Limnology and Oceanography

A recently published evaluation of bacterioplankton abundance and productivity in the bathypelagic North Pacific suggests that these properties are generally coupled with particulate organic carbon (POC) fluxes. In that analysis, bacterial biomass and productivity were several–fold greater in subarctic than subtropical waters, consistent with the basin–scale distribution of POC flux and suggestive of a sinking POC > DOC > bacteria transformation of the carbon. To test this hypothesis, we sought to determine whether the very strong spatial and temporal gradients in POC flux in the Arabian Sea would force similar deep–ocean gradients in bacterial variables. On both a within and between–cruise basis, there was variability in bacterial abundance and thymidine incorporation in the deep Arabian Sea, but correspondence was equivocal between these variables and several correlates to export: flux of biogenic carbon from the euphotic zone, state of the monsoon, and proximity to productive coastal upwelling zones. However, when annual mean bacterial abundance at 2,000 m was compared with annual POC flux at that depth, a strong correspondence emerged: high annual flux supported high bacterial abundance (such a correspondence was not found for bacterial productivity). This finding suggests that bathypelagic bacterial abundance responds to the long–term mean input of organic matter and less to episodic inputs. A comparative evaluation of the North Pacific revealed that although the bathypelagic bacteria there showed correspondence to deep POC flux, that variable alone would not account for the wide meridional variations in bacterial abundance that have been reported.

Benthic community responses to lake eutrophication are poorly understood relative to pelagic responses. We compared phytoplankton and periphyton productivity along a eutrophication gradient in Greenland, U.S., and Danish lakes. Phytoplankton productivity increased along the phosphorus gradient (total phosphorus [TP] = 2–430 mg m⁻³), but whole‐lake benthic algal productivity decreased, substantially depressing increases in primary productivity at the whole‐lake scale. In shallow, oligotrophic Greenland lakes, periphyton was responsible for 80–98% of primary production, whereas in Danish lakes with TP > 100 mg m⁻³, phytoplankton were responsible for nearly 100% of primary production. Benthic contributions ranged from 5 to 80% depending on morphometry and littoral habitat composition in lakes with intermediate phosphorus concentrations. Thus, eutrophication was characterized by a switch from benthic to pelagic dominance of primary productivity. Carbon stable isotope analysis showed that the redistribution of primary production entailed a similar shift from periphyton to phytoplankton in the diets of zoobenthos. Benthic and pelagic habitats were energetically linked through food web interactions, but eutrophication eroded the benthic primary production pathway.

Diurnal changes were studied at different times of the year in a variety of Louisiana ponds, swamps, ditches and streams by the following procedure. Collections were made between 0500 and 0600 and between 1500 and 1600 on one day, and at the same times several days later. Each sample was analyzed chemically and biologically. The times chosen measure the approximate high and low values of dissolved substances in the daily chemical cycle. The extent of change in chemical factors and plankton indicated the degree and role of oxidative processes in a shallow habitat, and provided a measure for comparing different habitats.

Plankton populations, predominantly protozoa, often changed during the day, and sometimes changed significantly within 2 hr after sunrise. The conditions and organisms found in shallow habitats may thus depend upon the time of sampling.

Bacterial growth and the chemical composition of dissolved organic matter (DOM) were followed during a 10ߚd decomposition experiment with fresh, algalߚ;derived DOM from an Arctic ice floe. During the experiment ~30% of the dissolved organic carbon (DOC) was used by bacteria, indicating the highly reactive nature of this fresh DOM. Over half of the DOC consumption was accounted for as losses of combined neutral sugars and amino acids. The initial composition of the DOM was characterized by high neutral sugar (14% DOC) and amino acid (7.4% DOC) yields and the dominance of glucose (~75 mol%) and glutamic acid (~25 mol%). During microbial degߚradation the neutral sugar and amino acid yields decreased, and the molecular composition of the DOM became more uniform. The relatively constant abundance of D amino acids and the dramatic changes in the neutral sugar and amino acid compositions indicated that bacteria were important in shaping the chemical composition of marine DOM by selectively removing bioreactive components and by leaving behind biorefractory components. Based on principal component analysis and other parameters, neutral sugars and amino acids were found to be excellent indicators of the diagenetic state and bioavailability of marine DOM.

