Linkage of small-scale spatial variations in DOC, inorganic nutrients and bacterioplankton growth with different coral reef water types

Abstract

This study focuses on links of dissolved organic carbon (DOC), inorganic nutrients (DIN and DIP) and bacterioplankton growth with specific coral reef water types. At 5 reef stations along the SW coast of Curacao (Netherlands Antilles), 6 different water types (4 reef water types (live coral surface contact water, reef crevice water, reef bottom water, reef overlying water) and 2 reference water types collected offshore from each station at 2 and 8 m depth) were analysed for DOC, DIN and DIP, bacterial production and abundance. Consistent patterns were found: reef waters were enhanced in DIN, DIP and specific bacterial growth rate (µn) compared to offshore waters. DOC, DIP, and µn were higher close to the reef bottom than in the reef overlying water. Coral surface contact water was significantly enhanced in DOC compared to reef crevice and reef bottom water, suggest- ing that the major source of DOC in reef waters are the live corals. DIN concentration was highest in reef crevice water, suggesting that crevices are the major net N-regenerating spaces on reefs, more important than the reef bottom between corals. Small-scale DIP distribution suggested net P regen- eration in reef crevice and coral surface contact water. The highest specific growth rates of bacterio- plankton were found in the coral surface contact water, with slightly lower rates in reef crevice water. Experiments showed that the labile DOC fraction (LDOC) in reef crevice water was substantially smaller than in coral surface contact and reef overlying water, which suggests that crevices are apparently sinks of LDOC. In reef crevice and reef bottom water the low substrate LDOC:DIN ratios suggest that LDOC might be the growth-limiting factor of bacterioplankton in these water types. In coral surface contact and reef overlying water the substrate LDOC:DIN ratios of more than 40 (and LDOC:DIP ratios of over 800) and the significant relations between µn and DIN (DIP) suggest nutrient limitation of bacterioplankton growth in these water types. This study clearly illustrates the close link in reef waters between coral reef benthos and bacterioplankton trophodynamics.