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fully developed immigrant Sargasso biota above and the remains of a refugee slope-water community below.
More generally, mesozooplankton species distributions echo the distribution of water types in this complex physical system. Using recurrent group and cluster analysis, Ashjian and Wishner (1993) examined how 22 categories of 18 species of copepods responded to the offshore downsloping of physical properties—pycnocline, nutricline, and downstream velocity profile—along a section across the Gulf Stream in May 1988. Four principal groups of taxa were identified, of which one (Group 3) comprised large, active diel migrants. Group 1 comprised epiplanktonicNannocalanus minor, Neocalanus gracilis, and Lucicutia ovalis(and their growth stages) that layered in the upper 100 m, in water of about 25C right across the stream. Group 2 (Calanus finmarchicus, Rhincalanus nasutus, and Metridia lucens) lay deep, in water<10C, and always below the downsloping pycnocline.
Group 4 (Calanus tenuicornis stages, and Lucicutia gemina) lay horizontally across the stream below Group 1, at 100–200 m, in water of 20–24C.
A seasonal series of mesoplankton samples at 36N reveals the general annual cycle of vertical migrations in the Gulf Stream system that will modify the foregoing simple pattern (Allison and Wishner, 1986). In winter, biomass in the upper 200 m is low both day and night in the slope water and in the adjacent north wall of the stream relative to biomass that is about one order of magnitude greater in spring and early summer.
In the Gulf Stream itself and the adjacent Sargasso water, there is little seasonality in near-surface biomass. In May, biomass is concentrated at 0–100 m both by day and night, progressively deepening offshore, and little diel migration occurs. By September, diel migration is strongly established from the slope to the Sargasso water, with a subsurface maximum at night at about 80–90 m in all areas, near the DCM that develops during the summer months. Such strong diel migration to 400–600 m in a region with such active horizontal advection must cause very significant horizontal redistribution of biomass.
Synopsis
Case 2—Nutrient-limited spring production peak. Zm undergoes moderate boreal winter excursion while Zeuremains fairly constant at 35–50 m, so that thermocline is illuminated from May to October. Rate increase of P begins in February, before a shoal (thermal) Zm is established in March–April, and reaches a (nutrient-limited?) maximum in April, subsequently declining progressively to an annual minimal rate in November (Fig. 9.8); a shoulder on this decline in August–October appears to be a response to the deepening of the mixed layer as it passes down out of the photic zone. Dynamic chlorophyll biomass range tracks the variations in P so that maximum accumulation occurs in April. Relations of biomass and P suggest that consumption and production are balanced, and that seasonal vertical migration of consumers is insignificant.
North Atlantic Subtropical Gyral Province
Atlantic Westerly Winds Biome 167
0.00 0.30 0.60 0.90 1.20 1.50
0.00 0.20 0.40 0.60 0.80 1.00
SeaWiFS (GFST): September 1997 - January 2002
Surface Chl (mg m -3)
Pt d-1
Chl m-3
1998 1999 2000 2001
Pt (gC m-2 d-1 ) 0 20 40 60 80 100 120
140 0
5 10 15 20 25 30
Climatology (years)
Depth (m) Production at DCM (%)Zm (sigma)
Zeu
Pt (at DCM)
Fig. 9.8 GFST: seasonal cycles of monthly surface chlorophyll and depth-integrated autotrophic production for the years 1997–2002 from SeaWiFS data together with characteristic seasonal cycles of mixed-layer depths from Levitus climatological data and photic depths computed from characteristic irradiance and the archive of chlorophyll profiles discussed in Chapter 1.
significant winter mixing, for which a useful marker would be the end-of-winter surface isotherm for 20C.
Defining Characteristics of Regional Oceanography
Errant cold-core eddies originating in Gulf Stream meanders may propagate into the NAST province beyond the average eddy field, which is considered to be part of GFST (see earlier discussion). Isolated seamounts support Taylor columns that may also spawn cyclonic, warm-core eddies that are observable in the SLA field. Such eddies occur, for instance, downstream from the Corner Rise seamounts (Richardson, 1980) and also from the Canaries, where we have good information on their biological effects (Aristeguiet al., 1997); here, eddies of both signs are generated downstream of the islands at intervals of several days to a few weeks at all seasons, often having elliptical or irregular form, suggesting that they are not yet in geostrophic balance.
This province represents that part of the anticyclonic subtropical gyre that lies below the influence of the westerly winds, although these are relatively weak at these latitudes.
Therefore, although winter mixing does occur, it is weaker than further to the north, not only because of lower wind stress there but also, as noted in Chapter 3, because wind stress at the sea surface is preferentially transformed into momentum rather than mixing progressively equatorward.
