Temporal Variability of Carbon and Nutrient Budgets from a Tropical Lagoon in Chiku, Southwestern Taiwan

Biogeochemical processes and budgets of carbon, nitrogen and phosphorus from the semi-enclosed Chiku Lagoon were constructed through periodic observations and modelling. During the investigation, samples were mostly collected bimonthly, and hydrochemical properties, inorganic and organic nutrients (DIN, DON, DIP, DOP, Dsi (dissolved silica)) and organic carbon (DOC, POC) from waters associated with the lagoon were measured. The water exchange time of Chiku Lagoon ranges from 1·0 d (June 1997) to 8·5 d (January 1997) with an annual mean of 5·0 d. The residence time of nutrients varies with water exchange time, and is about 2–5 d longer than the water exchange time. Terrestrial inputs and lagoon distributions of nutrients varied in time and space based on the time scale of sampling. Thus, carbon and nutrient budgets were prepared for each sampling period and then combined to form annual budgets, which differed significantly from those modelled from annual means of various parameters. The annual removal of terrestrial nutrient inputs to the lagoon system is 69·4, 47·0, 27·7 and 42·0%, respectively, for DIN, DON, DIP and DOP. Consequently, the nonconservative flux of dissolved inorganic phosphorus ( DIP) from the lagoon is around 0·1 mole m 2 yr 1, that is equivalent to an internal organic carbon sink of 11 mol C m 2 yr 1. This organic carbon budget indicates that the lagoon is an autotrophic system where photosynthesis exceeds respiration (p-r > 0). This carbon sink is one of largest reported from world’s lagoons, and its large size may result from the abundant nutrients in the lagoon. However, although the Chiku Lagoon is estimated to remove 4·7 mol C m 2 yr 1 carbonate through oyster calcification, it emits an equivalent amount of CO2 into the system. Despite net nitrogen fixation being observed during some periods, denitrification exceeds nitrogen fixation throughout the period of observation [(nfix-denit)= 1·4 mole N m 2 yr 1].