3.1. Growth, productivity and C:N:P ratios of Sargassum
The shipboard culture studies showed significantly (p b 0.0001;
Table 1) higher growth rates in neritic compared to oceanic waters
(Fig. 3). In neritic waters, growth rates for both S. natans and
S. fluitans were statistically similar (average ~0.06 doublings · day−1)
and ranged from 0.041 to 0.091 doublings · day−1 and 0.031 to 0.093
doublings · day−1, respectively (p = 0.9811; Table 1). The highest
growth rates for both species were observed at Dry Tortugas and Looe
Key in the Straits of Florida, which could reflect highly productive
populations from the GOMbeing advected by the Loop Current around
the Florida Keys and towards the Gulf Streamand Sargasso Sea. Growth
rates were onlymeasured for S. natans in oceanic waters, which ranged
from 0.005 to 0.020 doublings · day−1.
Gross productivity of pelagic Sargassum was also significantly
higher in neritic compared to oceanic populations (p b 0.0001;
Table 1, Fig. 4), with no significant difference between S. fluitans and
S. natans (p= 0.4271; Table 1). In neritic waters, gross productivity of
S. natans and S. fluitans averaged 2.37 ± 0.40 and 2.53 ± 0.45 mgC · g
dry wt. · h,−1 ranging from 1.84 to 2.81 mgC · g dry wt. · h−1 and
1.89 to 3.2 mgC · g dry wt. · h−1, respectively. In oceanic waters, lower
gross productivity rates of S. natans and S. fluitans averaged 1.15 ±
0.67 and 1.36 ± 0.64 mgC · g dry wt. · h−1, ranging from 0.55 to
1.59 mgC · g dry wt. · h−1 and 0.61 to 1.91 mgC · g dry wt. · h−1,