5. Conclusions
The historical view that pelagic Sargassum sustains itself in oligotrophic
waters of the Sargasso Sea (Børgesen, 1914; Parr, 1939) needs
revision in light of these findings. The low growth rates of pelagic
Sargassum in the Sargasso Sea, combined with relatively low gross productivity
and high C:Nand C:P ratios, indicate strong nutrient limitation
by both N and P. Significantly lower C:N and C:P ratios in neritic waters
reduce nutrient limitation, resulting in higher gross productivity and
rapid growth of Sargassum. The fastest growth rates for both S. natans
and S. fluitans were observed near Dry Tortugas and Looe Key in the
Straits of Florida, which could reflect highly productive, nutrientenriched
populations advected from the GOM. This was supported by
the relatively low C:N and C:P ratios of S. natans and S. fluitans at
the Dry Tortugas, which suggests nutrient enrichment in the GOM.
These findings are consistent with recent C and N isotope analysis
of Trichodesmium in neritic and oceanic regions of the GOM, which
similarly reported significant N-enrichment associated with river
discharges and terrestrial runoff (Dorado et al., 2012). Sargassumpopulations
advected by the Loop Current and transported around the
Florida Keys could also be retained in the Tortugas Gyre, where recruitment
of juvenile fishes and additional nutrient enrichment of Sargassum
could occur (Lee et al., 1994). The Florida Currentwould transport these
enriched Sargassum communities northwards to the Gulf Stream and
into the northern Sargasso Sea. This physical connectivity is supported
not only by our observations, but also those of Carpenter and Cox
(1974) who observed the most productive Sargassum populations of
the Sargasso Sea occur in the northern region where relatively
nutrient-enriched Sargassum populations are found.
Accordingly, we hypothesize that populations of pelagic Sargassum
in the Sargasso Sea may rely on sustained inputs of new production
that occurs during circulation between the Caribbean, GOM, Straits of
Florida, and Gulf Stream, which is facilitated by a mutualism involving