Evelyn J. Lessard

The contribution of microorganisms to particulate carbon and nitrogen in surface waters of the Sargasso Sea near Bermuda

Abstract Seawater samples were collected from the euphotic zone of the Sargasso Sea near Bermuda in August of 1989 and March–April of 1990. Microbial population abundances, chlorophyll concentration, particulate carbon and particulate nitrogen were measured. Calculations were performed to establish the relative and absolute importance of the various microbial assemblages. The choice of conversion factors (g C and N cell −1 , or g C and N μm −3 ) for the microbial populations dramatically affected the estimation of “living” and “detrital” particulate material in the samples, and the relative importance of the various microbial groups. Averaged over all samples on either of the two cruises, microbial biomass constituted a greater proportion of the total particulate carbon and nitrogen during March–April (55% and 63%, respectively), than during August (≈24% and 30%, respectively) using “constrained” conversion factors that were derived. Accordingly, detrital material constituted the bulk of the particulate material during August, but was similar to the amount of microbial biomass during March–April. The bacterial assemblage constituted the largest single pool of microbial carbon (35%) and nitrogen (45%) in the water, and a significant fraction of the total particulate carbon (≈10–20%) and nitrogen (≈15–30%). Phototrophic nanoplankton (microalgae 2–20 μm in size) were second in overall biomass, and often dominated the microbial biomass in the deep chlorophyll maxima that were present during both cruises. The results temper recent assertions concerning the overwhelming importance of bacterial biomass in the oligotrophic Sargasso Sea but still support a major role for these microorganisms in the open ocean as repositories for carbon and nutrients.

Species-specific grazing rates of heterotrophic dinoflagellates in oceanic waters, measured with a dual-label radioisotope technique

A dual-isotope method was developed to measure grazing rates and food preferences of individual species of heterotrophic dinoflagellates from natural populations, collected from the Slope, Gulf Stream, and Sargasso Sea and from a transect from Iceland to New England, in 1983. The isotope method measures the grazing rates of microzooplankton which cannot be separated in natural populations on the basis of size. Tritiated-thymidine and 14C-bicarbonate were used to label natural heterotrophic and autotrophic food, respectively. Nine oceanic dinoflagellate species in the genera Protoperidinium, Podolampas, and Diplopsalis fed on both heterotrophic and autotrophic food particles with clearance rates of 0.4 to 8.0 μl cell-1 h-1, based on 3H incorporation, and 0.0 to 28.3 μl cell-1 h-1, based on 14C incorporation. Two dinoflagellate species, Protoperidinium ovatum and Podolampas palmipes, fed only on 3H-labelled food particles. Several species of dinoflagellates fed on bacteria (<1 μm) which had been prelabelled with 3H-thymidine. The clearance rates of heterotrophic dinoflagellates and ciliates were similar and within the range of tintinnid ciliate clearance rates reported in the literature. As heterotrophic dinoflagellates and ciliates can have comparable abundances in oceanic waters, we conclude that heterotrophic dinoflagellates may have an equally important impact as microheterotrophic grazers of phytoplankton and bacteria in oceanic waters.

River Influences on Shelf Ecosystems: Introduction and synthesis

[1] River Influences on Shelf Ecosystems (RISE) is the first comprehensive interdisciplinary study of the rates and dynamics governing the mixing of river and coastal waters in an eastern boundary current system, as well as the effects of the resultant plume on phytoplankton standing stocks, growth and grazing rates, and community structure. The RISE Special Volume presents results deduced from four field studies and two different numerical model applications, including an ecosystem model, on the buoyant plume originating from the Columbia River. This introductory paper provides background information on variability during RISE field efforts as well as a synthesis of results, with particular attention to the questions and hypotheses that motivated this research. RISE studies have shown that the maximum mixing of Columbia River and ocean water occurs primarily near plume liftoff inside the estuary and in the near field of the plume. Most plume nitrate originates from upwelled shelf water, and plume phytoplankton species are typically the same as those found in the adjacent coastal ocean. River-supplied nitrate can help maintain the ecosystem during periods of delayed upwelling. The plume inhibits iron limitation, but nitrate limitation is observed in aging plumes. The plume also has significant effects on rates of primary productivity and growth (higher in new plume water) and microzooplankton grazing (lower in the plume near field and north of the river mouth); macrozooplankton concentration (enhanced at plume fronts); offshelf chlorophyll export; as well as the development of a chlorophyll ‘‘shadow zone’’ off northern Oregon.

