Yucatan Current. Near-bottom water on the bank is denser than water at similar depths in the Gulf of Mexico and Campeche Gulf. This dense near-bottom water is trapped within the euphotic zone on the bank for periods between days and weeks. Anomalies of temperature and chlorophylla at 30 m depth on the bank were directly proportional to density anomalies, indicating the mixing of subthermocline water with surface water. The development of surface patches of phytoplankton or elevated nitrate levels on the bank results from the vertical mixing of the stratified water column. During the summer, lateral transport of subthermocline water from the Yucatan Current appears to be an important source of nitrate nitrogen for the Campeche Bank.

DIEL SYNCHRONIZATION OF SPERM FORMATION IN THE DIATOM CHAETOCEROS CURVISETUM CLEVE1

During diel observations of spermatogenesis in Chaetoceros curvisetum Cleve, recognizable 4‐ and 8‐celled developmental stages were observed primarily during the daily dark period. The timing of the appearance of recognizable stages, 12–16 h after light period onset, was independent of the photoperiods used (8:16, 12:12, 16:8 h LD).