109
Primary Productivity
The measurement of phytoplankton primary production using on-deck simulation with
14C methodology has been subjected to considerable criticism in recent years (Gieskes
et ~. 1979· Eppley 1980). We emphasize ,therefore, that our results must be viewed
with reservation until they can be substantiated by other techniques.
In our study the measured areal photosynthetic rates ranged from 1.62 to 10.2
mgC.m- 2 .h- 1 (Fig. 6). Higher volumetric values (mgC.m- 3 .h- 1 ) were observed in the
richer nearshore waters but here total areal productivity was limited by depths which
were frequently less than 1% light attenuation depth. Assimilation numbers averaged
2.2 (s.d. ~ 2.1) mgC.mgChl- 1 .h- 1 .
Although accurate conversions from hourly to daily rates are not possible, rough
estimations based on effective sunlight hours give average daily values of primary
productivity of about 40 to 50 mgC.m- 2 .d- 1
These would imply that the annual net
photosynthetic carbon fixation in these waters might be somewhere between 10-20 gC.
m- 2 .y-l
Differential filtration indicated that, as with chlorophyll, the majority
of the photosynthetic activity was associated with organisms smaller than J ~m (Fig. 3).
However, the assimilation numbers of the picoplankton (~3 pm) were not generally higher
than those of the larger phytoplankton,in contrast to other observations (li et al.
1983) .
Conclusions
Despite their limited scope, our results show a consistent picture. The pelagic
waters of the Mediterranean Basin are highly oligotrophic, having extremely low
phytoplankton standing crops and apparent low rates of primary production as measured
by standard 14C methodology. Their transparency and optical characteristics reflect
the low algal populations. A deep chlorophyll maximum, usually at about 1% of incident light level, is generally observed. The nanoplankton (<:20 pm); and more especially, the picoplankton (<: 3 fJIil), constitute the majority of the chlorophyll biomass
and are responsible for most of the photosynthetic activity. The nearshore water
mass roughly overlying the narrow (10-20 kms) Israeli coastal shelf, has much higher
levels of chlorophyll standing crops and primary productivity and is often clearly
delineated from the pelagic water by a surface front of temperature and chlorophyll.
In the first part of this paper we briefly reviewed some recent observations and
discoveries which ,when more fully investigated,will undoubtedly greatly influence
our understanding of oceanic ecosystems. Our data indicate that the pelagic waters
of the Eastern Mediterranean Basin has many features characteristically associated
with nutrient poor oceans. This area, because of its relatively small size and the
proximity of shore based research facilities, may prove particularly convenient for
Primary Productivity
The measurement of phytoplankton primary production using on-deck simulation with
14C methodology has been subjected to considerable criticism in recent years (Gieskes
et ~. 1979· Eppley 1980). We emphasize ,therefore, that our results must be viewed
with reservation until they can be substantiated by other techniques.
In our study the measured areal photosynthetic rates ranged from 1.62 to 10.2
mgC.m- 2 .h- 1 (Fig. 6). Higher volumetric values (mgC.m- 3 .h- 1 ) were observed in the
richer nearshore waters but here total areal productivity was limited by depths which
were frequently less than 1% light attenuation depth. Assimilation numbers averaged
2.2 (s.d. ~ 2.1) mgC.mgChl- 1 .h- 1 .
Although accurate conversions from hourly to daily rates are not possible, rough
estimations based on effective sunlight hours give average daily values of primary
productivity of about 40 to 50 mgC.m- 2 .d- 1
These would imply that the annual net
photosynthetic carbon fixation in these waters might be somewhere between 10-20 gC.
m- 2 .y-l
Differential filtration indicated that, as with chlorophyll, the majority
of the photosynthetic activity was associated with organisms smaller than J ~m (Fig. 3).
However, the assimilation numbers of the picoplankton (~3 pm) were not generally higher
than those of the larger phytoplankton,in contrast to other observations (li et al.
1983) .
Conclusions
Despite their limited scope, our results show a consistent picture. The pelagic
waters of the Mediterranean Basin are highly oligotrophic, having extremely low
phytoplankton standing crops and apparent low rates of primary production as measured
by standard 14C methodology. Their transparency and optical characteristics reflect
the low algal populations. A deep chlorophyll maximum, usually at about 1% of incident light level, is generally observed. The nanoplankton (<:20 pm); and more especially, the picoplankton (<: 3 fJIil), constitute the majority of the chlorophyll biomass
and are responsible for most of the photosynthetic activity. The nearshore water
mass roughly overlying the narrow (10-20 kms) Israeli coastal shelf, has much higher
levels of chlorophyll standing crops and primary productivity and is often clearly
delineated from the pelagic water by a surface front of temperature and chlorophyll.
In the first part of this paper we briefly reviewed some recent observations and
discoveries which ,when more fully investigated,will undoubtedly greatly influence
our understanding of oceanic ecosystems. Our data indicate that the pelagic waters
of the Eastern Mediterranean Basin has many features characteristically associated
with nutrient poor oceans. This area, because of its relatively small size and the
proximity of shore based research facilities, may prove particularly convenient for