Historic changes in nutrient fluxes from the Yangtze River to the sea: Recent response to catchment regulation and potential linkage to maritime red tides

Abstract

Riverine nutrient fluxes are of great implications for marine ecosystems. However, there is a lack of knowledge on the source-to-sink process of nutrient fluxes, particularly the total phosphorus (TP) and total nitrogen (TN), in the Yangtze River and its adjacent sea. Thus, this study provides historic changes in riverine nutrient fluxes to the sea over the past ∼70 years, and tries to figure out their response to catchment-related factors and potential ecological influence on the sea during the past two decades. It is found that the process of the nutrient flux to the sea over the past ∼70 years can be divided into three stages as a low-level stage before 1980, a rapid developing stage after reform and opening-up, and a final stage under government regulation. Based on our detailed measurements in the recent two decades (2004–2020), the increasing trends of the riverine nutrient fluxes have slowed down, and the TP and TN fluxes averaged 0.09 ± 0.02 and 1.61 ± 0.23 mt/yr, respectively 12.2 % and 4.8 % smaller than the previous five-year (1999–2003) averages. The seasonal characteristics of the nutrient fluxes primarily reflected the controlling effect of the Yangtze River water discharge, and the overall interannual characteristics of the nutrient fluxes were related to the fertilizer use in the catchment. Over the past two decades, the annual frequency and area of red tides occurring in the East China Sea averaged ∼38 red tides and 5120 km2, respectively. The major red tides (single area >100 km2) mainly occurred between March and September, and peaked in May (50.0 % and 57.9 % of the total frequency and area, respectively). The intensity of red tides had a relatively decent reaction to the nutrient fluxes in May, but also exhibited some uncertainties affected by some other important influencing factors. This study supplies a more continuous, accurate and detailed field-measured dataset of TP and TN fluxes from the Yangtze River to the ocean, and assists in exploring the potential linkage between the maritime red tides and riverine nutrient fluxes.