Merritt Tuel

Seasonal patterns of ocean biogeochemistry at the U.S. JGOFS Bermuda Atlantic time-series study site

Seasonal patterns in hydrography, oxygen, nutrients, particulate carbon and nitrogen and pigments were measured on monthly cruises at the Bermuda Atlantic Time-series Study site, 80 km southeast of Bermuda. Between October 1988 and September 1990, the annual cycle was defined by the creation of 160–230 m-deep mixed layers in February of each year and a transition to strong thermal stratification in summer and fall. The 230 m mixed layer in February 1989 resulted in mixed-layer nitrate concentrations of 0.5–1.0 μmole kg−1, carbon fixation rates over 800 mg C m−2 day−1, and a phytoplankton bloom with chlorophyll concentrations over 0.4 mg m−3. Chlorophyll a, particulate organic matter, inorganic nutrients and primary production had returned to prebloom levels the following month with the exception of a chlorophyll maximum layer at 100 m. Particle fluxes at 150 m in February 1989 reached 56 mg C m−2 day−1 and 11 mg N m−2 day−1 (0.77 mmole N m−2 day−1). Estimates of new production during the bloom period calculated from changes in oxygen and nitrate profiles ranged from 100 to 240 mmoles N m−2, significantly higher than the sediment trap fluxes and approaching the measured total production rates. In spring of 1990, mixed layer depths did not exceed 160 m, nitrate was rarely detectable in the upper euphotic zone, chlorophyll a concentrations were similar to 1989, and particulate organic matter concentrations were lower. The period of elevated biomass lasted for 3 months in 1990, and phytoplankton pigment composition varied between cruises. The average rates of primary production and particle flux were higher in 1990 than those measured in the spring of 1989, despite the differences in mixed layer depth. Throughout both years, NO3 : PO4 ratios in the upper thermocline exceeded Redfield ratios. The maintenance of this pattern requires a net uptake of PO4 between 150 and 250 m, a depth range usually associated with net remineralization. The exact mechanism that maintains elevated PO4 uptake and its implication for the nutrient supply to the euphotic zone remain unknown.

Seasonal variability in primary production and particle flux in the northwestern Sargasso Sea: U.S. JGOFS Bermuda Atlantic time-series study

The relationship between primary production and sediment trap-derived downward flux of particulate organic matter was characterized over a 2 year period at the U.S. JGOFS Bermuda Atlantic Time-series Study (BATS) site to evaluate the importance of temporal variations in upper ocean biogeochemical processes. Water column-integrated primary production (∫PP), determined once each cruise using 14C incubations (in situ dawn-to-dusk), peaked in late winter/early spring of both 1989 and 1990. Smaller increases in ∫PP also occured in July 1989 and October–December 1990. Annual ∫PP was 9.2 mol C m−2 y−1 in 1989 and 12 mol C m−2 y−1 in 1990. This was higher than the 1959–1963 annual average (6.8 mol C m−2 y−1) determined at Station “S” located approximately 50 km northwest of the BATS site. Fluxes associated with sinking of total particulate mass, particulate organic carbon (POC) and particulate organic nitrogen (PON) were measured at 150, 200, 300 and 400 m using a free-floating sediment trap array generally deployed once each cruise for 72 h. Fluxes varied seasonally, and within our ability to resolve differences (i.e. monthly sampling), there was no distinguishable time offset between peaks in ∫PP and corresponding peaks in elemental flux. Fluxes generally decreased with increasing depth, and fluxes of POC and PON were positively correlated with particulate mass flux at all depths. POC/PON (C/N) ratios at 150 m during periods of high ∫PP were generally characteristics of live planktonic biomass. Higher C/N ratios in material collected by the deeper traps were consistent with more rapid losses of PON than POC from sinking particles. POC and PON fluxes at 150 m, nominally the base of the euphotic zone, were positively correlated with ∫PP. The fraction of ∫PP leaving the euphotic zone in the form of sinking particles (i.e. collected in traps) varied seasonally and was inversely proportionato ∫PP. Surface export of organic matter estimated by sediment traps at 150m was 0.78 mol C m−2y−1 (0.10 mol N m−2y−1) in 1989 and 0.77 mol C m−2y−1 (0.11 mol N m−2y−1) in 1990.

