Multiyear increases in dissolved organic matter inventories at Station ALOHA in the North Pacific Subtropical Gyre

The inventories and dynamics of dissolved organic matter (DOM) in the surface water at Station ALOHA were analyzed from the Hawaii Ocean Time-series (HOT) data set for the period 1989–1999. Euphotic zone, depthintegrated (0–175 m) concentrations of dissolved organic carbon (DOC), nitrogen (DON), and phosphorus (DOP) were temporally variable. In particular, during the period 1993–1999, concentrations of DOC and DON increased while inventories of DOP remained unchanged. DOC inventories increased by 303 mmol C m22 yr21, a value equivalent to approximately 2% of measured primary production (14C method) at this site. DON increased at 11 mmol N m22 yr21, resulting in a mean molar DOC : DON ratio of 27.5 for the accumulated DOM. Accumulation of DOC and DON without corresponding accumulation of DOP resulted in changes to the bulk organic C : N : P stoichiometry; bulk DOC : DOP ratios increased 16% and DON : DOP ratios increased by 17%. These results indicate that a small fraction of the annually produced organic matter escaped biological utilization on time scales of months to years. More importantly, the accumulated DOM inventories grew progressively enriched in C and N relative to P. Fundamental changes in the North Pacific Subtropical Gyre (NPSG) habitat appear to have altered microbial processes that regulate organic matter fluxes. Considered together, the long-term increases in DOC and DON inventories are consistent with previous observations, indicating that a recent reorganization of plankton community dynamics may have altered organic matter cycling in this ecosystem. Dissolved organic matter (DOM) in seawater constitutes one of the largest exchangeable pools of reduced carbon on Earth, so accurate quantification of DOM inventories and fluxes is essential for understanding oceanic carbon cycling (Carlson et al. 1994; Ducklow et al. 1995; Williams 1995). 1 Corresponding author ([email protected]). Acknowledgments We are indebted to the many hard-working scientists who have made the HOT program a remarkable oceanographic achievement. In particular, L. Tupas, T. Walsh, H. Nolla, K. Selph, and L. Fujieki provided expert technical assistance and data accessibility. C. Winn, D. Hebel, L. Tupas, and F. Santiago-Mandujano coordinated many of the cruises examined in this study. Two anonymous reviews and comments provided by J. Cole greatly improved the manuscript. HOT program support comes from the National Science Foundation (OCE 96-17409 to D.M.K. and OCE 98-11921 to R. Lukas); M.J.C. is currently supported by a research fellowship from the Virginia Institute of Marine Science. This is SOEST contribution 5831 and U.S. JGOFS contribution 692. Efforts to quantify carbon fluxes in oligotrophic marine ecosystems indicate that significant fractions of organic carbon may cycle through DOM pools (Carlson et al. 1994; Emerson et al. 1997; Carlson et al. 1998). At Station ALOHA in the North Pacific Subtropical Gyre (NPSG), the dissolved carbon (C), nitrogen (N), and phosphorus (P) inventories in the photic zone represent 98, 95, and 94% of the total organic matter inventories, respectively. Three primary classes of bulk DOM have been defined based on their temporal lability in the marine environment: (1) labile DOM with turnover times of hours to days, (2) semilabile DOM that turns over on seasonal to annual time scales, and (3) nonlabile DOM inventories that cycle on time scales of hundreds to thousands of years (Kirchman et al. 1993). The ultimate source of DOM in the open ocean is marine primary production so across finite time and space scales, DOM production should covary with primary production. However, the exact pathways of DOM production and utilization are still poorly constrained (Ducklow and Carlson 1992). DOM is an operationally defined term used here to describe measured pools of organic carbon, nitrogen,