172
Chapter 9: The Atlantic Ocean
made at OWS S, for the individual data vary from 23 kg m
−2 wet weight down to a
few milligrams per square meter. Associated with the floating gulf weed are epiphytic,
phycoerythrin-rich cyanobacteria of which at least some are capable of N fixation. The
mean C turnover time of Sargassum biomass is about 40 days but the relative productivity
of both gulf-weed and its associated epiphytes contributes no more than 0.5% to total
regional primary production in the western Sargasso Sea (Carpenter and Cox, 1974).
A specialized fauna of fish and epibionts is, of course, associated with Sargassum
and this varies regionally: for instance, stronger encrustation of epibionts (hydroids,
calcareous algae, spirorbid polychaetes, etc.) occurs in cooler water. Young-of-the-year
amberjack (Seriola dumerlii) are associated with offshore Sargassum, and these individuals
have faster growth rates than those that remain close to their coastal spawning region.
Seasonal mesozooplankton biomass at BATS in the period 1994–98 was maximal in
late spring and (in some years) again in autumn and early winter while, between years,
biomass is positively correlated with autotrophic biomass and productivity (Madin et al.,
2001). The diel migration factor is from 1.7 to 3.4 and, as is usually the case, migration
is more prominent in the larger size fractions: mean biomass values (0–200 m) for this
period were 418 and 659 mg dw m
−2 for day and night respectively: diel migration, then,
penetrates rather deep in these clear waters. Migrants are dominated by copepods (Pleuromamma, Euchirella), euphausiids (Thysanopoda, Nematobrachion), amphipods (Scina,
Anchylomera), and shrimps (Sergestes, Sergia).
The mesozooplankton from BATS have not yet, I think, been formally published
taxonomically, but previous studies in the NAST province indicate the existence of a
typical warm-water assemblage dominated numerically by small epiplanktonic copepods:
Clausocalanus, Oithona, Farranula, Ctenocalanus, and Lucicutia. In the northern Sargasso
Sea, Grice and Hart (1962) found 92 epiplanktonic copepod species, 9 chaetognaths,
20 euphausiids, 23 siphonophores, and so on—a typical warm-water zooplankton. Smaller
species dominate the summer zooplankton, more efficient at using the small autotrophic
cells and protists that then abound.
It is in this province that we first meet the large deep-water tunas in any abundance.
Fontenau (1997) shows that there is a significant east-west difference in tuna habitat.
In NAST-W, albacore (Thunnus alalunga) is present year-round, but maintains a larger
population during the cooler months, whereas swordfish (Xiphias gladiator), bluefin
(Thunnus thynnus) and bigeye (Th. obesus) together formed only 10–30% of the longline
catches in the period 1984–1993. This is a rather unselective gear and we may assume the
catches indicate rather well the actual species association of large predatory fish present.
But, in NAST-E, albacore are a minority component and the most abundant species
are bluefin and swordfish. Adult albacore, of course, occur preferentially around the
subtropical sectors of the principal oceanic gyral systems and are ecologically distinct in
their habitat requirements from the cooler-water adult bluefin. I shall reserve discussion
of their ecology until we get to the discussion of the Pacific subtropical provinces.
Synopsis
Case 3—Winter-spring production with nutrient limitation. Z m undergoes a moderate
boreal winter excursion, slightly deeper in the west, while Zeu remains at 45–55 m, so
that the thermocline is illuminated for 6–7 months. An increase in P is initiated in
December, continuing until the maximum P rate is reached in April–May (Fig. 9.10) when
thermal stratification is fully reestablished at 25–30 m. Dynamic range of chlorophyll
biomass is small, reaching an annual maximum in March–April with higher values in
the east (<40 mg m
−3 ) than in the west (<25 mg m
−3 ), after which the standing stock is
rapidly depleted. The chlorophyll cycle is therefore consistent with a close match between
consumption and production, and with the seasonality in zooplankton biomass.
