Temporal dynamics of total microbial biomass and particulate detritus at Station ALOHA

Particulate adenosine-5′-triphosphate (P-ATP) and particulate carbon (PC) concentrations were measured on approximately monthly intervals throughout the upper water column (0–1000 m) over a 30-yr (1989–2018) period at Station ALOHA to track the seasonal-to-decadal variability in total microbial biomass and the dynamics of living-to-nonliving particulate organic matter pools. On selected cruises, samples were also collected to a depth of ∼4800 m. P-ATP concentrations were relatively uniform (27–34 ng l⁻¹) throughout the upper euphotic zone (0–100 m) with a distinct peak at 45 m. P-ATP concentrations were significantly higher (p < 0.001) in summer (Jun-Aug) than in winter (Dec-Feb), especially between 45 and 100 m where the seasonal differences averaged 28%. Below 100 m, P-ATP concentrations decreased rapidly with depth to a 30-yr mean value of 3.5 ng ATP l⁻¹ at 250 m, and then decreased more gradually to a 30-yr mean value of 0.9 ng ATP l⁻¹ at 1000 m. Between 125 and 175 m, the seasonal peak in P-ATP shifted to spring (Mar-May), with minima in fall (Sep-Nov) and winter (Dec-Feb). No consistent seasonal variations in P-ATP were detected at depths >175 m, suggesting a temporally stable habitat. Assuming a PC:P-ATP ratio of 250:1 (g g⁻¹), the 0–100 m, 100–250 m, and 250–1000 m depth-integrated microbial biomass estimates were 775, 425, and 350 mg C m⁻², respectively. Bathypelagic zone (>1000 m) P-ATP concentrations, based on a more limited data set than the upper portions of the water column, were low (0.4–0.7 ng ATP l⁻¹). However, when integrated over the entire deep water habitat (1000–4800 m), bathypelagic zone microbial biomass was substantial (425 mg C m⁻²). Expressed as a percentage of total PC, microbial biomass ranged from ∼30% in the upper euphotic zone to ∼3% at depths >3000 m, emphasizing the preponderance of detrital PC throughout the entire water column. The total water column inventory of microbial biomass at Station ALOHA was 2 g C m⁻² compared to ∼18 g C m⁻² for total suspended PC; approximately 50% of the total microbial biomass is resident in the aphotic zone (>175 m). Although daily gross primary production at Station ALOHA is on par with the total euphotic zone (0–175 m)-integrated microbial biomass (∼1 g C m⁻² d⁻¹ and ∼1 g C m⁻², respectively), the sources of C and energy fueling the substantial aphotic zone microbial biomass (∼1 g m⁻²) are not well understood at the present time.