The role of sinking fecal pellets in stratified euphotic zones

Abstract The euphotic zone at the VERTEX II oligotrophic sediment trap station (18°N, 108°W) can be viewed for simplicity a as two-layered system with (1) an upper mixed layer (0–40 m) of relatively high primary production, low chlorophyll concentration, and low zooplankton biomass, and (2) a lower layer (40–100 m) containing the chlorophyll and particulate organic carbon maxima, higher zooplankton biomass, and relatively low primary production. Particle traps at 30 m yielded no particulate carbon flux, while traps at 120 m registered 38 mg C m−2 d−1, with many visible fecal pellets. Zooplankton in the upper layer apparently produced small pellets which were recycled in situ. Fecal pellet carbon production by 200–2000 μm zooplankton in the lower layer, however, was 30% of trap-measured C flux. Tiny pellets produced by 53–200 μm zooplankton were mostly recycled within the euphotic zone, regardless of which euphotic layer the animals were inhabiting when collected. Comparisons are made between the relative contributions of sinking fecal pellets when the VERTEX II euphotic zone is considered as a single uniform layer rather than as a two-layered system. In addition, comparison of the VERTEX II data with data from the eutrophi VERTEX Vc site (35°50′N, 122°30′W) suggests that zooplankton can be more significantly coupled to carbon flux out of the bottom layers of oligotrophic euphotic zones than to carbon flux out of unlayered eutrophic euphotic zones.

Satellite Assessment of Bio-Optical Properties of Northern Gulf of Mexico Coastal Waters Following Hurricanes Katrina and Rita

The impacts of major tropical storms events on coastal waters include sediment resuspension, intense water column mixing, and increased delivery of terrestrial materials into coastal waters. We examined satellite imagery acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color sensor aboard the Aqua spacecraft following two major hurricane events: Hurricane Katrina, which made landfall on 29 August 2005, and Hurricane Rita, which made landfall on 24 September. MODIS Aqua true color imagery revealed high turbidity levels in shelf waters immediately following the storms indicative of intense resuspension. However, imagery following the landfall of Katrina showed relatively rapid return of shelf water mass properties to pre-storm conditions. Indeed, MODIS Aqua-derived estimates of diffuse attenuation at 490 nm (K_490) and chlorophyll (chlor_a) from mid-August prior to the landfall of Hurricane Katrina were comparable to those observed in mid-September following the storm. Regions of elevated K_490 and chlor_a were evident in offshore waters and appeared to be associated with cyclonic circulation (cold-core eddies) identified on the basis of sea surface height anomaly (SSHA). Imagery acquired shortly after Hurricane Rita made landfall showed increased water column turbidity extending over a large area of the shelf off Louisiana and Texas, consistent with intense resuspension and sediment disturbance. An interannual comparison of satellite-derived estimates of K_490 for late September and early October revealed relatively lower levels in 2005, compared to the mean for the prior three years, in the vicinity of the Mississippi River birdfoot delta. In contrast, levels above the previous three year mean were observed off Texas and Louisiana 7-10 d after the passage of Rita. The lower values of K_490 near the delta could be attributed to relatively low river discharge during the preceding months of the 2005 season. The elevated levels off Texas and Louisiana were speculated to be due to the presence of fine grain sediment or dissolved materials that remained in the water column following the storm, and may also have been associated with enhanced phytoplankton biomass stimulated by the intense vertical mixing and offshore delivery of shelf water and associated nutrients. This latter view was supported by observations of high chlor_a in association with regions of cyclonic circulation.