Eastern Pacific Coral Reef Ecosystem
19.5 Nutrient Cycling, Carbon Production
and Trophic Relationships
295
Perhaps the greatest gaps in our knowledge of eastern Pacific coral reef
ecosystem function relate to nutrient cycling, community metabolism,
organic production, and trophic relationships. These sorts of studies have
not yet been undertaken, although data on inorganic carbon production,
i.e. rates of calcium carbonate accumulation, have recently been forthcoming. Unlike the oft quoted simile of highly productive reef oases surrounded by desert-like oligotrophic environments, nearshore eastern Pacific reefs are generally exposed to waters of high nutrient and suspended
sediment inputs. This holds true for reefs near drainage basins receiving
high river discharge as well as for reefs in upwelling centers, such as the
Gulf of Tehuantepec and the Panama Bight. Whereas dissolved nitrate concentrations in waters surrounding oceanic reefs are typically 0.1-0.3flg
atom 1- 1 , eastern Pacific reefs are commonly exposed to inorganic N levels
an order of magnitude or higher during upwelling pulses. Since coral reefs
are present under these conditions, it can be concluded that the requisite
elements for reef growth are met.
A pivotal question begging attention is why vigorous algal growth in
nutrient-rich environments does not exclude coral cover and prevent net
reef accretion? Invertebrate and fish herbivore populations probably play
an important role in preventing benthic algae from outcompeting corals.
The high abundances of grazers on reefs in eastern Pacific upwelling areas
may be sufficient to control algal growth and thus allow corals a competitive advantage, especially rapidly calcifying species such as Pocillopora. In
a comparative study of the intensity of fish grazing on sponges in eastern
Pacific and Caribbean nearshore areas, Birkeland ( 1987) demonstrated
that grazing pressure was 25 times greater at Pacific than Caribbean sites.
He attributed this interoceanic difference to high Pacific nutrient input
that supports high phyto- and zooplankton production, which in turn
favors large population densities of grazing and browsing fishes. Since
some coral communities are occasionally overwhelmed by benthic algae,
e.g. at Uva Island, Panama (Glynn and Mate 1997) and Urvina Bay, Galapagos Islands (Macintyre et al. 1993), it is possible that such situations are
simply temporary responses to nutrient pulses and not long-lived. A study
designed to elucidate rates of algal production and consumption relative to
coral recruitment and survival should shed light on the dynamics of reef
growth in the eastern Pacific region.
Core-drilling and radiocarbon dating reveal that the best developed coral
reefs in Panama and Costa Rica range between 4 and 10 m thick and are
19.5 Nutrient Cycling, Carbon Production
and Trophic Relationships
295
Perhaps the greatest gaps in our knowledge of eastern Pacific coral reef
ecosystem function relate to nutrient cycling, community metabolism,
organic production, and trophic relationships. These sorts of studies have
not yet been undertaken, although data on inorganic carbon production,
i.e. rates of calcium carbonate accumulation, have recently been forthcoming. Unlike the oft quoted simile of highly productive reef oases surrounded by desert-like oligotrophic environments, nearshore eastern Pacific reefs are generally exposed to waters of high nutrient and suspended
sediment inputs. This holds true for reefs near drainage basins receiving
high river discharge as well as for reefs in upwelling centers, such as the
Gulf of Tehuantepec and the Panama Bight. Whereas dissolved nitrate concentrations in waters surrounding oceanic reefs are typically 0.1-0.3flg
atom 1- 1 , eastern Pacific reefs are commonly exposed to inorganic N levels
an order of magnitude or higher during upwelling pulses. Since coral reefs
are present under these conditions, it can be concluded that the requisite
elements for reef growth are met.
A pivotal question begging attention is why vigorous algal growth in
nutrient-rich environments does not exclude coral cover and prevent net
reef accretion? Invertebrate and fish herbivore populations probably play
an important role in preventing benthic algae from outcompeting corals.
The high abundances of grazers on reefs in eastern Pacific upwelling areas
may be sufficient to control algal growth and thus allow corals a competitive advantage, especially rapidly calcifying species such as Pocillopora. In
a comparative study of the intensity of fish grazing on sponges in eastern
Pacific and Caribbean nearshore areas, Birkeland ( 1987) demonstrated
that grazing pressure was 25 times greater at Pacific than Caribbean sites.
He attributed this interoceanic difference to high Pacific nutrient input
that supports high phyto- and zooplankton production, which in turn
favors large population densities of grazing and browsing fishes. Since
some coral communities are occasionally overwhelmed by benthic algae,
e.g. at Uva Island, Panama (Glynn and Mate 1997) and Urvina Bay, Galapagos Islands (Macintyre et al. 1993), it is possible that such situations are
simply temporary responses to nutrient pulses and not long-lived. A study
designed to elucidate rates of algal production and consumption relative to
coral recruitment and survival should shed light on the dynamics of reef
growth in the eastern Pacific region.
Core-drilling and radiocarbon dating reveal that the best developed coral
reefs in Panama and Costa Rica range between 4 and 10 m thick and are