Diatom growth and productivity in an oligo-trophic midocean gyre: A 3-yr record from the Sargasso Sea near Bermuda

Abstract

We measured the rate of biogenic silica production in the upper 160 m at the Bermuda Atlantic Time-series Study (BATS) site in the western Sargasso Sea on 12 cruises between August 1991 and May 1994 using a 30Si tracer method. Previously initiated time series of particulate silica concentrations and the gravitational flux of siliceous particles were extended to produce an October 1988-December 1994 time series for biogenic silica concentrations and August 1991-December 1994 time series for lithogenic silica concentrations and fluxes of biogenic and lithogenie silica. Specific rates of biogenic silica production during nonbloom periods indicate active diatom growth, with a mean doubling time of 4.5 d for biogenic silica. The annual silica production rate is conservatively estimated to be 240 mmol Si m-2, which corresponds to a diatom productivity of -20-45 g C m-2 yr-I, -15-25% of the mean annual primary productivity. Comparison of silica production rates with sinking fluxes from the upper 150 m (26-53 mmol Si m-2 yr-I) indicates that -80% of the silica produced in the euphotic zone dissolves in the upper 150 m. Sinking of diatoms and their remains can account for -30% of the particulate organic carbon export at the BATS site on an annual basis and nearly 100% during the annual winter/spring diatom bloom. Atmospheric dust inputs each summer, indicated by July-August maxima in the concentration and vertical flux of lithogenic silica, do not stimulate either biogenic silica production or primary productivity, implying that iron availability limits neither diatom growth nor total primary production in the western' Sargasso Sea.Most of the primary production in the oceans occurs in midocean gyres and other oligotrophic systems-a seemingly contradictory situation that results from those systems' enormous spatial extent (e.g. Ryther 1969). The phytoplankton in the midocean gyres is comprised mostly of small, nonsiliceous forms, with <l p,rn prokaryotes (e.g. Synechococcus and Prochlorococcus) numerically dominant and comprising a significant fraction of the autotrophic biomass (e.g. Chisholm 1992). Although diatoms are found in oligotrophic oceans at all times of year they are seldom a major component of the phytoplankton assemblage except during spring blooms and other brief episodes of high productivity (e.g. Hulburt 1990). Additionally, diatom opal is generally absent from, or at most a trace component of, modern openocean sediments at low latitudes (e.g. Broecker and Peng 1982). Such observations have led to a general view that diatoms are of minor importance in the pelagic ecosystem of the central oceans. AcknowledgmentsWe thank Julie Arrington, Kristie Klose, Aude Leynaert, Linda O'Bryan, and Becky Pregger for assisting us, both at sea and with the shore-based analyses. Ann Close, Alice Doyle, and Kjell Gundersen at the Bermuda Biological Station for Research (BBSR) collected samples of suspended and sinking particulate matter for us on the BATS cruises in which we did not participate directly, and Tony ...

Biogeochemical responses to late-winter storms in the Sargasso Sea, III—Estimates of export production using 234Th:238U disequilibria and sediment traps

“…Depth integrated (0 -140 m) bSiO 2 production rates averaged 1.03 ± 0.96 (n = 17) and 2.28 ± 2.06 (n = 9) mmol Si m -2 d -1 for the West and East Transects, respectively, a substantial increase over previous Sargasso Sea measurements (Brzezinski and Kosman, 1996;Brzezinski and Nelson, 1995;Nelson and Brzezinski, 1997). For example, Nelson and Brzezinski (1997) in a three year study (1992)(1993)(1994) at BATS. During our cruise, integrated bSiO 2 stocks were also enhanced by a factor of 2.5 in the East relative to the West Transects coincident with a temporal increase in vertically integrated bSiO 2 (2.5 mmol m -2 on YD68 to ~4.5 mmol m -2 on YD70) and NO 3 -concentrations (from ~18 mmol m -2 to ~55 mmol m -2 ) (Krause et al, 2008;Lomas et al, 2008a).…”

“…Krause et al (2008) measured bSiO 2 silica production rates during this study using the 32 Si tracer and incubation method described by Brzezinski and Phillips (1997) and Brzezinski et al (2001). Depth integrated (0 -140 m) bSiO 2 production rates averaged 1.03 ± 0.96 (n = 17) and 2.28 ± 2.06 (n = 9) mmol Si m -2 d -1 for the West and East Transects, respectively, a substantial increase over previous Sargasso Sea measurements (Brzezinski and Kosman, 1996;Brzezinski and Nelson, 1995;Nelson and Brzezinski, 1997). For example, Nelson and Brzezinski (1997) in a three year study (1992)(1993)(1994) at BATS.…”

“…Average bSiO (Nelson and Brzezinski, 1997). Krause et al (2008) measured bSiO 2 silica production rates during this study using the 32 Si tracer and incubation method described by Brzezinski and Phillips (1997) and Brzezinski et al (2001).…”

“…Although it is generally accepted that these short-term weather induced mixing events may contribute to changes in phytoplankton biomass (e.g. Cullen et al, 2002;Dugdale, 1998;Nelson and Brzezinski, 1997;Smayda, 1998), their specific impact on community structure and particulate organic carbon (POC) export to depth remain enigmatic. The transient nature of such storm events simply make them difficult to predict and sample.…”

Phytoplankton distribution patterns in the northwestern Sargasso Sea revealed by small subunit rRNA genes from plastids

“…These and other systemic biological responses to nutrient variations are well described (see Michaels and Knap, 1996;Steinberg et al, 2001 and citations within). Patterns of primary production (Menzel and Ryther, 1960;Lohrenz et al, 1992;Michaels and Knap, 1996;Steinberg et al, 2001) and the temporal variability of specific phytoplankton groups have been studied extensively at BATS with a variety of approaches, including radiotracers (Brzezinski and Nelson, 1995;Nelson and Brzezinski, 1997), photosynthetic pigment proxies (Bidigare et al, 1990;Goericke, 1998;Steinberg et al, 2001;Lomas and Bates, 2004), flow cytometric measurements of phytoplankton abundance (Olson et al, 1990;Li et al, 1992;DuRand et al, 2001) and growth (Li et al, 1992;Worden and Binder, 2003a, b;Cuvelier et al, 2010). Most notably, a recent study at BATS showed a 450% increase in chlorophyll a (Chl a) data between 1996 and 2007 (Lomas et al, 2010).…”