CHARACTERIZATION OF THE PELAGIC ECOSYSTEM IN SURFACE WATERS OF THE NORTHERN GULF OF ST-LAWRENCE IN EARLY SUMMER - THE LARVAL REDFISH CALANUS MICROPLANKTON INTERACTION
Ja. Runge et Y. Delafontaine, CHARACTERIZATION OF THE PELAGIC ECOSYSTEM IN SURFACE WATERS OF THE NORTHERN GULF OF ST-LAWRENCE IN EARLY SUMMER - THE LARVAL REDFISH CALANUS MICROPLANKTON INTERACTION, Fisheries oceanography, 5(1), 1996, pp. 21-37
In June 1989, the water column along a transect in the north-central G
ulf of St Lawrence was thermally stratified (10-14 degrees C at the su
rface; 0-1 degrees C at 30 m). In the surface layer, nitrate and chlor
ophyll concentrations were very low; the mean concentration of chlorop
hyll a > 5 mu m in the subsurface maximum was 0.26 mu g l(-1). Autotro
phic and (presumably) heterotrophic flagellates and dinoflagellates we
re the most abundant microplankton. In this system, redfish (Sebastes
spp.) larvae and the planktonic copepod Calanus finmarchicus overwhelm
ingly dominated the ichthyoplankton and zooplankton, respectively. Red
fish larvae, Calanus females and Calanus eggs were most abundant in th
e surface layer (0-25 m) day and night. Daily specific egg production
rates of Calanus, calculated from shipboard incubations of females, ap
proached the predicted maximal level for this species at the ambient t
emperature of the surface layer, indicating no or little food limitati
on. The redfish larvae were feeding almost exclusively on the Calanus
eggs' and were found in greatest abundance along the transect where Ca
lanus egg production rates (eggs m(-2) day(-1)), calculated from the p
roduct of the specific egg production rate and female concentration, w
ere highest. The monospecificity of the larval redfish diet and the co
-dominance with Calanus Finmarchicus are consistent with the findings
of previous research in regions of the North Atlantic, including the I
rminger Sea and Flemish Cap. Contrary to predictions of the traditiona
l view of the relationship between C. finmarchicus and phytoplankton b
looms, however, Calanus was not dependent on high phytoplankton concen
trations for egg production in this region, at least at this time of y
ear. These observations lay the foundation for characterization of the
surface layer of the north-central Gulf in late lune as a summer, pos
t-bloom environment in which production of autotrophic and heterotroph
ic microplankton supports the Calanus-larval redfish interaction.