Implications of spatial and temporal variation for biogeochemical budgets of estuaries

The analysis of nutrient budgets is a common method for assessing the bioge ochemical function of estuaries including denitrification and nutrient rete ntion rates. The Land Ocean Interactions in the Coastal Zone (LOICZ) guidel ines for constructing such budgets concentrate on the simplest case where a n estuary or embayment is treated as a single box which is well-mixed both vertically and horizontally and at steady-state. We show that these simplif ying assumptions can lead to significant and sometimes very large errors in estimates of internal retention (or production) rates. If the load to an e stuary varies significantly through the year, the use of time-averaged conc entrations of nutrient and salinity to calculate retention rates is shown t o cause errors of up to 30% depending on the circumstances. The second case considered examines the consequences of treating an estuary which has sign ificant long-estuary salinity and nutrient gradients as a well-mixed box. I n a simple case considered, the calculated internal production from a distr ibuted source is underestimated by a factor of two. The errors are shown to depend on the estuary's mixing and geometrical characteristic as well as o n the location of the nutrient source. The third case considers the errors in the calculated internal retention/production rates of treating an estuar y with a two-layer circulation as a single-box system. The potential errors are severe. A comparison between a one-box analysis and a two-box analysis which accounts for the true estuarine circulation shows that the two analy ses can yield calculated retention rates of opposite sign. In this situatio n, one configuration for the estuary would appear to sequester nutrient, wh ereas the other would be a net exporter of nutrient.