Characteristics of Danish estuaries

We review various aspects of the structure and functioning of Danish estuar ies from data collected by the National Monitoring Program and from informa tion in published sources. We present data on the physical, chemical, and b iological characteristics of estuaries in Denmark, we evaluate the function ing of these systems as filters and transformers of nutrients, and we evalu ate the outlook for Danish estuaries in the future. Danish estuarine system s are for the most part shallow (< 3 m deep), have short residence times, a nd tend to be heavily loaded with nutrients primarily from agricultural sou rces. Total average loads from land per unit watershed area are 112 kg P km (2) yr(-1) and 2,400 kg N km(-2) yr(-1) during the period 1989-1995. The t otal phosphorus (TP) load in estuaries has been significantly reduced over the last decade, following implementation of the 1987 Action Plan for the A quatic Environment (Vandmiljoplan in Danish) that prescribed that nitrogen loads to the total aquatic environment should be reduced by 50% and phospho rus loads by 80%. Reductions in the total nitrogen (TN) load have been more modest. Nutrient loading is one of the primary determinants of estuarine n utrient concentration with 70% of the annual variation in TN concentration and 55% of the annual variation in TP concentration explained by variation in the load. Many Danish estuaries have rich communities of macrophytes and benthic filter feeders, such as Mytlis edulis and Ciona intestinalis, that can control water column chlorophyll concentrations by their filter feedin g activities. Many of the estuaries experience hypoxia and anoxia, especial ly during warm and calm summer months. Further reductions in nutrient loadi ng are expected following implementation of the Action Plan for the Aquatic Environment II, with predicted improvements in oxygen concentrations and i n the functioning of these shallow, dynamic estuarine systems.