The occurrence of alkaline phosphatase activity (APA) that hydrolyses organic phosphorus into phosphate (PO₄) is commonly related to PO₄ deficiency of oceanic, coastal and fresh waters. APA is almost never investigated in PO₄‐rich estuaries, since very low activities are expected to occur. As a consequence, microbial mineralization of organic phosphorus into PO₄ has often been ignored in estuaries. In this study, we examined the importance of potential APA and the associated microbial dynamics in two estuaries, the Aulne and the Elorn (Northwestern France), presenting two different levels of PO₄ concentrations. Unexpected high potential APA was observed in both estuaries. Values ranged from 50 to 506 nmol L⁻¹ h⁻¹, which range is usually found in very phosphorus‐limited environments. High potential APA values were observed in the oligohaline zone (salinity 5–15) in spring and summer, corresponding to a PO₄ peak and a maximum bacterial production of particle‐attached bacteria. In all cases, high potential APA was associated with high suspended particulate matter and total particulate phosphorus. The low contribution of the 0.2–1 μm fraction to total APA, the strong correlation between particulate APA and bacterial biomass, and the close relationship between the production of particle‐attached bacteria and APA, suggested that high potential APA is mainly due to particle‐attached bacteria. These results suggest that the microbial mineralization of organic phosphorus may contribute to an estuarine PO₄ production in spring and summer besides physicochemical processes.

Seawater samples from several depths in the Sargasso Sea and equatorial Pacific and from surface stations in Biscayne Bay were analyzed for dissolved free amino acids and dissolved combined amino acids by a ligand‐exchange chromatography technique. Enantiomeric ratios of dissolved total amino acids from Atlantic and Pacific samples were also determined. On the basis of these ratios, a bacterial source is postulated for the origin of the dissolved d‐amino acids in seawater. The possible contributions from chemical racemization are compared to biological production as a source for these d‐amino acids.

Sedimentation poses a significant threat to stream ecosystems throughout the world, and increases in bedload can be especially detrimental to benthic communities. To examine how increased bedload affects algal‐ and detrital based stream communities, we manipulated sediment (via daily sediment addition to 0.25‐m² areas of stream bottom) and top‐down effects of macroconsumers (fishes and crayfish, via electric exclusion) in two factorial experiments, one using tiles and one using leaf packs as sampling substrates. Sediment addition had significant effects on both algal‐ and detrital‐based stream benthic communities, most notably via the alteration of macroconsumer‐mediated biotic interactions; these effects largely were due to sediment transport across the stream bottom, rather than sediment deposition. In the tile experiment, macroconsumers reduced total insect biomass and biomass of several dominant insect taxa under ambient sediment conditions. Sediment addition eliminated all macroconsumer effects except their reduction of chironomid biomass. In the leaf pack experiment, sediment addition eliminated macroconsumer effects on fungal accumulation rates; in general, however, leaf packs were not as affected by sediment addition as tiles. Direct effects of sediment addition were minimal in both experiments: algal composition was altered on tiles, while dipteran predator biomass tended to decline in leaf packs. These experiments demonstrate that small, environmentally realistic increases in bedload affect benthic communities, primarily via the alteration of macroconsumer effects. Although indirect effects of sedimentation have been examined less frequently than direct biotic responses, this study demonstrates the importance of sediment regime in determining the outcome of fish and crayfish‐mediated species interactions.

