Distinct peaks of UV-absorbing compounds in CDOM and particulate absorption spectra, of near-surface Great Barrier Reef coastal waters, associated with the presence of Trichodesmium spp. (NE Australia)

During a field study in The Fitzroy River/Estuary and Keppel Bay region of the Great Barrier Reef in September 2003, samples collected for CDOM and particulate absorption were found to have peaks in the UV region of the absorption spectra. These peaks are assumed to be due to the presence of Mycosporine Amino Acids (MAAs) and are not often observed, especially in the dissolved or CDOM fraction. The data provided shows the effect the UV-absorbing compounds (MAAs) have on the absorption recorded in the visible region. This in turn ultimately affects the accuracy of satellite remote sensing imagery to accurately depict the presence of phytoplankton blooms. \nLineage: A field survey was carried out in the Fitzroy River Estuary – Keppel Bay region within the Great Barrier Reef (GBR) Lagoon in NE Australia between 05 – 12 September 2003. Around 60 sites were sampled to determine various optical and biogeochemical parameters, but this data set is only from sites within Keppel Bay and does not include sites sampled within the Fitzroy River or near the river mouth. \nDiscrete water samples were collected, at several sites, from the surface waters using a 10 L plastic carboy held approximately 10 -20 cm under the surface (nominally referred to as “surface” samples). From this bulk water sample, subsamples were taken for the determination of total suspended matter (TSM) concentration, HPLC pigment concentration and composition, and the absorption coefficient of the particulate and dissolved fractions. At two sites (38 and 48), Trichodesmium cells/colonies were visible in the surface waters. In an attempt to capture these cells/colonies, an additional sample was collected by holding a 2 L plastic bottle within the top 2 - 3 cm of the surface layer, such that the mouth of the bottle was not fully submerged (nominally referred to as “near-surface” samples) and subsamples were taken for all discrete measurements.\n\nIn situ continuous measurements of total absorption and spectral attenuation were collected along a horizontal transect using an ac-9 (WETLabs, 9 wavelengths, 25 cm path length). The water intake for the ac-9 was about 1 meter below the water’s surface and the water was pumped, using low pressure, through a stainless steel filter mesh (pore size 350 um) and a debubbling system into the ac-9 chamber at a flow rate of 6 litres per minute. Samples were collected from the outflow of the ac-9 only at site 9. \nPigment analysis - sample water was filtered through a 47 mm glass-fibre filter (Whatman GF/F) using low vacuum and then stored in liquid nitrogen until analysis. Pigment extracts were analysed using a published method with a HPLC (Waters) and photo-diode array detection. The separated pigments were detected at 436 nm and identified against standard spectra using Waters Millenium software. Concentrations of chl-a, chl-b, and B,B-carotene were determined from standards (Sigma) and all other pigment concentrations were determined from standards of purified pigments isolated from algal cultures. \nParticulate and detrital absorption - sample water was filtered through a 25 mm glass-fibre filter (Whatman GF/F) and then stored flat in liquid nitrogen until analysis. Optical density (OD) spectra for total particulate and detrital matter were obtained using a GBC 916 UV/VIS dual beam spectro-photometer equipped with integrating sphere. The OD spectrum of the phytoplankton pigment was obtained as the difference between the OD of the total particulate and detrital components. The optical density scans were converted to absorption spectra by first nor-malising the scans to zero at 830 nm and then correcting for the path length amplification. \nCDOM absorption -samples were generally analysed within 24 – 48 hours after collection. At the laboratory, the pre-filtered samples were stored under subdued light until they reached room temperature (3 – 4 hours) and then filtered through a 0.2 um Durapore filter (25 mm, Millipore) immediately prior to analysis. The CDOM absorbance was measured in a 10 cm path length quartz cell, from 200–900 nm, using the normal cell compartment of the GBC 916 UV/VIS spectrophotometer, with Milli-Q water as a reference. \n