Michael R. Roman

Physical and biological controls on carbon cycling in the equatorial pacific.

The equatorial Pacific is the largest oceanic source of carbon dioxide to the atmosphere and has been proposed to be a major site of organic carbon export to the deep sea. Study of the chemistry and biology of this area from 170� to 95�W suggests that variability of remote winds in the western Pacific and tropical instability waves are the major factors controlling chemical and biological variability. The reason is that most of the biological production is based on recycled nutrients; only a few of the nutrients transported to the surface by upwelling are taken up by photosynthesis. Biological cycling within the euphotic zone is efficient, and the export of carbon fixed by photosynthesis is small. The fluxes of carbon dioxide to the atmosphere and particulate organic carbon to the deep sea were about 0.3 gigatons per year, and the production of dissolved organic carbon was about three times as large. The data establish El Ni�o events as the main source of interannual variability.

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.

Mesozooplankton grazing and metabolism at the equator in the central Pacific: Implications for carbon and nitrogen fluxes

Primary productivity and chlorophyll in the equatorial Pacific are lower than expected based on ambient nutrient concentrations. We tested the hypothesis that these conditions are due to a balance between phytoplankton growth and mesozooplankton grazing. Grazing rates and biomasses of three size classes of zooplankton in the size range of 200-200 μm were measured during March-April and October of 1992 at the equator (140°W). El Nino conditions prevailed in March–April, whereas in October a tropical instability wave (TIW) passed through the study area. Weight-specific pigment ingestion rates of mesozooplankton tended to be higher in March–April than in October while the opposite was the case for biomass. There were no discernible temporal trends in mesozooplankton community grazing rates in March–April, but there were changes in October associated with the passage of the TIW. Mesozooplankton grazing removed 1–9% day−1 of the total chlorophyll and 1–12% of the primary production within the euphotic zone. Therefore, the grazing hypothesis as stated above is rejected, although a simple chlorophyll budget suggests that grazing (mostly by microzooplankton) and phytoplankton growth are roughly in balance. Most of the phytoplankton was 2-μm phytoplankton, we estimate removal rates of the > 2-μm chlorophyll standing stock of up to 27% day−1. Another question of biogeochemical importance is the rate of the production of large diatoms. Mesozooplankton grazing could have balanced growth of the large (> 20-μm) diatoms in March–April, but not in October. From estimates of respiration and excretion based on water temperature and body size, we infer that: (1) > 80% of the carbon ingested by mesozooplankton is not phytoplankton; (2) mesozooplankton excretion supports < 7% of the nitrogen demands of phytoplankton; (3) the flux of carbon passing through the mesozooplankton would be equivalent to 23% of the primary production; (4) mesozooplankton fecal carbon could account for 100% of the sinking POC flux in this region. Finally, a simple food chain analysis suggests that a significant fraction of the microzooplankton production probably passes through mesozooplankton.

Downward export of respiratory carbon and dissolved inorganic nitrogen by diel-migrant mesozooplankton at the JGOFS Bermuda time-series station

Published calculations suggest that downward fluxes of respiratory carbon (RC) and dissolved inorganic nitrogen (DIN) associated with diel-migrant zooplankton are significant forms of export production. In this study, we examined variability in downward fluxes of RC and DIN due to diel-migrant mesozooplankton (200–2000 μm) on seven occasions between 25 March and 8 April 1990 at the JGOFS (Joint Global Ocean Flux Study) station off Bermuda. Downward fluxes due to migrators crossing 150 m ranged from 6 to 41 mgC m−2 day−1 (RC) and 2 to 5 mgN m−2 day−1 (DIN). In comparison to gravitational fluxes of POC and PON measured with particle traps at 150m during the same period, mesozooplankton RC flux was 18–70% of POCflux and mesozooplankton DIN flux was 17–82% of PON flux. If the RC and DIN fluxes due to migrators are considered in calculations of export production, the percentage of the production exported below 150 m over the study period would increase by an average of 25% for carbon and 21% for nitrogen. These results support the contention that metabolic activities of diel-migrant zooplankton should be included in calculations of export of carbon and nitrogen out of the mixed layer.

