Intratidal Variation and Net Transport of Dissolved Inorganic Nutrients, POC and Chlorophyll a in the Camboriú River Estuary, Brazil

Abstract

The aim of this study was to evaluate the intratidal variability of dissolved inorganic nutrients (NO−3, NO−2, NH+4, PO3−4 and Si), Particulate Organic Carbon (POC) and chlorophyll a (chla) in the estuary of the Camboriú River, and to quantify their transport into the adjoining coastal zone. During a 1-day spring tide sampling in March 1998, continuous records of direction and velocity of currents within the estuary channel over a period of 25h were obtained, covering two complete tidal cycles. Vertical profiles of salinity and temperature were observed and samples of surface and bottom water were collected at hourly intervals. The transport of these substances through the estuary was calculated from the data on: (1) concentration of nutrients, POC and chla; (2) velocity and direction of surface and bottom currents and (3) the area of the channel cross section. The values obtained for residual transport of nutrients to the bay were as follows: 2·9×104mol of DIN (401kg N); 6·6×102mol of PO3−4 (20·4kg P- PO3−4); and 4·5×104mol of Si (1264kg Si). The main form of DIN was NH+4, which may be related to the decomposition of organic material in the estuary, as well as to the entry of effluents from the Balneário Camboriú sewage treatment plant, located about 3km upstream from the collection site. The N:P ratio between the nutrients was high (45), which may have contributed to the higher values for this ratio in the bay. The results also showed an export of 532kg of POC and 5kg of chla into the bay. The maximum values of chla always occurred right after the periods of tide inversion, at the beginning of the ebb tide or of the flood tide, invariably with salinities between 25 and 35. The highest of these peaks (23μgl−1) was recorded in the afternoon period, during the ebb tides. This pattern suggests the existence of a belt of productivity located close to the mouth of the estuary, whose precise location is related to the tide, sometimes entering the estuary (flooding) and sometimes exiting to the bay (ebbing). The high values for cla may be explained as being the result of the balance between the high supply of nutrients provided by the Camboriú River, and the increase of light penetration due to the higher transparency of salt water. The prevalence of coastal phytoplankton species during the peaks of cla confirms the idea that, when entering the estuary, they find ideal conditions of light and nutrients for their optimum development. The result of the export of cla in this case may have been due to the fact that the growth of organisms occurred within the estuary after the entry of a smaller innoculum of organisms during the high tide which, in this study, was registered early in the morning.