Atlantic Coastal Biome
221
space: the Atlantic-boreal C. finmarchicus is restricted to the lower layer, the temperate C.
helgolandicus is restricted to the upper layer, and the euphausiids (Nyctiphanes couchi and
Meganyctiphanes norvegica) make diel migrations between the two layers. In the western
English Channel two chaetognaths, like the two species of Calanus, partition the water
column: Sagitta elegans occupies the cold, lower layer and S. setosa the upper layer. None
of these findings are at variance with observations of vertical profiles in summer in the
central North Sea or in the FLEX box in the northern North Sea, where C. finmarchicus
is the dominant calanoid at all depths.
The OMEX I investigations of the shelf-break ecosystem west of Brittany (e.g., Joint
et al., 2001a,b) revealed daily productivity of ∼150 gC m
−2 , with an f-ratio of <025 during summer months, but higher during the spring bloom. During winter, heterotrophic
demand (bacteria to mesozooplankton) was significantly lower than autotrophic production, although during summer the heterotrophic demand exceeded production of
plant cells. The annual budget suggests that 40–60% of total autotrophic production
is not utilized by the heterotrophs and is available for export to midwater organisms
and the benthos. These direct measurements during OMEX I were compared by Joint
et al. (op. cit.) with computations that they made from SeaWiFS data: the two methods
produced very similar results. Joint and Williams had already estimated in 1985 that the
demands of herbivores in the Celtic Sea could only be met on the assumption that they
can take particles in the 1- to 5-m range; microplankton appear to consume 10–40%
of the standing stock of autotrophs daily.
The pelagic ecosystem of the Baltic is spatially complex (Segerstrale, 1957). In this
small sea, there is a gradient from estuarine organisms in the west (Kattegat: salinity,
10–15‰) to lacustrine organisms in the east (Gulf of Bothnia: 2–3‰). The glacial relic
copepod Limnocalanus grimaldii occurs in >97% of plankton tows in the Gulf of Bothnia
and in <10% in the Belt Sea east of Denmark where the salinity gradient is sharpest. The
marine medusa Aurelia aurita is abundant in the western Baltic but occurs rarely and does
not reproduce in the Gulf of Finland. Another glacial relic mysid (Mysis relicta) occurs in
deep water. Brackish water copepods (Eurytemora hirundoides and Acartia bifilosa) and
cladocera (Bosmina maritima) are an important component of the fauna, with the last
organism at times being the most abundant planktonic crustacean in summer and an
important food for herring. Other cladocera (Podon and Evadne) and rotifers are especially
important in the gulfs in summer. Large cells of the phytoplankton are dominated by
diatoms and Cyanophycae, especially Nodularia, Anabaena, and Aphanizomena.
In the Landsort Deep, in the main basin of the Baltic Sea, the mesozooplankton
exhibit some resource partitioning in a water column having a pycnocline at 70–80 m,
associated with a nutricline and (presumably) a DCM (Ackefors, 1966). Small abundant copepods are specialized to depth horizons: Acartia spp., 15 m; Temora longicornis,
20–25 m; P. elongatus, 50–100 m.
Fisheries science in this region has been much concerned with the consequences of
between-year changes in the physical environment for the recruitment to fish populations,
mediated through the variability of the pelagic ecosystem that is physically forced. This
concern goes back to Johann Hjort’s hypothesis, expressed in 1914, of the probability
of massive mortality of larval fish in the event that their food was—in some way—
insufficient or unavailable. I mention these problems here only because it was in the
NECS province that the issue of the determination of year-class strength first attracted
serious attention. It is, of course, a global problem at the roots of the current fishery
crisis.
It is increasingly evident that uncertainty in the survival of larval fish populations, and
hence in subsequent recruitment to the adult stock, is closely tied to uncertainty in the
seasonality and strength of the plankton cycle. This, in turn, is a consequence of variability
221
space: the Atlantic-boreal C. finmarchicus is restricted to the lower layer, the temperate C.
helgolandicus is restricted to the upper layer, and the euphausiids (Nyctiphanes couchi and
Meganyctiphanes norvegica) make diel migrations between the two layers. In the western
English Channel two chaetognaths, like the two species of Calanus, partition the water
column: Sagitta elegans occupies the cold, lower layer and S. setosa the upper layer. None
of these findings are at variance with observations of vertical profiles in summer in the
central North Sea or in the FLEX box in the northern North Sea, where C. finmarchicus
is the dominant calanoid at all depths.
The OMEX I investigations of the shelf-break ecosystem west of Brittany (e.g., Joint
et al., 2001a,b) revealed daily productivity of ∼150 gC m
−2 , with an f-ratio of <025 during summer months, but higher during the spring bloom. During winter, heterotrophic
demand (bacteria to mesozooplankton) was significantly lower than autotrophic production, although during summer the heterotrophic demand exceeded production of
plant cells. The annual budget suggests that 40–60% of total autotrophic production
is not utilized by the heterotrophs and is available for export to midwater organisms
and the benthos. These direct measurements during OMEX I were compared by Joint
et al. (op. cit.) with computations that they made from SeaWiFS data: the two methods
produced very similar results. Joint and Williams had already estimated in 1985 that the
demands of herbivores in the Celtic Sea could only be met on the assumption that they
can take particles in the 1- to 5-m range; microplankton appear to consume 10–40%
of the standing stock of autotrophs daily.
The pelagic ecosystem of the Baltic is spatially complex (Segerstrale, 1957). In this
small sea, there is a gradient from estuarine organisms in the west (Kattegat: salinity,
10–15‰) to lacustrine organisms in the east (Gulf of Bothnia: 2–3‰). The glacial relic
copepod Limnocalanus grimaldii occurs in >97% of plankton tows in the Gulf of Bothnia
and in <10% in the Belt Sea east of Denmark where the salinity gradient is sharpest. The
marine medusa Aurelia aurita is abundant in the western Baltic but occurs rarely and does
not reproduce in the Gulf of Finland. Another glacial relic mysid (Mysis relicta) occurs in
deep water. Brackish water copepods (Eurytemora hirundoides and Acartia bifilosa) and
cladocera (Bosmina maritima) are an important component of the fauna, with the last
organism at times being the most abundant planktonic crustacean in summer and an
important food for herring. Other cladocera (Podon and Evadne) and rotifers are especially
important in the gulfs in summer. Large cells of the phytoplankton are dominated by
diatoms and Cyanophycae, especially Nodularia, Anabaena, and Aphanizomena.
In the Landsort Deep, in the main basin of the Baltic Sea, the mesozooplankton
exhibit some resource partitioning in a water column having a pycnocline at 70–80 m,
associated with a nutricline and (presumably) a DCM (Ackefors, 1966). Small abundant copepods are specialized to depth horizons: Acartia spp., 15 m; Temora longicornis,
20–25 m; P. elongatus, 50–100 m.
Fisheries science in this region has been much concerned with the consequences of
between-year changes in the physical environment for the recruitment to fish populations,
mediated through the variability of the pelagic ecosystem that is physically forced. This
concern goes back to Johann Hjort’s hypothesis, expressed in 1914, of the probability
of massive mortality of larval fish in the event that their food was—in some way—
insufficient or unavailable. I mention these problems here only because it was in the
NECS province that the issue of the determination of year-class strength first attracted
serious attention. It is, of course, a global problem at the roots of the current fishery
crisis.
It is increasingly evident that uncertainty in the survival of larval fish populations, and
hence in subsequent recruitment to the adult stock, is closely tied to uncertainty in the
seasonality and strength of the plankton cycle. This, in turn, is a consequence of variability