Cultured isolates of Prochlorococcus from the Mediterranean Sea (MED4) and Sargasso Sea (SS120) have been shown to have dramatically different pigment composition and growth rate responses when grown over a range of irradiances. Moreover, analyses of field populations in the North Atlantic have shown that distinct ecotypes can coexist in the same water column. These and other observations have led to the hypothesis that Prochlorococcus is comprised of genetically distinct ecotypes that collectively expand the range of light intensities over which the genus can thrive. In this paper, we explore this hypothesis by comparing the photophysiology of 10 different Prochlorococcus isolates from diverse oceanographic regimes. We found that the 10 isolates could be grouped into two loose clusters based on their growth response to varying light intensity and their chlorophyll b/a₂ (Chl b/a₂) ratios. Although both groups photoacclimate when grown over a range of light intensities, isolates with distinctly higher Chl b/a₂ ratios (high B/A ecotype) reach maximal growth rates at lower irradiances (I_k,g), have high growth efficiencies (α_g), and are inhibited in growth at irradiances where isolates with low Chl b/a₂ ratios (low B/A ecotype) are growing maximally. High Chl b/a₂ ratios resulted in higher spectrally weighted average Chl a₂‐specific absorption coefficient (ā_chl^*), Chl a₂ specific light‐harvesting efficiency (α_chla), and quantum yield (φ_m) under low growth irradiances for the isolates of the high B/A ecotype relative to the others. The distinction between the high and low B/A ecotypes is supported by molecular phylogenies constructed using the 16S ribosomal ribonucleic acid (rRNA) gene. The physiological differences between the ecotypes most likely result in different relative distributions in a given water column and in fluctuations in their relative abundances as a function of seasonal dynamics and water‐column stability.

Picoplankton, i.e., cells smaller than 2–3 μm, dominate in most open oceanic regions, such as in the Pacific Ocean. In these areas, the dominant carotenoid of photosynthetic eukaryotes is 199‐hexanoyloxyfucoxanthin (19HF), considered to be a diagnostic marker for prymnesiophytes. This suggests that this class could be a major component of eukaryotic picoplankton, despite the fact that virtually no prymnesiophyte has been described to date from this size class. To address this question, we assessed prymnesiophyte diversity and abundance in natural picoplankton coμmunities, using a molecular approach. Total genomic DNA was isolated from 3‐μm‐filtered samples collected in the Pacific Ocean. Small subunit (18S) ribosomal RNA genes (rDNA) were amplified by the polymerase chain reaction (PCR) using universal eukaryotic primers. The relative abundance of 18S rDNA from prymnesiophytes was quantified using group‐specific and eukaryotic 18S rDNA probes. The percentage of the prymnesiophyte versus total 18S rDNA was much lower than the percentage of prymnesiophytes calculated on the basis of pigment analyses for the same samples. 18S rDNA libraries from five samples were screened using a prymnesiophyte‐specific oli‐gonucleotide probe, and 14 nearly complete 18S rDNA sequences were retrieved. Phylogenetic analysis of these sequences established the presence of several prymnesiophyte lineages with no equivalent among cultivated species.

Phytoplankton cell size is believed to be closely regulated by the nutrient regime of water masses in the western North Atlantic Ocean. Since particle size affects attenuation of light in ocean water, we argue that a bridge between classical ocean optics and microbial ecology has formed whereby the physics of ocean color must include consideration of ecological factors important to the diversity of phytoplankton species.

Observations of the specific absorption coefficient, a*, have been made during 10 oceanographic cruises in the western North Atlantic. The range in Chl a concentration for ocean samples from the various water masses in this region was 0.1–8.0 µg liter⁻¹. Diffuse attenuation spectra of particles measured by a glass‐fiber filter technique show strong correlations between extracted chlorophyll and blue (440 nm) or red (670 nm) absorption. Of the two, a*440 is the most variable. Average specific absorption coefficients are calculated to be 0.049 m² mg⁻¹ at 440 nm and 0.028 at 670 nm, but both relationships are seen to be nonlinear. The nonlinearity observed is attributed to differences in cell size “packaging” within natural phytoplankton populations. Variation in cell size and a* are primarily related to the availability of nitrate‐N. In addition, other factors contribute to the variability in a*440, such as detritus and short wavelength UV‐absorbing pigments.