Sargassum horneri is one of the most important Sargassum species forming floating seaweed rafts. Drag force is the most important factor involved in detaching seaweeds from substrates. We conducted field experiments on drag force on S. horneri thalli > 40 cm by towing them with a boat at steady speeds of 0.5-4.0 m s-1 in Nabeta Cove, Japan, from in April 2014 and February 2015. The holdfast of thallus was attached to the end of a non-stretching fishing line; the other end was attached to a spring scale running through a steel pipe attached to the side of the boat and on low-friction pulleys fixed at both ends of the pipe. The lower end of the pipe was projected into the sea at a depth of approximately 1 m to keep the thallus submerged. The thallus was towed while force was measured on the spring scale. The relative speed of the boat towing the thallus versus seawater was continuously recorded by a flow meter attached to the end of the steel pipe. The results showed that drag force on the thallus was increased with thallus length and current speed; the drag force was roughly proportional to flow speed3/2; the drag coefficient, Cd, of
S. horneri was expressed with the following equation: Cd = 18.295Re-0.571 in a range of Reynolds number (Re) between 104 and 106.
Seasonal variations in the increase rate of chlorophyll (Chl.) a and the primary production rate were measured by in situ seawater incubation in the Shinkawa River estuary in the Seto Inland Sea, Japan. Nutrient concentrations in the seawater were always sufficient for the growth of phytoplankton. The study site was very shallow, but photo-inhibition, which occurs at high light intensities, such as > 1,000 μmol/m²/s, was not observed in photosynthesis test samples. Therefore, light intensity was a minor factor to explain primary production in this area throughout the year. Chl. a concentration reduced a little or was almost constant at low water temperatures. In contrast, Chl. a concentration increased significantly in the period of high water temperature (July to September), showing a maximum specific increase rate of 0.3-0.4/h. The primary production rate was also high while water temperature was high. Monthly primary production rates, estimated from water temperature and PAR, varied from 0.01±0.00 to 1.13±0.24 gC/m²/day, which was similar to that reported for other estuaries and coastal seas. The most influential factor in primary productivity in this study area was water temperature, with peak temperatures associated with extremely high primary productivity, with up to 460μgC/l/h recorded in summer.
To clarify the occurrence of upwelling and its effect on primary production in Shimanto
Canyon of western Tosa Bay, the vertical distributions of nutrients and chlorophyll a were
investigated seasonally in 2010. Maximum nutrient concentrations during each survey occurred
at the bottom layer in Shimanto Canyon. The vertical distribution of nutrients showed that
upwelling of the nutrient-rich deep waters of the sea bottom to the upper layers occurred only in
August. Simultaneously, the sharp chlorophyll a maximum, being the annual highest value,
appeared in the subsurface layer on the rim of Shimanto Canyon when the Kuroshio Current
flowed near the shore. This suggests that the upwelling occurrence in Shimanto Canyon is related
to the Kuroshio Current path, which plays an important role in supplying nutrient-rich waters of
the submarine canyon to the euphotic zone.