The bacterial component of the oceanic euphotic zone

Bacteria in the open sea remote from land are sustained strictly on local sources of organic production which should make understanding their nutrition and growth regulation easier than in nearshore systems, estuaries and lakes. Until now, a paucity of data from geographically isolated oceanic sites prevented ready :interpretation. In the past decade investigation of bacterial properties in oceanic systems has increased rapidly, stimulated in part by large oceanographic programs like the Joint Global Ocean Flux Study. Here I review comprehensive investigations of bacterial biomass and production dynamics in the subarctic north Atlantic and north Pacific, oligotrophic gyres in both oceans, upwelling provinces in the equatorial Pacific and northwest Arabian Sea, and in the Ross Sea, Antarctica. Euphotic zone bacterial stocks are remarkably similar across all except the last regime, averaging about 1 g C m(-2). Production and growth rates vary more widely, suggesting independent regulation of biomass and production. The seasonal to annual mean ratio of bacterial to primary production is usually below 20%. (C) 1999 Federation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved

Bacterial growth in experimental plankton assemblages and seawater cultures from the Phaeocystis antarctica bloom in the Ross Sea, Antarctica

A series of seawater culture experiments was carried out during the Phaeocystis antarctica bloom in the Ross Sea polynya (76.5 degrees S, 180 degrees W; November to December 1994 and December 1995 to January 1996) to examine bacterioplankton growth and derive empirical factors for estimating bacterial production rates. Bacterial growth was exponential over 3 to 10 d in all experiments, at rates of ca 0.1 to 0.7 d(-1), even in persistently cold waters (-2 to + 1 degrees C). Growth rates were lower in the early part of the bloom (early to mid-November) and highest during the period of peak primary productivity (2 to 4 g C m(-2) d(-1) in late November through December). Apparent lag phases in the growth curves lasting 1 to 7 d could be accounted for mathematically by subpopulations in the bacterial assemblages growing exponentially at different rates, with no need to invoke inactive, nondividing or nonviable populations. Lags were absent during the period of peak primary production, suggesting adaptation of the bacteria to ambient DOM. Growth was not stimulated by small temperature increases (Delta+2 to 4 degrees C), and was not balanced by removal processes in untreated \u27whole\u27 water samples. Growth rates were broadly similar to other directly observed bacterial growth rates in the Antarctic and did not appear to differ from rates in warmer waters. Conversion factors for thymidine and leucine averaged 8 and 0.8 x 10(17) cells mol(-1), respectively, not dissimilar to estimates from temperate waters. These findings suggest that bacteria were growing actively in 0 to -2 degrees C waters under rich bloom conditions, and lend strong sup port to the hypothesis that bacterioplankton metabolism controls DOC accumulation in Antarctic waters, at least at the low rates of DOM supply we infer from field and experimental observations. Bacterioplankton responded within 10 to 20 d to the evolving P. antarctica bloom and did not appear to behave substantially differently from lower latitude bloom systems

Temperature effects on export production in the open ocean

A pelagic food web model was formulated with the goal of developing a quantitative understanding of the relationship between total production, export production, and environmental variables in marine ecosystems. The model assumes that primary production is partitioned through both large and small phytoplankton and that the food web adjusts to changes in the rate of allochthonous nutrient inputs in a way that maximizes stability, i.e., the ability of the system to return to steady state following a perturbation. The results of the modeling exercise indicate that ef ratios, defined as new production/total production = export production/total production, are relatively insensitive to total production rates at temperatures greater than ∼25°C and lie in the range 0.1‐0.2. At moderate to high total production rates, ef ratios are insensitive to total production and negatively correlated with temperature. The maximum ef ratios are ∼0.67 at high rates of production and temperatures of 0°−10°C. At temperatures less than ∼20°C, there is a transition from low ef ratios to relatively high ef ratios as total production increases from low to moderate values. This transition accounts for the hyperbolic relationship often presumed to exist between ef ratios and total production. At low rates of production the model predicts a negative correlation between production and ef ratios, a result consistent with data collected at station ALOHA (22°45′N, 158°W) in the North Pacific subtropical gyre. The predictions of the model are in excellent agreement with results reported from the Joint Global Ocean Flux Study (JGOFS) and from other field work. In these studies, there is virtually no correlation between total production and ef ratios, but temperature alone accounts for 86% of the variance in the ef ratios. Model predictions of the absolute and relative abundance of autotrophic and heterotrophic microorganisms are in excellent agreement with data reported from field studies. Combining the ef ratio model with estimates of ocean temperature and photosynthetic rates derived from satellite data indicates that export production on a global scale is ∼20% of net photosynthesis. The results of the model have important implications for the impact of climate change on export production, particularly with respect to temperature effects

Simulation of annual plankton productivity cycle in the Black Sea by a one-dimensional physical-biological model

The annual cycle of the plankton dynamics in the central Black Sea is studied by a one-dimensional vertically resolved physical-biological upper ocean model, coupled with the Mellor-Yamada level 2.5 turbulence closure scheme. The biological model involves interactions between the inorganic nitrogen (nitrate, ammonium), phytoplankton and herbivorous zooplankton biomasses, and detritus. Given a knowledge of physical forcing, the model simulates main observed seasonal and vertical characteristic features, in particular, formation of the cold intermediate water mass and yearly evolution of the upper layer stratification, the annual cycle of production with the fall and the spring blooms, and the subsurface phytoplankton maximum layer in summer, as well as realistic patterns of particulate organic carbon and nitrogen. The computed seasonal cycles of the chlorophyll and primary production distributions over the euphotic layer compare reasonably well with the data. Initiation of the spring bloom is shown to be critically dependent on the water column stability. It commences as soon as the convective mixing process weakens and before the seasonal stratification of surface waters begins to develop. It is followed by a weaker phytoplankton production at the time of establishment of the seasonal thermocline in April. While summer nutrient concentrations in the mixed layer are low enough to limit production, the layer between the thermocline and the base of the euphotic zone provides sufficient light and nutrient to support subsurface phytoplankton development. The autumn bloom takes place some time between October and December depending on environmental conditions. In the case of weaker grazing pressure to control the growth rate, the autumn bloom shifts to December-January and emerges as the winter bloom or, in some cases, is connected with the spring bloom to form one unified continuous bloom structure during the January-March period. These bloom structures are similar to the year-to-year variabilities present in the data

WAP-1D-VAR v1.0: development and evaluation of a one-dimensional variational data assimilation model for the marine ecosystem along the West Antarctic Peninsula

The West Antarctic Peninsula (WAP) is a rapidlywarming region, with substantial ecological and biogeochemical responses to the observed change and variability for the past decades, revealed by multidecadal observationsfrom the Palmer Antarctica Long-Term Ecological Research (LTER) program. The wealth of these long-term observations provides an important resource for ecosystem modeling, but there has been a lack of focus on the development of numerical models that simulate time-evolving plankton dynamics over the austral growth season along the coastal WAP. Here, we introduce a one dimensional variational data assimilation planktonic ecosystem model (i.e., the WAP-1D-VAR v1.0 model) equipped with a model parameter optimization scheme. We first demonstrate the modified and newly added model schemes to the pre-existing food web and biogeochemical components of the other ecosystem models that WAP-1D-VAR model was adapted from, including diagnostic sea-ice forcing and trophic interactions specific to the WAP region. We then present the results from model experiments where we assimilate 11 different data types from an example Palmer LTER growth season (October 2002–March 2003) directly related to corresponding model statevariables and flows between these variables