Seasonal response of zooplankton to monsoonal reversals in the Arabian Sea

Abstract The US JGOFS Arabian Sea Process Study was designed to provide a seasonally and spatially resolved carbon budget for a basin exhibiting some of the highest and lowest concentrations of plant biomass in the world’s ocean. During the US JGOFS Process Study in the Arabian Sea (September 1994–January 1996), the absolute maximum in biomass of epipelagic zooplankton in the entire study was observed during the Southwest Monsoon season inshore of the Findlater Jet in the area of upwelling. The greatest contrast between high and low biomass in the study area also was observed during the Southwest Monsoon, as was the strongest onshore–offshore gradient in biomass. Lowest biomass throughout the study was observed at the most offshore station (S15), outside the direct influence of the monsoon forcing. The greatest day/night contrasts in biomass were observed nearshore in all seasons, with nighttime biomass exceeding daytime in the Northeast Monsoon season, but daytime exceeding nighttime in the Southwest Monsoon season. The diel vertical migration patterns in general reversed between the monsoons at all stations in the southern part of the study area. Virtually, no diel vertical migration of zooplankton took place in any season at the station with strong, persistent subsurface suboxic conditions (N7), suggesting that these conditions suppress migration. Based on the distribution of biomass, we hypothesize that inshore of the Findlater Jet, zooplankton grazing on phytoplankton is the dominant pathway of carbon transformation during both monsoon seasons, whereas offshore the zooplankton feed primarily on microplankton or are carnivorous, conditions that result in reduction of the carbon flux mediated by the zooplankton. Predation by mesopelagic fish, primarily myctophids, may equal daily growth of zooplankton inshore of the Findlater Jet during all seasons. This suggests that the food web inshore of the Findlater Jet is well integrated, may have evolved during past periods of intensified upwelling, and has a distinctly annual cycle.

Zooplankton variability on the equator at 140°W during the JGOFS EqPac study

The vertical distributions of zooplankton biomass and community composition were measured on the equator at 140°W during March/April (19 days) and October 1992 (21 days). El Nino conditions prevailed during the March/April time series. The average integrated (200 m) zooplankton (>64 μm) biomass was 32 mM C m−2 in March/April and 41 mM C m−2 in October. The overall cruise means were not significantly different; however during October zooplankton were more aggregated in the upper euphotic zone, there were more copepods >200 μm, and there were more day/night differences in the vertical distribution of zooplankton biomass as a result of vertical migration behavior. Cyclopoid copepods were more abundant during the El Nino conditions of March compared to October. There were no apparent trends in integrated zooplankton biomass during the March/April time series. However, the species composition of the zooplankton community changed towards the end of the time series when meridional currents flowed to the south. Zooplankton biomass during the October cruise increased nearly five-fold with the passage of a tropical instability wave. A lag in the increase of zooplankton compared to nitrate and chlorophyll as well as changes in the zooplankton community composition over the time series reflect the response times of zooplankton to upwelling events.

Latitudinal variations in mesozooplankton grazing and metabolism in the central tropical Pacific during the U.S. JGOFS EqPac study

