Dynamics of nutrient elements in three estuaries of North China: The Luanhe, Shuangtaizihe, and Yalujiang

Concentrations of nutrients (NO₃^-, NO₂^-, NH₄⁺, PO₄^5-, and dissolved SiO₂) were examined in three North China estuaries-the Luanhe, Shuangtaizihe, and Yalujiang. These riverine-estuarine systems provide distinct geographic and hydrodynamic conditions, that is, a shallow water zone embraced by shoals and sandbars (Luanhe), the confluence of two streams in the upper estuary with different water anti sediment loads, and a turbidity maximum in the upper estuarine mixing zone (Yalujiang). Nutrient element concentrations in these rivers are high in comparison with large, less disturbed systems but similar to those from polluted and/or eutrophic European and North American rivers. This is attributed to intensive weathering and erosion and extensive use of chemical fertilizers. In the fresh-marine waters mixing zone, nutrient species can behave either conservatively or nonconservatively, or both. Wherever nonconservative behaviours of nutrient elements are observed, remobilization from solid phases is probably the predominant mechanism. The extrapolation of dilution curves to the fresh water end-members gives estimated riverine concentrations, which can be between two and ten times higher than those from field observations. Taking into account the high N:P ratios (10²10⁴) from North China rivers and very low concentrations of nitrogen species in the Northwest Pacific coastal oceans (e.g., Yellow Sea), the estuaries in this study may act as regions in which production is limited by phosphorus to regions in which production is limited by nitrogen.