Bacterivory: A Novel Feeding Mode for Asteroid Larvae

Planktotrophic larvae that occur beneath the annual sea ice in the Antarctic assimilate organic solutes and preferentially ingest bacteria, whereas they actively exclude phytoplankton. In regions where phytoplankton biomass is temporally limited by light or nutrient concentrations, the growth and development of planktotrophic larvae may not be directly coupled to phytoplankton production.

Sorption of organic matter from four phytoplankton species to montmorillonite, chlorite and kaolinite in seawater

The sorption of organic matter from four species of phytoplankton to clean mineral surfaces was studied using sequential adsorption experiments. The soluble intracellular components from four phytoplankton species, Phaeocystis globosa, Gymnodinium sanguineum, Scrippsiella trochoidea, and Ditylum brightwellii, were repeatedly exposed to three minerals; montmorillonite, kaolinite, and chlorite. The surface-reactive fraction of organic matter in phytoplankton dissolved organic carbon (DOC) solutions ranged from 47% to 85% of the total. Adsorption partition coefficients (Kd) ranged from 53 to 175 l kg−1. These partition coefficients are significantly higher than the partition coefficients commonly observed for sedimentary porewater organic matter (≤30 l kg−1), and suggest that phytoplankton exudates contain a component of organic matter with considerable surface reactivity. S. trochoidea and D. brightwellii consistently had the highest Kd values and the greatest reactive organic matter component relative to P. globosa and G. sanguineum. On average, the highest Kd values were associated with chlorite (average Kd of 119 l kg−1) and montmorillonite (average of 105 l kg−1) rather than kaolinite (average Kd of 89 l kg−1). Organic matter from all four phytoplankton species interacted most with montmorillonite (average=82% reactive), followed by chlorite (72% reactive) and kaolinite (63% reactive). Significant variation in both the extent of surface-reactive material and the partition coefficients of the reactive material using this matrix of source organisms and mineral surfaces suggest that both the mineral and organic matter sources are influential in the sorption of organic matter to sediment surfaces.

Spatial and temporal changes in the partitioning of organic carbon in the plankton community of the Sargasso Sea off Bermuda

The vertical distribution of plankton (bacteria, nanozooplankton, microzooplankton, mesozooplankton, macrozooplankton and salps) biomass in the photic zone near the JGOFS time series station off Bermuda was examined during 2–3 week periods in August 1989 and in March/April 1990. The amount of phytoplankton carbon in the photic zone was lower in August as compared to March/April (398 and 912 mg C m−2, respectively). Total heterotrophic biomass in the photic zone was also lower in August as compared to March/April (1106 and 1795 mg C m−2, respectively). Taken together, bacteria and nanozooplankton constituted approximately 70% of the total heterotrophic carbon in the photic zone on both cruises. Considering their high weightspecific carbon demand relative to micro-, meso-, and macrozooplankton, it is clear that most of the carbon in the surface waters of the Sargasso Sea near Bermuda cycles through bacteria and flagellates—the “microbial loop”. However, both seasonal (August vs. March/April) and withincruise variations in the vertical flux of organic material were related to the biomass of macrozooplankton. Macrozooplankton biomass was lower in August than March/April (93 and 267 Mg C m−2, respectively). There was more non-living carbon (detritus) than living carbon in the photic zone during the August cruise (70% of total organic matter) but about equal amounts of detritus and living carbon in March/April.

Estuary-enhanced upwelling of marine nutrients fuels coastal productivity in the U.S. Pacific Northwest

© 2014. American Geophysical Union. All Rights Reserved. The Pacific Northwest (PNW) shelf is the most biologically productive region in the California Current System. A coupled physical-biogeochemical model is used to investigate the influence of freshwater inputs on the productivity of PNW shelf waters using realistic hindcasts and model experiments that omit outflow from the Columbia River and Strait of Juan de Fuca (outlet for the Salish Sea estuary). Outflow from the Strait represents a critical source of nitrogen to the PNW shelf-accounting for almost half of the primary productivity on the Vancouver Island shelf, a third of productivity on the Washington shelf, and a fifth of productivity on the Oregon shelf during the upwelling season. The Columbia River has regional effects on the redistribution of phytoplankton, but does not affect PNW productivity as strongly as does the Salish Sea. A regional nutrient budget shows that nitrogen exiting the Strait is almost entirely (98%) of ocean-origin - upwelled into the Strait at depth, mixed into surface waters by tidal mixing, and returned to the coastal ocean. From the standpoint of nitrogen availability in the coastal euphotic zone, the estuarine circulation driven by freshwater inputs to the Salish Sea is more important than the supply of terrigenous nitrogen by rivers. Nitrogen-rich surface waters exiting the Strait follow two primary pathways - to the northwest in the Vancouver Island Coastal Current and southward toward the Washington and Oregon shelves. Nitrogen flux from the Juan de Fuca Strait and Eddy Region to these shelves is comparable to flux from local wind-driven upwelling.