Winter deepening of the surface mixed layer of this province is initiated by the passage of atmospheric cold fronts across the subtropical ocean in autumn, eroding the seasonal
168 Chapter 9: The Atlantic Ocean
thermocline. The consequent convective mixing deepens Zm to 125–150 m, a process leading to the formation of the 18C subtropical mode water (Worthington, 1986);
in winter, this extends from 30N almost to 40N, then sinks and spreads, effectively separating the seasonal from the permanent thermocline. Toward the center of the Sargasso Sea, in the vicinity of Bermuda, the upper 200 m of the water column may become thermally uniform from January to April, after which stratification is reimposed not only by increasing irradiance and reduced wind stress at the surface, but also by rainfall that induces a shallow, freshened layer that is enriched in atmospheric nitrogen (Michaels et al., 1993). As the summer progresses, the increasing frequency of tropical storms progressively modifies this simple shoal mixed layer; cooling occurs rapidly after early October.
Even though here we are far from the unpredictable higher latitudes, there remains significant between-year variability in seasonal mixing and stratification: maximum winter Zm varied over a 9-year period from >300 m in 1993–94 to only 170 m in several later winters, although in each year the maximum occurred predictably in February. Surface nitrate in most years briefly exceeds 10mol kg−1 at the surface at this time, but this enrichment is not observed in those years when winter mixing is relatively shallow.
Many of the observations on which we depend for understanding the NAST province have originated from the Bermuda Atlantic Time Series (BATS) program, but it must be emphasized that BATS is placed right on the edge of the region of the North Atlantic where winter mixing penetrates sufficiently deeply to induce a weak spring bloom; as Nelson et al. (2004) point out, just south of the BATS site there is a strong meridional gradient in surface properties, beyond which the tropical conditions of the NATR province obtain. Because the BATS site is marginal to the winter-mixing regime of the North Atlantic, we should not be surprised that between-year variability here is strong, as has been observed.
Along the southern limit of the province, the transition between midlatitude wester- lies and trade winds from the east drives convergence of surface water along the STC (Iselin, 1936; Voorhuis, 1969). The thermal fronts comprising the STC thus separate the anticyclonic gyre into a northern subtropical and a southern tropical portion at about 25–30N. To the south of Bermuda, such a transition has long been known to occur, across which subtropical, winter-mixing conditions are replaced by tropical conditions in which surface water in winter never falls below 20C. Schroeder (1965) traced the thermal front of the STC from 70W eastward to 40W, always between 20 and 31N;
at 30W, to the south of the Azores, Gould (1985) mapped the same front carefully.
The STC is found to be rich in eddy activity at all scales; shear instability and random superposition of internal waves produce small-scale structure along the front (Voorhuis and Bruce, 1982; Toole and Schmidt, 1987). Wavelike baroclinic eddies (of scale 800 km, 200 days) induce persistent westward-propagating sea surface temperature anomalies at the STC (Halliwell et al., 1991), whose movement along the front is spaced 3–5 days apart (Leetma and Voorhis, 1978). The instantaneous location of the STC can readily be found in basin-scale TOPEX-Poseidon data.
The topography of the Mid-Atlantic Ridge constrains the main recirculation of west- ern boundary current water within the western basin (e.g., Richardson, 1985), and the biological properties of the eastern and western basins differ significantly. Therefore, for some purposes, it may be useful to consider treating the western and eastern basins as distinct subprovinces: NAST-E and NAST-W. In this case, we should draw the boundary along the topography of the Mid-Atlantic Ridge, running southwest from the Azores.
Response of the Pelagic Ecosystems
Because the geographical Sargasso Sea corresponds with the subprovince NAST-W, there is a very appropriate marker to demonstrate that the line between the eastern and the
Atlantic Westerly Winds Biome 169
western subprovinces is not arbitrary, but biologically significant. Floating aggregations of gulf weed (Sargassumspp.) are abundant to the west but sparse to the east of this line, or approximately over the Mid-Atlantic Ridge. It is thought that this is consequent on the partially closed nature of circulation within the Sargasso Sea. There are also significant morphological and ecological differences betweenSargassumin this province and in the NATR to the south of the STC (Niermann, 1986; Butler et al., 1983, and references therein).
As mentioned earlier, there are>300 individual seamounts (not all associated with the Mid-Atlantic Ridge) of sufficient topography to sustain Taylor columns, so it may be expected that some areas of enhanced surface chlorophyll will be topographically controlled, even in midocean. Saltzman and Wishner (1997a) briefly review the ecological effects that may be anticipated where seamounts populate a deep ocean region. Uplifting of isotherms and upwelling of nutrients, interaction with diel vertical migrant zooplankton and consequent patch formation, and induction of relatively high biomass of pelagic fish are among the more frequently noted effects.
