Phytoplankton biomass and photosynthetic carbon fixation in a tropical coral reef bay (Sanya Bay) of the northern South China Sea, with special reference to the effects of Solar UVB

data collected in Sanya Bay from Tropical Marine Biological Research Station in Hainan, including twelve monitoring stations were designed and a reference station of river input.Water samples were collected using Niskin bottles at the surface and bottom layer. A YSI 6600 instrument (YSI Inc., Yellow Spring, OH, USA) was used at a station to take vertical profiles of temperature and salinity. To measure the turbidity of the waters, a white Secchi disk (30 cm diameter) is used to measure the Secchi depth at each station. Photosynthetically active radiation (PAR, 400-700 nm) and UVB intensity in the air and water column were measured with submersible, broad-band, and photodiode PAR and UVB detectors attached to a IL-1700 Research Radiometer (International Light Inc., MA, USA). Vertical penetration of UVB and PAR in the water column subjects to the exponential attenuation law, IZ = I0e -KZ, where IZ and I0 are the light intensity at a depth Z and at the surface, respectively. K (m-1) is the light extinction coefficient (Yin et al., 2004). KPAR and KUVB are the extinction coefficients for PAR and UVB, respectively. The 1% light level depth of PAR and UVB were calculated from KPAR and KUVB, respectively.Stns W1, W4, W6, W9, W11 and W12 were selected to conducted photosynthetic carbon fixation incubations. Water samples for phytoplankton photosynthetic carbon fixation (or primary productivity) incubations were prescreened through a 200 µm mesh net to remove larger zooplankton, and then transferred to 50 ml quartz tubes, inoculated with 2 µCi labeled sodium bicarbonate (NaH14CO3) and incubated for 4-6 h between 08:00 and 16:00 h. All experimental treatments were in duplicate and the tubes were placed horizontally (i.e., parallel to the water surface) in a water bath on board the ship. The horizontal placement helped to mix the incubated water samples. The bath was flushed with surface sea water to maintain in situ temperature. Duplicate 50 ml quartz tube samples were incubated under: (a) NS-UVB (natural sunlight with UVB); and (b) PAR (natural sunlight without UVB), achieved by shielding UVB with Mylar film (Melinex film 389, DuPont products).After the incubation, samples from the 14C addition experiments were filtered through Whatman GF/F filters, and were kept frozen (-20 °C) until analyzed within one month following the JGOFS protocols (Knap et al., 1996). The filters were put into scintillation vials containing 0.2 ml of 0.5 N HCl for 12 h in order to remove inorganic carbon. After the addition of 10 ml of scintillation cocktail (Hi-Safe) to each vial, samples were counted on a Beckman 2000 CA/LL Liquid Scintillation Counter and primary productivity (14C uptake rates) were calculated according to standard procedures (Knap et al., 1996). The relative photoinhibition ratio due to UVB was calculated as follows:Inh (%) = (Pno UVB - PUVB) / Pno UVB × 100%Chl a was measured fluorometrically by a Turner Designs 10-AU fluorometer after the method by Parsons et al. (1984). The filtrate (30 ml) of precombusted (450 °C, 5 h) Whatman GF/F filter was collected and frozen for nutrient analyses in laboratory. Nutrients (dissolved inorganic nitrogen, DIN = NO3 + NO2 + NH4, PO4 and SiO4) were analyzed by a Skalar San Autoanalyzer after the colorimetric methods described in Yin et al. (2001). Dissolved oxygen (DO) was determined using the Winkler titration method. Chemical oxygen demand (COD), five-day biochemical oxygen demand (BOD5) and dissolved organic carbon (DOC) were tested according to “Specifications for Oceanographic Survey, Part 4: Survey of chemical parameters in sea water” (GB/T12763.4, China).