A new method for live-staining protists with DAPI and its application as a tracer of ingestion by walleye pollock (Theragra chalcogramma (Pallas)) larvae

The efficacy of DAPI (2,4-diamadino-6-phenylindole), a fluorescent DNA-specific stain, to live-stain protists was evaluated for use as tracers in laboratory studies of feeding by larval pollock, Theragra chalcogramma (Pallas). At 4 μg · ml−1, DAPI effectively live-stained all eight species of heterotrophic protists examined. DAPI also stained four species of autotrophic protists although staining was more variable. No toxic effects of DAPI were detected. DAPI did not appear to alter the swimming behavior of stained protists. Further, the growth rates of a ciliate, Euplotes sp. and an autotrophic flagellate, Rhodomonas salina (Wislouch) Hill and Wetherbee, exposed to DAPI, did not differ significantly from growth in controls. When DAPI-stained Euplotes sp. were presented to larval pollock, ingested cells were easily seen in larval guts. This is the first report of ingestion of an aloricate ciliate by larval pollock. The lack of toxicity of DAPI and the easy visualization of ingested DAPI-stained protists, make this method a useful new tool for examining the ingestion of protists by metazoa and other protists.

Distribution, abundance and size composition of heterotrophic dinoflagellates and ciliates in the Sargasso Sea near Bermuda

We examined the temporal variability in vertical distribution, abundance, size composition and biomass of heterotrophic dinoflagellates and ciliates at the JGOFS time-series station off Bermuda during August 1989 and March-April 1990. Abundances of heterotrophic dinoflagellates and ciliates ranged from 1.3 to 14.2 ml−1 in August and 2.0 to 28.6 ml−1 in number in March-April. Total biomass of these two groups ranged from 0.2 to 2.9 μg C 1−1; integrated euphotic zone biomass was twice as high in March-April than in August. Heterotrophic dinoflagellates were always more numerous than ciliates (0.9–28.6ml−1 compared to 0.1–5.0ml−1). On average, heterotrophic dinoflagellates constituted 50% of the total biomass of these two groups (46–59% in August and 29–64% in March-April) in the upper 150 m. Greater than 92% of the dinoflagellate cells were 5 gmm heterotrophic protists in the oligotrophic subtropical Sargasso Sea.

Does the Columbia River plume influence phytoplankton community structure along the Washington and Oregon coasts

[1] As part of the River Influences on Shelf Ecosystems (RISE) program, we examined the influence of the Columbia River plume on the composition and biomass of phytoplankton communities on the Washington and Oregon coasts. We determined the taxonomic composition, size structure, and biomass of phytoplankton assemblages in near-surface shelf waters during four 3-week cruises in spring and/or summer of 2004–2006. As the Columbia plume is very dynamic, the influence of the plume was examined in three ways. Two comparisons of the entire data set were made: (1) a geographical comparison of the Washington, Oregon, and Columbia River mouth regions based on latitude and (2) a ‘‘plume’’ versus ‘‘nonplume’’ comparison based on salinity. A third comparison focused on samples taken in and outside of three specific plumes under different upwelling/downwelling conditions. In whole data set comparisons, there were no significant differences in chlorophyll, carbon biomass, or diatom community structure between regions or between plume and nonplume samples. However, within some cruises there were regional and plume differences in chlorophyll and biomass. Diatom community composition differed between cruises, but within a cruise it was similar across regions and in plume/nonplume samples, indicating there was no unique plume community. On finer time and space scales, differences in community structure as well as biomass were evident between samples in and outside of specific plumes. Over broader scales, the Columbia plume acts to make coastal phytoplankton communities more homogeneous. Specific impacts will depend on the history of upwelling and nutrient status of the coastal waters which the plume encounters.