NAST(E) includes a sector of the offshore Canary Current, clear of the field of eddies and filaments generated within the coastal boundary zone. Here, mixed-layer depths are shoaler, and enhanced chlorophyll biomass is indicated by the satellite images, particularly south of the Canaries, though it will be important to separate the effects of eddies from the filaments of high chlorophyll induced by tidal mixing (and other coastal processes) at the islands. Vertical nutrient flux in cyclonic eddies occurs centrally by isopycnal transport, and therefore it is strongest where the pycnocline dome is shoalest and where some diapycnal mixing may occur across the shoaled pycnocline. As in many upwelling situations, chlorophyll enhancement occurs most strongly a little downstream of the surface nutrient maximum and so, in this case, chlorophyll is highest around the edge of the cyclonic eddies. Anticyclonic eddies frequently interact with filaments of water having enhanced chlorophyll, causing the chlorophyll signal to spiral inward toward the center of the eddy, as also occurs in Gulf Stream rings.
The STC front along the southern flank of NAST has been studied to the south of the Azores by Fernandez and Pingree (1996), who show that it supports local enhancement of primary production and biomass accumulation, as is generally the case for oceanic fronts. Winter productivity is in the range 08–09 g C m−2d−1 or about twice the rate for those parts of this province not influenced by frontal or mesoscale eddy processes.
These authors suggest that because of the great spatial extent of the frontal signature in the Azores Current system, primary production within the subtropical front is of major significance for regional carbon budgets. A DCM occurs everywhere across the NAST province at about 50–100 m across the province, following the depth of the nutricline rather than that of the pycnocline in the density gradient. It lies progressively deeper to the south and east across the province. As usual, the chlorophyll maximum is somewhat deeper than the depth of maximum photosynthetic rate and of the biomass maximum for photosynthetic cells. This must follow from the generalization that the C/chl ratio decreases with increasing depth of the DCM. Li (1995) shows that the DCM for cyanobacteria and prochlorophytes can be traced oceanwide across NAST (Fig. 9.9).
It is in the west of the province, in the Sargasso Sea, that the most detailed studies of ecosystem response have been performed, over a long period of years, because of the proximity of the “Oceanographic” at Woods Hole, and the existence of the Bermuda Station for Biological Research at St. George’s. Gordon Riley (1957) obtained data for 2 years at OWS E at 35N 48W, to the northeast of Bermuda, and laid the groundwork for much of what has followed. The most comprehensive description of pelagic ecosystem here derives from the Bermuda Atlantic Time Series Study (BATS) studies at 31N, just south of Bermuda over deep water, at a station that was occupied for several days on each of 111 occasions from 1989 to 1994. This extraordinary data set allows almost weekly
170 Chapter 9: The Atlantic Ocean
Fig. 9.9 This zonal JGOFS section across the NAST province from Nova Scotia to Morocco shows how the depth of maximum abundance of cyanobacteria deepens eastward.
Source: Redrawn from Li, 1995.
precision over this 6-year period (Steinberg et al., 2001): seasonal and between-year signals are unambiguous in the data. Because the BATS station is very close to OWS S, also studied since the 1950s, decadal variability in pelagic production is also recoverable for this region.
The seasonal evolution of the pelagic ecosystem of NAST responds to the seasonal evolution of mixed-layer depth and has been followed with precision in the multidepth profiles of bulk properties obtained at BATS (Steinberg et al., 2001). As noted earlier, nitrate is available in the mixed layer briefly (usually 05–10M, and for <30 days) when mixing is deepest. The NO3/PO4ratio is anomalous because it exceeds the Redfield ratio of 16, both generally in the Sargasso Sea and specifically at the BATS station.
It is thought that balance must be obtained by nitrogen fixation, either by vertically migrating nitrogen-fixingTrichodesmiumcolonies that obtain phosphate at depth, or by vertical transport of deep nitrogen by vertically migratingRhizoseleniamats, perhaps an important source of new nitrogen in other oceans.
A short winter bloom of varying duration occurs between February and April, and the period when chlorophyll is enhanced is usually shorter than the period of enhanced rate of primary production. During this brief seasonal bloom, high chlorophyll values extend from the surface to 150 m depth, this episode being imposed, as it were, on a permanent DCM at about 100 m. McGillicuddy and Robinson (1997) have suggested that the observed rate of new production in the mixed layer here in the Sargasso Sea is consistent with an independently derived net flux of 05 mol N m−2year into the mixed layer. Most of this flux is thought to occur during the formation and intensification of cyclonic eddies in a manner consistent with the observed eddy field; about 30% may result from the interaction between mesoscale flow and wind-driven surface currents. These authors point out that it is important to consider the interaction between the light field in the oligotrophic ocean and vertical motions, of both signs, induced within eddy fields.
