Case Studies - Coastal Lagoons: Ecosystem Processes and Modeling for Sustainable Use and Development - page 411

distributed. Aurelia aurita , the only autochthonous species, is the less abundant and
has its maximum abundance in spring (April and May). Rhyzostoma pulmo started
to increase in May, while Cotylorhiza tuberculata peaked in abundance in June and
July, reaching more than 12 individuals per 100 m 3 . The total population of jellyfishes
estimated in the lagoon by mid-summer of 1997 was on the order of 40 million.
Presence of large-celled phytoplankton in response to elevated nitrate concen-
trations has implications for the structure and function of the whole planktonic food
web. The distribution of biomass in aquatic systems generally shows a regular decline
in biomass with increasing organism size arranged in logarithmically equal size
intervals. 55 The spectra slope has been related to the energy flow through the plank-
tonic food web 56-58 and to the trophic state of the ecosystem. 59 Although the seasonal
variation of the Mar Menor Lagoon's trophic state was also explained in the biomass
size spectra study, 14 it was most interesting, nevertheless, to find that the interannual
comparison of planktonic size distribution showed an almost invariable slope within
a size range from 2- to 1000-

m equivalent spherical diameter. 47 The size range used
for comparison excluded the jellyfish fraction (up to 40 cm in diameter). It can be
seen, nevertheless, that this size fraction plays a major role in controlling the biomass
spectra parameters. Jellyfish gut contents indicate clearly their preference for large
diatoms (62-86%), tintinnids (3-33%), and copepods (1-3%). 47 High removal rates
of larger plankton were expected in the Mar Menor Lagoon due to the large number
of jellyfishes and their size-selective diet.
It is generally believed that nutrient loads stimulate primary production, thereby
increasing planktonic biomass, but that was not completely true in our lagoon because
of the top-down control of the planktonic food web by jellyfishes. It was paradoxical
to find that chlorophyll a ( Figure 9.3.18) and the total biovolume considered in the
2- to 1000-

µ

m size range ( Figure 9.3.19 ) at the same sampling station was always
lower in 1997 (with higher nitrate loads, lower phosphate concentration, and very
high densities of jellyfishes) compared to 1988 (with lower nitrate levels, higher
phosphate concentrations, and where jellyfishes were not found) in the four spectra
compared. The size spectra comparison suggests that jellyfishes can be an efficient
top-down agent controlling the consequences of a eutrophication process.
In systems where nutrients are scarce, fast-growing small cells can provide avail-
able food for certain size ranges of grazers, resulting in relatively high densities of
copepods. At higher concentrations of nutrients, large diatoms dominate, which cope-
pods cannot eat. This means that the density of copepods is low to feed the largest
zooplankton, such as fish larvae and jellyfish. As jellyfish gut contents indicate, high
removal rates of larger plankton are expected in the Mar Menor Lagoon due to the
large number of jellyfishes and their size-selective diet. While the origin of large
diatoms in the water column can be explained as a direct consequence of nitrate loads,
abundance of tintinnids, the second most numerically important of gut contents, feed
mainly on bacteria, heterotrophic flagellates, and small phytoplankton cells. The effect
of jellyfishes removing tintinnids can be seen as an indirect top-down control mech-
anism on small size fractions. By eating copepods, jellyfish also act indirectly on small
phytoplankton, reducing the top-down control exerted by copepods on this fraction.
Trade-offs between direct and indirect effects may explain why some eutrophic systems
support viable populations of small-celled phytoplankton and large populations of

µ