Ejh. Head et al., PLANKTON DYNAMICS AND CARBON FLUX IN AN AREA OF UPWELLING OFF THE COAST OF MOROCCO, Deep-sea research. Part 1. Oceanographic research papers, 43(11-12), 1996, pp. 1713
A carbon flux study was carried out off the coast of Morocco, at 31 de
grees N, in a region characterized by the presence of a persistent cyc
lonic eddy. Two short-term (4 and 3 day) deployments of free-floating
sediment traps were combined with water column sampling and rate proce
ss measurements as the ship followed the traps. For a period of 36 h b
etween trap deployments, a hydrographic section was run along 31 degre
es 30'N as part of a larger scale survey being carried out simultaneou
sly on the R.V. A. von Humboldt. The first trap deployment was near th
e eastern margin of the eddy and the traps moved to the north and west
in a frontal jet associated with its northern boundary. After the sec
ond deployment, which was at the recovery point of the first, the trap
s moved to the west and then to the southwest. Throughout the study, c
hlorophyll concentrations varied between 27 and 125 mg m(-2) (0-100 m)
, with highest concentrations in the upwelled water nearest the coast
and in upwelled water generated within the cyclonic eddy. Particulate
organic carbon (POC) and particulate organic nitrogen (PON) concentrat
ions were relatively uniform (13.6+/-1.8 and 1.63+/-0.28 g m(-2)), wit
h phytoplankton carbon accounting for 16-85% of total POC. Bacterial c
arbon was similar to 5% of total POC and mesozooplankton carbon concen
trations were equivalent to similar to 9% of total POC. Microzooplankt
on biomass was not assessed but POC:PON ratios in the water column wer
e often high, suggesting there was sometimes a large detrital componen
t in the POC. Primary production rates varied between 1.0 and 2.5 g C
m(-2) day(-1). Bacterial consumption accounted for similar to 50% of p
rimary production. Metabolic rates suggested that copepods were ingest
ing more than 0.4 g C m(-2) day(-1) while filtration rates suggested t
hat ingestion of phytoplankton carbon was only similar to 0.2 g C m(-2
) day(-1), even when phytoplankton constituted similar to 85% of the P
OC.f-ratios (based on uptake rates for N-15-nitrate and ammonia) were
between 0.1 and 0.4, and excretion by mesozooplankton could account fo
r similar to 40% of the dairy ammonium uptake by phytoplankton. HPLC p
igment analysis showed that when chlorophyll biomass was high, diatoms
were dominant, whereas when it was low, small prymnesiophytes, chloro
phytes and diatoms were all important. The composition of the fluorese
cent pigments in material in the sediment traps indicated that intact
phytoplankton and copepod faecal pellets were the main sources but the
relative rates of sedimentation of pigment, POC and PON for the two t
rapping periods did not reflect differences that were observed in the
overlying water column. This was likely to be the result of spatial he
terogeneity and strong horizontal currents within the euphotic zone. T
hus, material collected at 100 m probably did not originate in the wat
er column immediately overlying the traps and trapping efficiencies mi
ght also have been variable. Copyright (C) 1996 Elsevier Science Ltd