162 Productivity and Benthic Organisms
algae; thus, they are limiting nutrients. Algal material, and also animal matter, sinks
below the photic zone and undergoes bacterial decay. Nutrients are released in the
process, that is, remineralization occurs. The nutrients are not being used at depth, in
the absence of light. They become concentrated, therefore.
Thus, the ocean has a deep reservoir of nutrients below the surface waters. The
boundary between this reservoir and the nutrient-poor sunlit waters is the top of the
thennocline, usually between 50 m and 100 m deep.
When the thennocline is eroded by stonns or when the deep water comes to the
surface through the pumping effects of eddies or through upwelling (Sect. 4.3.3), the
nutrients return to the surface water, into the sunlit zone. This fertilization by admixture of deep waters results in increased productivity, both in the algae and in the
animals that feed on them. All the good fishing areas are in such regions of vertical
mixing.
6.1.4 Salinity. Life in the sea depends on other factors besides sunlight and nutrients.
A large number of organisms can tolerate salinity fluctuations only if salinity stays
between 30 and 40 %0 (stenohaline forms). Examples are radiolarians, reef corals,
cephalopods, brachiodops, and echinodenns. In general, their remains indicate
marine conditions for the sediment that contains them. However, there are exception.
A few representatives of the above groups may have relatively wide tolerances: for
instance, there is a starfish that lives in the brackish waters of the Baltic Sea. Also,
fossils embedded in sedimentary layers do not necessarily show the environment of
deposition: they may be redeposited.
With increasing salinity, for example in lagoons of arid regions, fewer and fewer
species are able to survive, until only a few representatives remain - an improverished fauna and flora. Some ostracods (bi-valved crustaceans of millimeter size) can
tolerate salinities of more than 100 %0. As these hardy form have few competitors (or
predators), impoverished faunas can be very rich in individuals. Quite generally, of
course, species-poor but individual-rich faunas indicate special, restricted environments. Restricted does not apply to space here, but to unusual temperature ranges,
oxygen deficiencies, and other stress-producing factors.
6.1.5 Temperature. In the open ocean, where salinities are well within the tolerance
of all marine organisms, temperature plays a decisive role in controlling distributions
(Fig. 6.6).
In polar areas the temperature can fall to -1.5 °C; in marginal seas it can rise to
well over +30 °C, as in the Red Sea and the Persian Gulf. Outside the tropics
temperature can vary strongly with the seasons in the upper 100 to 200 m of the water
column. Below this depth it varies little, however, being rather low throughout the
year.
Most of the ocean is extremely cold: in the deep sea the temperature stays below
4 0c. The cold temperature in itself does not reduce diversity: it has recently been
shown that an astounding variety of small benthic organisms occur even at abyssal
depths. Large, unpredictable fluctuation of environmental factors appears to be much
more restrictive than just extreme cold. Such fluctuations work damage, especially on
eggs, larvae, and the young recruits of a population.
algae; thus, they are limiting nutrients. Algal material, and also animal matter, sinks
below the photic zone and undergoes bacterial decay. Nutrients are released in the
process, that is, remineralization occurs. The nutrients are not being used at depth, in
the absence of light. They become concentrated, therefore.
Thus, the ocean has a deep reservoir of nutrients below the surface waters. The
boundary between this reservoir and the nutrient-poor sunlit waters is the top of the
thennocline, usually between 50 m and 100 m deep.
When the thennocline is eroded by stonns or when the deep water comes to the
surface through the pumping effects of eddies or through upwelling (Sect. 4.3.3), the
nutrients return to the surface water, into the sunlit zone. This fertilization by admixture of deep waters results in increased productivity, both in the algae and in the
animals that feed on them. All the good fishing areas are in such regions of vertical
mixing.
6.1.4 Salinity. Life in the sea depends on other factors besides sunlight and nutrients.
A large number of organisms can tolerate salinity fluctuations only if salinity stays
between 30 and 40 %0 (stenohaline forms). Examples are radiolarians, reef corals,
cephalopods, brachiodops, and echinodenns. In general, their remains indicate
marine conditions for the sediment that contains them. However, there are exception.
A few representatives of the above groups may have relatively wide tolerances: for
instance, there is a starfish that lives in the brackish waters of the Baltic Sea. Also,
fossils embedded in sedimentary layers do not necessarily show the environment of
deposition: they may be redeposited.
With increasing salinity, for example in lagoons of arid regions, fewer and fewer
species are able to survive, until only a few representatives remain - an improverished fauna and flora. Some ostracods (bi-valved crustaceans of millimeter size) can
tolerate salinities of more than 100 %0. As these hardy form have few competitors (or
predators), impoverished faunas can be very rich in individuals. Quite generally, of
course, species-poor but individual-rich faunas indicate special, restricted environments. Restricted does not apply to space here, but to unusual temperature ranges,
oxygen deficiencies, and other stress-producing factors.
6.1.5 Temperature. In the open ocean, where salinities are well within the tolerance
of all marine organisms, temperature plays a decisive role in controlling distributions
(Fig. 6.6).
In polar areas the temperature can fall to -1.5 °C; in marginal seas it can rise to
well over +30 °C, as in the Red Sea and the Persian Gulf. Outside the tropics
temperature can vary strongly with the seasons in the upper 100 to 200 m of the water
column. Below this depth it varies little, however, being rather low throughout the
year.
Most of the ocean is extremely cold: in the deep sea the temperature stays below
4 0c. The cold temperature in itself does not reduce diversity: it has recently been
shown that an astounding variety of small benthic organisms occur even at abyssal
depths. Large, unpredictable fluctuation of environmental factors appears to be much
more restrictive than just extreme cold. Such fluctuations work damage, especially on
eggs, larvae, and the young recruits of a population.