Implications of spatial and temporal variation for biogeochemical budgets of estuaries

The analysis of nutrient budgets is a common method for assessing the biogeochemical function of estuaries including denitrification and nutrient retention rates. The Land Ocean Interactions in the Coastal Zone (LOICZ) guidelines for constructing such budgets concentrate on the simplest case where an 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 simplifying assumptions can lead to significant and sometimes very large errors in estimates of internal retention (or production) rates. If the load to an estuary varies significantly through the year, the use of time-averaged concentrations of nutrient and salinity to calculate retention rates is shown to cause errors of up to 30% depending on the circumstances. The second case considered examines the consequences of treating an estuary which has significant long-estuary salinity and nutrient gradients as a well-mixed box. In a simple case considered, the calculated internal production from a distributed 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 on the location of the nutrient source. The third case considers the errors in the calculated internal retention/production rates of treating an estuary 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 analyses can yield calculated retention rates of opposite sign. In this situation, one configuration for the estuary would appear to sequester nutrient, whereas the other would be a net exporter of nutrient.