Nutrient pulses driven by internal solitary waves enhance heterotrophic bacterial growth in the South China Sea

This study demonstrated the potential effects of internal waves (IWs) on heterotrophic bacterial activities for the first time. Nine anchored studies were conducted from 2009–2012 in the South China Sea areas with different physical conditions, i.e. areas subjected to elevation IWs, to depression IWs, and to weak/no IWs. The latter two areas were treated as the Control sites. Field survey results indicated that within the euphotic zone, the minima of the depth-averaged bacterial production (IBP; ∼1.0 mgC m⁻³ d⁻¹) and growth rate (IBμ; ∼0.1 d⁻¹) at all sites were similar. Except for one case, the maxima of IBP (6–12 mgC m⁻³ d⁻¹) and IBμ (0.55–1.13 d⁻¹) of the elevation IWs areas were ∼fivefolds higher than those of the Control sites (IBP 1.7–2.1 mgC m⁻³ d⁻¹; IBμ 0.13–0.24 d⁻¹). Replicate surveys conducted at the north-western area of the Dongsha atoll during spring-to-neap (NW1 survey) and neap-to-spring (NW2 survey) tide periods showed a great contrast to each other. Low variation and averages of IBμ in NW1 survey were similar to those of the Control sites, while those in NW2 were similar to the other elevation IWs sites with larger variation and higher averages of IBμ. This finding suggests that bacterial activities may be a function of the lunar fortnightly (14-day) cycle. Enrichment experiments suggested more directly that the limiting inorganic nutrients introduced by the elevation waves (EIWs) may contribute a higher IBμ within the euphotic zone.