ALBERT

Description: Cellular nutrient ratios are often applied as indicators of nutrient limitation in phytoplankton studies, especially the so-called Redfield ratio. For periphyton, similar data are scarce. We investigated the changes in cellular C: N: P stoichiometry of benthic microalgae in response to different levels and types of nutrient limitation and a variety of abiotic conditions in laboratory experiments with natural inocula. C: N ratios increased with decreasing growth rate, irrespective of the limiting nutrient. At the highest growth rates, the C: N ratio ranged uniformly around 7.5. N: P ratios 〈13 indicated N limitation, while N: P ratios 〉22 indicated P limitation. Under P limitation, the C: P ratios increased at low growth rate and varied around 130 at highest growth rates. For a medium with balanced supply of N and P, an optimal stoichiometric ratio of C: N: P = 119 : 17 : 1 could be deduced for benthic microalgae, which is slightly higher than the Redfield ratio (106 : 16 : 1) considered typical for optimally growing phytoplankton. The optimal ratio was stable against changes in abiotic conditions. In conclusion, cellular nutrient ratios are proposed as an indicator for nutrient status in periphyton.

Description: According to Connell�s intermediate disturbance hypothesis (IDH), diversity within a community is maximal at intermediate frequencies and intensities of disturbances. In order to test the IDH, disturbances of different frequencies and intensities were imposed on natural plankton communities in controlled field experiments. These disturbances consisted of an artificial deepening of the mixed layer, leading to the dilution of epilimnetic populations and to a higher level of nutrients. Intervals between disturbances ranged from 2 to 12 d. Different intensities of disturbance were caused by differences in the experimental mixing depth (150 and 225% of the original epilimnion depth). Investigation focused on the effect that disturbances had on the diversity of natural phytoplankton communities. Additionally, we were interested in determining the effect of grazing by zooplankton. The results of the field experiments show for the first time the applicability of the IDH to phytoplankton within complete planktonic communities. Diversity showed a clear maximum at the intermediate disturbance interval of 6 d. Similarly, species number peaked at intermediate interval length (6-10 d).

Description: We deployed CO2 and O2 sensors on the U.S. continental shelf off Cape Hatteras, North Carolina, during late summer 1994. A continuous 32‐d gas record was obtained at 20 m in 25 m of water, below the thermocline for most of the period. Analysis of the correlation between CO2 and O2 indicates that biological and advective processes dominated the gas variability, with small or insignificant fluxes due to air–sea exchange, vertical eddy diffusion, and carbonate dissolution or formation. The observed O2 : CO2 correlation was 1.39, within the range predicted for the photosynthetic quotient. Photosynthesis and respiration appeared to be tightly coupled, resulting in no net community production in these waters during the late summer. It is evident from these results that the combination of mooring‐based CO2 and O2 measurements will be a powerful tool for studying the marine carbon cycle.

Description: The effect of variable concentrations of dissolved molecular carbon dioxide, [CO2,aq], on C:N:P ratios in marine phytoplankton was studied in batch cultures under high light, nutrient-replete conditions at different irradiance cycles. The elemental composition in six out of seven species tested was affected by variation in [CO2,aq]. Among these species, the magnitude of change in C:N:P was similar over the experimental CO2 range. Differences in both cell size and day length-dependent growth rate had little effect on the critical CO2 concentration below which a further decrease in [CO2,aq] led to large changes in C:N:P ratios. Significant CO2-related changes in elemental ratios were observed at [CO2,aq] 〈 10 mu mol kg-l and correlated with a CO2-dependent decrease in growth rate. At [CO2,aq] typical for ocean surface waters, variation in C:N:P was relatively small under our experimental conditions. No general pattern far CO2-related changes in the elemental composition could be found with regard to the direction of trends. Either an increase or a decrease in C:N and C:P with increasing [CO2,aq] was observed, depending on the species tested. Diurnal variation in C:N and C:P, tested in Skeletonema costatum, was of a similar magnitude as CO2-related variation. In this species, the CO2 effect was superimposed on diurnal variation, indicating that differences in elemental ratios at the end of the photoperiod were not caused by a transient buildup of carbon-rich storage compounds due to a more rapid accumulation of carbohydrates at high CO2 concentrations. If our results obtained under high light, nutrient-replete conditions are representative for natural phytoplankton populations, CO2-related changes in plankton stoichiometry are unlikely to have a significant effect on the oceanic carbon cycle

Description: Diatoms have evolved a multitude of morphologics, including highly elongated cells and cell chains. Elongation and chain formation have many possible functions, such as grazing protecticn or effects on sinking. Here, a model of diffusive and advective nutrient transport is used to predict impacts of cell shape and chain length on potential nutrient supply and uptake in a turbulent environment. Rigid, contiguous, prolate spheroids thereby represent the shapes of simple chains and solitary cells. At scales larger than a few centimeters, turbulent water motions produce a more or less homogeneous nutrient distribution. At the much smaller stall: of diatom cells, however, turbulence drcates a roughly linear shear and nutrients can locally become strongly dl=pleted because of nutrient uptake by phytoplankton cells. The potential diffusive nutrient supply is greater for elongated than for spherically shaped cells of similar volume but lower for chains than for solitary cells. Although the relative increase in nutrient transport due to turbulence is greater for chains, single cells still enjoy a greater total nutrient supply in turbulent cnvironmerits. Only chains with specialized structures, such as spaces between the cells, can overcome this disadvantage and even obtain a higher nutrient supply than do solitary cells. The mod=1 results are compared to laboratory measurements of nutrient uptake under turbulent conditions and to effects ol’ sinking

