Nitrogen
Fixation
Several recent analyses show
deficiencies in N inputs relative to outputs in several ocean basins (Gruber and
Sarmiento, 1997, Michaels et al. 1996, Karl et al. 1992, Sambrotto 1993, Bates et al. 1996). New production estimates in the
upper water column of tropical regions often exceed
known new N inputs. In several tropical systems, d15N of pools of PON and NO3- in the upper ocean have been
reported to be close to that of atmospheric N2, evidence of a biological N2 fixation source.
Reanalysis of existing data suggests a much larger role for
N2 fixation than is currently given. Trichodesmium, is the most
prominent planktonic marine nitrogen fixer (Capone et al. 1997). It occurs throughout the
open waters of oligotrophic tropical and
subtropical oceans. Research through the 1970s indicated that it can be of regional significance but its contribution to
the larger marine N and C cycle wasconsidered minimal. However, on recent research cruises
in several diverse locations, including the
tropical N. Atlantic, Arabian Sea and Eastern Indian Ocean, it has been confirmed that N2 fixation by Trichodesmium
can, at times, match or exceed, NO3- flux into the upper water
column. In addition, anecdotal data is widespread
with regard to the extensive surface blooms it forms, sometimes covering in excess of
100,000 km2. Open ocean N2 fixation by Trichodesmium therefore represents a major
source of new N input in the oceans and it is a globally significant source of fixed N in
the biosphere. We need more information on the
global distribution of Trichodesmium biomass and activity.
One of the reasons for the
uncertainty in the importance of Trichodesmium to global N budget is that
reliable quantitative information on the abundance
and activity of Trichodesmium is relatively scarce. Trichodesmium has a
few physiological properties that make it unique in the marine ecosystem and these have contributed to a severe
underestimation of its biomass and production in the upper water column (Capone 1997). Trichodesmium is a colonial cyanobacterium
that is large enough to be visible to the naked eye. Colonies of Trichodesmium
are made up of about 50 to 200 trichomes -
filaments, each made up of about 100 cells. Thus, a colony of Trichodesmium
containing about 5000 to 20,000 cells, is a quantized packet
of carbon biomass and traditional phytoplankton sampling techniques suitable for
homogenous distribution of phytoplankton cells are not appropriate.
Typical concentrations of colonies in oligotrophic tropical waters are about 1 per L
(Carpenter & Romans 1991).
In addition, Trichodesmium
is of interest to coastal zone managers because it forms massive noxious blooms close to
the coast. Trichodesmium thiebautii contains
a type of neurotoxin (Codd, 1994) and has been reported as causing fish kills
(Nagabushanam, 1967) and respiratory difficulties (Trichodesmium
fever, Sato et al. 1963). Suvapepun (1992) reported a loss of over $1 million worth of
farm-fish harvest due to Trichodesmium blooms in
the Gulf of Thailand. Sellner (1997) presents other evidence for Trichodesmium-induced
toxicity and ecosystem damage. Tester et al. (1993) mention
the co-occurance of Trichodesmium blooms and red tides due to Gymnodinium
breve off the Carolina coast. There is accendotal evidence of Trichodesmium blooms preceeding red tides
in the Gulf of Mexico. Devassy et al. (1979) clearly detail the succession of species
following a Trichodesmium bloom.
Suvapepun (1989) reported that blooms of Noctiluca followed Trichodesmium blooms.
Thus it is quite obvious that Trichodesmium
blooms are both significant noxious bloom formers in themselves and could be related to
other phytoplankton blooms.
Trichodesmium blooms extensively along the south eastern U.S.
coast and in the Gulf of Mexico (Biddanda, 1995 (1); Guo & Tester, 1994; Tester et al.
1993; Paerl &Bebout, 1988(2); Eleuterius, 1981(6); Dunstan and Hosfood, 1977(7); J. Nelson (3); K.Carder
(5); J. Pennock (6), E. Haugin personal
communication, Subramaniam et al.(in prep) (4)). Dunstan
and Hosfood (1977) found that T. thiebautii accounted for over 50% of the particulate
organic material in the July surface samples of
the Georgia Bight. Biddanda (1995) reported
encountering dense surface accumulations of this organism
for almost the entire length of a transect across the Gulf
of Mexico from Florida to Texas. Elsewhere in
the world, reports of spatially extensive, persistent
and recurrent coastal blooms of Trichodesmium come from the Gulf of Thailand, south of the Izu peninsula,
Japan, in summer, off much of the north and
north-west coast of Australia including the Great Barrier Reef region, in and around the Indian Ocean and Arabian Sea and have
been summarized by Carpenter and Capone (1992). For all the above reasons, we need a better
understanding of the processes that promote Trichodesmium
blooms and the consequences of these blooms.
By being able to monitor its abundance from space, we should be able to determine some of
the factors (i.e. aeolian iron deposition in
dust, wave action, currents, upwelling etc.) that lead to the formation and dispersion of
blooms.