Chapter 9: The Atlantic Ocean
made at OWS S, for the individual data vary from 23 kg m
−2 wet weight down to a
few milligrams per square meter. Associated with the floating gulf weed are epiphytic,
phycoerythrin-rich cyanobacteria of which at least some are capable of N fixation. The
mean C turnover time of Sargassum biomass is about 40 days but the relative productivity
of both gulf-weed and its associated epiphytes contributes no more than 0.5% to total
regional primary production in the western Sargasso Sea (Carpenter and Cox, 1974).
A specialized fauna of fish and epibionts is, of course, associated with Sargassum
and this varies regionally: for instance, stronger encrustation of epibionts (hydroids,
calcareous algae, spirorbid polychaetes, etc.) occurs in cooler water. Young-of-the-year
amberjack (Seriola dumerlii) are associated with offshore Sargassum, and these individuals
have faster growth rates than those that remain close to their coastal spawning region.
Seasonal mesozooplankton biomass at BATS in the period 1994–98 was maximal in
late spring and (in some years) again in autumn and early winter while, between years,
biomass is positively correlated with autotrophic biomass and productivity (Madin et al.,
2001). The diel migration factor is from 1.7 to 3.4 and, as is usually the case, migration
is more prominent in the larger size fractions: mean biomass values (0–200 m) for this
period were 418 and 659 mg dw m
−2 for day and night respectively: diel migration, then,
penetrates rather deep in these clear waters. Migrants are dominated by copepods (Pleuromamma, Euchirella), euphausiids (Thysanopoda, Nematobrachion), amphipods (Scina,
Anchylomera), and shrimps (Sergestes, Sergia).
The mesozooplankton from BATS have not yet, I think, been formally published
taxonomically, but previous studies in the NAST province indicate the existence of a
typical warm-water assemblage dominated numerically by small epiplanktonic copepods:
Clausocalanus, Oithona, Farranula, Ctenocalanus, and Lucicutia. In the northern Sargasso
Sea, Grice and Hart (1962) found 92 epiplanktonic copepod species, 9 chaetognaths,
20 euphausiids, 23 siphonophores, and so on—a typical warm-water zooplankton. Smaller
species dominate the summer zooplankton, more efficient at using the small autotrophic
cells and protists that then abound.
It is in this province that we first meet the large deep-water tunas in any abundance.
Fontenau (1997) shows that there is a significant east-west difference in tuna habitat.
In NAST-W, albacore (Thunnus alalunga) is present year-round, but maintains a larger
population during the cooler months, whereas swordfish (Xiphias gladiator), bluefin
(Thunnus thynnus) and bigeye (Th. obesus) together formed only 10–30% of the longline
catches in the period 1984–1993. This is a rather unselective gear and we may assume the
catches indicate rather well the actual species association of large predatory fish present.
But, in NAST-E, albacore are a minority component and the most abundant species
are bluefin and swordfish. Adult albacore, of course, occur preferentially around the
subtropical sectors of the principal oceanic gyral systems and are ecologically distinct in
their habitat requirements from the cooler-water adult bluefin. I shall reserve discussion
of their ecology until we get to the discussion of the Pacific subtropical provinces.
Synopsis
Case 3—Winter-spring production with nutrient limitation. Z m undergoes a moderate
boreal winter excursion, slightly deeper in the west, while Zeu remains at 45–55 m, so
that the thermocline is illuminated for 6–7 months. An increase in P is initiated in
December, continuing until the maximum P rate is reached in April–May (Fig. 9.10) when
thermal stratification is fully reestablished at 25–30 m. Dynamic range of chlorophyll
biomass is small, reaching an annual maximum in March–April with higher values in
the east (<40 mg m
−3 ) than in the west (<25 mg m
−3 ), after which the standing stock is
rapidly depleted. The chlorophyll cycle is therefore consistent with a close match between
consumption and production, and with the seasonality in zooplankton biomass.