Description: In this study, the impact of oceanic data assimilation on ENSO simulations and predictions is investigated. The authors’ main objective is to compare the impact of the assimilation of sea level observations and three-dimensional temperature measurements relative to each other. Three experiments were performed. In a control run the ocean model was forced with observed winds only, and in two assimilation runs three-dimensional temperatures and sea levels were assimilated one by one. The root-mean-square differences between the model solution and observations were computed and heat content anomalies of the upper 275 m compared to each other. Three ensembles of ENSO forecasts were performed additionally to investigate the impact of data assimilation on ENSO predictions. In a control ensemble a hybrid coupled ocean–atmosphere model was initialized with observed winds only, while either three-dimensional temperatures or sea level data were assimilated during the initialization phase in two additional forecast ensembles. The predicted sea surface temperature anomalies were averaged over the eastern equatorial Pacific and compared to observations. Two different objective skill measures were computed to evaluate the impact of data assimilation on ENSO forecasts. The authors’ experiments indicate that sea level observations contain useful information and that this information can be inserted successfully into an oceanic general circulation model. It is inferred from the forecast ensembles that the benefit of sea level and temperature assimilation is comparable. However, the positive impact of sea level assimilation could be shown more clearly when the forecasted temperature differences rather than the temperature anomalies themselves were compared with observations.

Description: Periphyton grazing by the marine isopod Idothea chelipes was studied by exposing periphyton grown on glass slides to a gradient of grazer densities. An analysis of the algal growth rates and their relationships to grazer density revealed two groups of algae. The unicellular diatoms Licmophora ehrenbergii, Fragilaria tabulata, Navicula spp., Cocconeis costata, and the green alga Ulothrix implexa had high maximal growth rates (0.90–1.47 d−1) and suffered high grazing losses (0.41–0.68 d−1 per grazer ind.). The tube dwelling diatom Amphipleura rutilans and the cyanobacteria Lyngbya confervoides and Spirulina subsalsa had low maximal growth rates (0.38–0.81 d−1) and suffered only moderate grazing losses (0.10–0.27 d−1 per grazer ind.). The species of the first group seemed to be less strongly resource limited than did the species of the second group. Grazing by I. chelipes has the potential to drive succession from the well‐edible to the less edible periphyton species.

Description: The seasonal cycle over the tropical Pacific simulated by 11 coupled ocean–atmosphere general circulation models (GCMs) is examined. Each model consists of a high-resolution ocean GCM of either the tropical Pacific or near-global means coupled to a moderate- or high-resolution atmospheric GCM, without the use of flux correction. The seasonal behavior of sea surface temperature (SST) and eastern Pacific rainfall is presented for each model. The results show that current state-of-the-art coupled GCMs share important successes and troublesome systematic errors. All 11 models are able to simulate the mean zonal gradient in SST at the equator over the central Pacific. The simulated equatorial cold tongue generally tends to be too strong, too narrow, and extend too far west. SSTs are generally too warm in a broad region west of Peru and in a band near 10°S. This is accompanied in some models by a double intertropical convergence zone (ITCZ) straddling the equator over the eastern Pacific, and in others by an ITCZ that migrates across the equator with the seasons; neither behavior is realistic. There is considerable spread in the simulated seasonal cycles of equatorial SST in the eastern Pacific. Some simulations do capture the annual harmonic quite realistically, although the seasonal cold tongue tends to appear prematurely. Others overestimate the amplitude of the semiannual harmonic. Nonetheless, the results constitute a marked improvement over the simulations of only a few years ago when serious climate drift was still widespread and simulated zonal gradients of SST along the equator were often very weak.

Description: Diatoms have evolved a multitude of morphologies, including highly elongated cells and cell chains. Elongation and chain formation have many possible functions, such as grazing protection or effects on sinking. Here, a model of diffusive and advective nutrient transport is used to predict impacts of cell shape and chain length on potential nutrient supply and uptake in a turbulent environment. Rigid, contiguous, prolate spheroids thereby represent the shapes of simple chains and solitary cells. Ar scales larger than a few centimeters, turbulent water motions produce a more or less homogeneous nutrient distribution. At the much smaller scale of diatom cells, however, turbulence creates a roughly linear shear and nutrients can locally become strongly depleted because of nutrient uptake by phytoplankton cells. The potential diffusive nutrient supply is greater for elongated than for spherically shaped cells of similar volume but lower for chains than for solitary cells. Although the relative increase in nutrient transport due to turbulence is greater for chains, single cells still enjoy a greater total nutrient supply in turbulent environments, Only chains with specialized structures, such as spaces between the cells, can overcome this disadvantage and even obtain a higher nutrient supply than do solitary cells. The model results are compared to laboratory measurements of nutrient uptake under turbulent conditions and to effects of sinking.