Variability and tidal exchange of ecological properties in a coastal lagoon

Abstract

Sea level, current velocity, temperature, salinity, oxygen, inorganic phosphate, chlorophyll a, seston and its organic and inorganic fractions, phytoplankton species abundance, particulate organic carbon and nitrogen, and primary productivity time series were generated for the mouth of San Quintin Bay, Baja California, Mexico, for 18 days during the summer of 1977. This was done to elucidate the main factors that cause variability of these ecological properties and to frame some generalizations about the offshore waters during the summer season. San Quintin Bay is of considerable interest because of its developing mariculture potential, and because it is representative of a type of coastal lagoon that is rapidly being latered by man's activities.

Alternation of upwelling events was the main cause of variability for all properties except temeprature. Semi-diurnal tides were the main cause of variability for temeprature. Conservative variables such as temperature and salinity had a semi-diurnal component greater than the diurnal one. Most non-conservative variables (chlorophyll a and phosphate concentrations, for example) had equal diurnal and semi-diurnal variability components, due to biological processes that strongly depend on the solar radiation cycles. The relationship between variability components for oxygen was intermediate to those relationships for the conservative and non-conservative variables. Seston variability was mainly due to turbulence induced by winds and tidal currents. Lack of correlation between particulate organic carbon and nitrogen, and the wide range of C:N ratios in particles, indicate that bay-derived seston of widely varying organic and inorganic content represents most of the suspended particulate matter.

Mean transports of all properties measured at the bay mouth, over the complete sampling period and within each upwelling event, were positive, indicating net movement into the bay. During the second upwelling event (the last 7 days of the sampling period) mean fluctuation fluxes for temperature, oxygen and chlorophyll a at the bay mouth were significantly negative, indicating exports of these properties from the bay. For all other variables, and during the first upwelling period and the upwelling relaxation period, there were no significant fluctuation fluxes at the bay mouth.

Diatoms were always the most abundant phytoplankton group. Primary productivity was always greatest at the surface, with a mean value of 27 mg C m⁻³ h⁻¹ through the sampling period. Maximum surface productivity values were obtained for the upwelling relaxation period (up to 44 mg C m⁻³ h⁻¹). A mean value of 122 mg C m⁻² h⁻¹ was found for the entire water column during the 18-day period. These values are comparable to the productivity maxima of other upwelling areas. The mean surface assimilation ratio was 6·6 mg C (mg chl a)⁻¹ h⁻¹, which indicates nutrient-rich waters.