“Cold” cyclonic eddies import nutrients up into the euphotic zone, whereas “warm”
anticyclonic eddies carry phytoplankton-rich, nutrient-depleted water down below the lighted layer.
At OWS S, also near Bermuda but less comprehensively monitored than the BATS station, depth of winter mixing varies (as at BATS) from 150 to>250 m and the duration of the bloom (as indicated by enhanced chlorophyll) varies from 40 to 120 days; mixing was especially deep and of long duration in 1991–1993, apparently associated with the ENSO events of those years. During the period 1958–1971, because it was as deep as
Atlantic Westerly Winds Biome 171
350–450 m, mixed-layer nitrates in the range of 2–5M kg−1have been hindcast for that period. HPLC analysis of phytoplankton accessory pigments at BATS enables charac- terization of the taxa that contribute seasonally to blooms and to the permanent deep maximum. It is, of course, the prokaryotic picoplankton that dominate the autotrophic biomass almost year-round, with some seasonal segregation between groups. Prochloro- phytes are seasonally variable, being important from late spring, and through summer, while cyanobacteria are more regular in their biomass, never falling to such low values as prochlorophytes (DuRandet al., 2001).
Coccolith blooms occur in late winter and spring (<100 cells liter−1) at OWS S, with much lower concentrations during summer and with some species succession:Emiliana huxleyi dominates near-surface in spring and Florisphaera profundaat the DCM in late summer (Haidar and Thiersen, 2001). Prymnesiophytes and pelagophytes numerically dominate the eukaryote phytoplankton and dinoflagellates are relatively less important.
Diatom blooms occur, often late in the summer but, even so, their cell numbers may be quite high. It has been suggested that the phytoplankton community, and especially the eukaryotes, are resistant to changes in oceanographic conditions as these change seasonally. Perhaps this is an observation of general relevance in warm open oceans?
The DCM is a site of especially intense trophic activity in this province as it is elsewhere, and it is a preferred depth zone for herbivorous mesozooplankton. Consumption follows a strong diel cycle not only for diel migrant micronekton and copepods but also for those resident in the photic zone. For the diel vertical migration of micronekton and large copepods, consumption is lowest in late afternoon and peaks around midnight. It is in this province that the role of small heterotrophic nanoflagellates in both “nurturing and grazing on planktonic bacteria,” as Sieburth and Davis (1984) put it, was worked out. As elsewhere, the bulk profiles at the BATS show that bacteroplankton dominate (if cyanobacteria are included) the living biomass, and also that the heterotrophic bacterial biomass is maximal in the deep chlorophyll maximum layer, though it is somewhat more seasonal there than the autotrophic cells. Likewise, most of the biomass of heterotrophic organisms at BATS comprises the nanozooplankton (2–5m): acantharia, radiolaria, and foraminifera are, as usual, the most abundant forms. As elsewhere, this was a difficult community to assess at BATS, given the high level of production by autotrophic symbionts within the cells of these organisms.
The seasonal biological cycle at the 1989 JGOFS station (32N 20W) in the NAST- E subprovince reported by Jochem and Zeitzschel (1993) appears to be similar to the weak spring bloom found at the BATS mooring. However, we must be very careful when comparing data sets not obtained simultaneously, because the 1955–1994 data from OWS S show strong decadal and between-year variability, principally forced by the relative length of the winter mixing period and the depth of mixing (Michaels and Knap, 1996); these factors determine the nutrient levels in the mixed layer when stratification is reestablished.
One may be surprised that modern ecological studies, such as those derived from the BATS series of observations, do not appear to consider seriously the macroalgae. Here, the existence of floating Sargassum (Fucacae) should not pass without notice, for it is a unique feature of this province. Several species (S. natans, S. fluitans, S. hystrix, and S. polyceratus) are involved, having differential distributions (e.g., Stoner, 1983), while different morphological forms of at least one species are recognized. The occurrence of masses of gulf weed dense enough to stop sailing ships were probably only the stuff of old sailors’ yarns, and Sargassumnormally occurs in yellow, disc-shaped clumps 10–50 cm in diameter at the surface of the ocean. These may be randomly dispersed (as I have seen them) or arranged in windrows, accumulated by the Langmuir circulation, as much as 50 m wide, and aligned parallel to the direction of the wind. It has not been easy to derive satisfactory estimates of biomass from surface net tows, though many have been