Nutrient dynamics in tropical rivers, lagoons, and coastal ecosystems of eastern Hainan Island, South China Sea

Abstract

Abstract. Nutrient dynamics based on field observations made along the eastern Hainan Island during the period 2006–2009 were investigated to understand nutrient biogeochemical processes, and to provide an overview of human perturbations of coastal ecosystems in this tropical region. The rivers showed seasonal variations in nutrient concentrations, with enrichment of dissolved inorganic nitrogen and dissolved silicate, and depletion of PO43−. High riverine concentrations of nitrate mainly originated from agricultural fertilizer inputs. The DIN : PO43− ratios ranged from 37 to 1063, suggesting preferential depletion of PO43− relative to nitrogen in rivers. Chemical weathering in the drainage area might explain the high levels of dissolved silicate. Aquaculture ponds contained high concentrations of NH4+ and dissolved organic nitrogen. The particulate phosphorus concentrations in the study area were lower than those reported for estuaries worldwide. The particulate silicate levels in rivers and lagoons were lower than the global average level. Nutrient biogeochemistry in coastal areas was affected by human activities (e.g., aquaculture, agriculture), and by natural phenomena including typhoons. The nutrient concentrations in coastal waters were low because of dispersion of land-derived nutrients in the sea. Nutrient budgets were built based on a steady-state box model, which showed that riverine fluxes are magnified by estuarine processes (e.g., regeneration, desorption) in estuaries and Laoyehai Lagoon, but not in Xiaohai Lagoon. Riverine and groundwater inputs were the major sources of nutrients to Xiaohai and Laoyehai lagoons, respectively, and riverine inputs and aquaculture effluents were the major sources for the eastern coast of Hainan Island. Nutrient inputs to the coastal ecosystem increased with typhoon-induced runoff of rainwater, elucidating the important influence of typhoons on small tropical rivers.

Evaluation of Water Residence Time, Submarine Groundwater Discharge, and Maximum New Production Supported by Groundwater Borne Nutrients in a Coastal Upwelling Shelf System

“…The activities of 226 Ra in river/estuary waters have a range of 2.0–407 dpm 100 L −1 , with an average of 32.9 ± 46.0 dpm 100 L −1 , and 228 Ra from 2.8 to 504 dpm 100 L −1 , with an average of 92.4 ± 98.8 dpm 100 L −1 (supporting information Table S1). The river/estuary discharge of the eastern Hainan coastline is 2.06 × 10 7 m 3 d −1 (Li et al, ; Zhang et al, ). Thus, the fluvial radium loading of 226 Ra and 228 Ra is estimated to be 2.2 × 10 10 and 1.9 × 10 10 dpm d −1 , respectively.…”

“…The infiltration coefficients, which are retrieved from 1: 50,000 and 1: 2,000,000 regional hydrogeological map sheets as described above, range from 0.1 to 0.6, with an average of 0.3 ± 0.11. The river discharge of the five main rivers emptying into the eastern Hainan shelf is 2.1 × 10 7 m 3 yr −1 (Li et al, ; Liu et al, ). The groundwater exploitation of the eastern Hainan Island is 1.1 × 10 8 m 3 yr −1 or 2.9 × 10 5 m 3 d −1 (Zhou, ).…”

“…There are several estuaries, bays, and lagoons located at the eastern coasts. The project studied the SGD and the derived nutrient loadings to these coastal bays and lagoons (Ji et al, ; Li et al, ; Liu et al, ; N. Su et al, ; Wang & Du, ). Extended SGD study by Wang et al () revealed that SGD serves as a significant contributor to the acidification of coral reef system in Sanya Bay at the southeast coast of Hainan Island.…”

“…Besides the field data from the cruises, the inland groundwater data are sourced from the previous publications (Jahnke et al, ; Li et al, ; Liu et al, ; N. Su et al, ; Wang et al, ; Wang & Du, ). Along the east coast, five coastal areas (sites B–F in Figure ) have groundwater data of radium isotopes and nutrients from previous studies.…”

Submarine groundwater discharge into typical tropical lagoons:Acase study in easternHainanIsland,China

“…The DIP concentration was even higher than the DIN concentration in the well water around the XH Lagoon. This may be because of the dissolution of minerals containing P in the soil and P accumulation in the drainage channels of shrimp and fish ponds [Li et al, 2013[Li et al, , 2014Herbeck et al, 2013] and these might also be reasons for the extremely low N/P ratio reported by Ji et al [2013] since the feed of the ponds has a high concentration of P [Herbeck et al, 2013].…”