Abstract Pigment ingestion rates by three size classes of mesozooplankton (200–500 μm, 500–1000 μm and 1000–2000 μm) within the euphotic zone were measured during the Survey l (February/March) and Survey 2 (August/September) cruises of the 1992 United States Joint Global Ocean Flux Study (U.S. JGOFS) in the central equatorial Pacific (EqPac). Survey 1 was characterized by El Nino conditions while Survey 2 was characterized by typical climatological conditions. The small animals (200–500 μm) contributed more than 50% (range: 34–80%) to the total mesozooplankton grazing reported here. Mesozooplankton grazing was higher within the equatorial region (5°S–5°N) than at higher latitudes (5°S–12°S, 5°N–12°N). The carbon-specific ingestion rates of both the large and small animals tended to be higher during Survey 1 than during Survey 2. In contrast to the carbon-specific ingestion rates, the mesozooplankton biomass during Survey 1 was lower than that during Survey 2. Thus, the higher ingestion rates during Survey 1 were offset by lower biomass and the mesozooplankton grazing was quite similar during Surveys 1 and 2. Mesozooplankton removal of chi a was higher within the high-phytoplankton-biomass equatorial region than at higher latitudes during Survey 1, but regional differences were not significant during Survey 2. Mesozooplankton community grazing was equivalent to an average daily removal of 3.0% (range: 0.5–7.7%) and 2.2% (range: 0.8–3.5%) of the total chi a standing stock within the euphotic zone during Surveys 1 and 2, respectively. However, mesozooplankton grazing was equivalent to a daily removal of 47% (range: 15–91%) and 36% (range: 23–66%) of the > 5μm chl a standing stock within the euphotic zone during Surveys 1 and 2, respectively. Assuming a carbon-to-chi a ratio of 58, we estimate that mesozooplankton grazing removed an average of 6% (range: 2–12%) of 14 C primary production during Survey 1, and 5% (range: 2–10%) during Survey 2. Removal rates were not significantly different between the equatorial region and the higher latitudes. Rates of respiration and ammonium excretion were estimated from empirical models based on animal weight and water temperature. Phytoplankton ingestion could not satisfy the estimated daily maintenance-carbon demands of mesozooplankton. The shortages were more pronounced in the large size fraction than in the small size fraction. The estimated ammonium excretion by the mesozooplankton could support 4–15% and 3–17% of 14 C primary production during Surveys 1 and 2, respectively.

Grazers and Associated Organisms of Trichodesmium

Colonies of the pelagic cyanobacterium Trichodesmium consist not only of filamentous trichomes, but of other types of associated organisms as well, including bacteria, diatoms, dinoflagellates, fungi, protozoa, hydrozoans and copepods. These associated organisms use Trichodesmium as a physical substrate and as a food source. Pelagic harpacticoid copepods (the genera Macrosetella and Miracia in particular), are the only organisms that have been quantitatively shown to use Trichodesmium as a food source. There have been anecdotal reports of other organisms feeding on Trichodesmium including salps, crabs and fish. These micro-communities, with tight nutrient coupling, may be an adaptation for existence in oligotrophic tropical and subtropical seas. Associated organisms may cause physiological changes in Trichodesmium, which could account for much of the inter-colony variation when measuring various physiological parameters of this cyanobacteria. Trichodesmium can make a significant contribution to “new production” of nitrogen and the flux of organic matter from surface waters. Therefore, the consumption and fate of Trichodesmium has important consequences for understanding the inputs and outputs of carbon and nitrogen in the open ocean.

Latitudinal gradients in zooplankton biomass in the tropical Pacific at 140°W during the JGOFS EqPac study: Effects of El Niño

Abstract In 1992, as part of the equatorial Pacific study (EqPac) of the U.S. JGOFS Program, we collected zooplankton samples during a set of cruises that crossed the equator in the Pacific Ocean roughly along 140°W from 12°N to 12°S. The first cruise (Survey 1) occurred during El Nino conditions in February-March when anomolously warm surface waters (>28°C) were present over most of the transect. The second cruise (Survey II) was during August-September when surface temperatures had returned to near or below the climatological mean over much of the region, and inorganic nitrogen and chlorophyll concentrations had increased compared with Survey I. Zooplankton biomass was higher in the equatorial region during Survey II compared with Survey I. For both cruises, chlorophyll and zooplankton biomass generally increased in the epipelagic zone (0–200 m) towards the equator. However, whereas primary production and chlorophyll were highest on or near the equator, zooplankton biomass was usually highest several degrees to the north and south of the equator. Zooplankton biomass was reduced below 100 m in newly upwelled water near the equator during both cruises, while biomass was distributed more evenly with depth in the presence of a weak thermocline at higher latitudes. The size structure of the zooplankton community changed between cruises, with more biomass in the >1000 μm size fraction during Survey 2 in waters near (but not on) the equator. As noted by previous investigators, zooplankton biomass in this region appears to be in a dynamic balance between utilization of a relatively rich food supply and advection out of the area. Physical changes caused by El Nifio interrupt this balance.

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.