Dale V. Hebel

The role of nitrogen fixation in biogeochemical cycling in the subtropical North Pacific Ocean

Seven years of time-series observations of biogeochemical processes in the subtropical North Pacific Ocean gyre have revealed dramatic changes in the microbial community structure and in the mechanisms of nutrient cycling in response to large-scale ocean–atmosphere interactions. Several independent lines of evidence show that the fixation of atmospheric nitrogen by cyanobacteria can fuel up to half of the new production. These and other observations demand a reassessment of present views of nutrient and carbon cycling in one of the Earth′s largest biomes.

Seasonal and interannual variability in primary production and particle flux at Station ALOHA

A 5-year time-series study of primary production and euphotic-zone particle export in the subtropical North Pacific Ocean near Hawaii (Sta. ALOHA, 22°45′N, 158°W) with measurements collected at approximately monthly intervals has revealed significant variability in both ecosystem processes. Depth-integrated (0–200 m) primary production averaged 463 mg C m−2 day−1 (s = 156, n = 54) or 14.1 mol C m−2 year−1. This mean value is greater than estimates for the North Pacific Ocean gyre made prior to 1984, but conforms to data obtained since the advent of trace metal-clean techniques. Daily rates of primary productivity at Sta. ALOHA exhibited interannual variability including a nearly 3-year sustained increase during the period 1990–1992 that coincided with a prolonged El Nifio-Southern Oscillation (ENSO) event. Export production, defined as the particulate carbon (PC) flux measured at the 150 m reference depth, also varied considerably during the initial 5 years of the ongoing field experiment. The PC flux averaged 29 mg C m−2 day−1 (s = 11, n = 43) or 0.88 mol Cm−2 year−1. A 5-fold variation between the minimum and maximum fluxes, measured in any given year, was observed. During the first 3 years of this program (1989–1991), a pattern was resolved that included two major export events per annum one centered in late winter and the other in late summer. After 1991, export production exhibited a systematic decrease with time during the prolonged ENSO event. When expressed as a percentage of the contemporaneous primary production, PC export ranged from 2 to 16.9%, with a 5-year mean of 6.7% (s = 3.3, n = 40). Contrary to existing empirical models, contemporaneous primary production and PC flux were poorly correlated, and during the ENSO period they exhibited a significant inverse correlation. This unexpected decoupling of particle production and flux has numerous implications for oceanic biogeochemical cycles and for the response of the ocean to environmental perturbations.

Trichodesmium Blooms and New Nitrogen in the North Pacific Gyre

A method of protecting plant life from injury due to frost or sub-freezing temperatures which comprises applying to the plant an effective, but non-phytotoxic amount, having regard to the plant being treated of a compound of the formula: wherein R1 is lower-alkyl; R2 is selected from the group consisting of:

Freezing as a method of sample preservation for the analysis of dissolved inorganic nutrients in seawater

It is often desirable or necessary to store collected seawater samples prior to analysis for dissolved inorganic nutrients. It is therefore important to establish preservation and storage techniques that will ensure sample integrity and will not alter the precision or accuracy of analysis. We have performed a series of experiments on the storage of nutrient samples collected at the oligotrophic North Pacific benchmark Station ALOHA, using both standard autoanalyses and low-level techniques. Our results reveal that for oligotrophic oceanic waters, the immediate freezing of an unfiltered water sample in a clean polyethylene bottle is a suitable preservation method. This procedure is simple, it avoids potentially contaminating sample manipulations and chemical additions, and it adequately preserves the concentrations of nitrate + nitrite, soluble reactive phosphate, and soluble reactive silicate within a single water sample.

Seasonal variability in the phytoplankton community of the North Pacific Subtropical Gyre

Time series measurements of in situ fluorescence, extracted particulate chlorophyll a, primary productivity, extracted adenosine 5′-triphosphate, and fluorescence per cell, as measured by flow cytometry, demonstrate seasonal cycles in fluorescence and chlorophyll concentrations in the North Pacific Subtropical Gyre (22° 45′N, 158° 00′W). Two opposing cycles are evident. In the upper euphotic zone (0–50 m), chlorophyll a concentrations increase in winter, with a maximum in December, and decrease each summer, with a minimum in June or July. In contrast, chlorophyll a concentrations in the lower euphotic zone (100–175 m) increase in spring, with a maximum in May, and decline in fall, with a minimum in October or November. The winter increase in chlorophyll a concentration in the upper 50 m of the water column appears to be a consequence of photoadaptation in response to decreased average mixed-layer light intensity rather than a change in phytoplankton biomass. In the lower euphotic zone, however, the seasonal cycle in pigment concentration does reflect a change in the rate of primary production and in phytoplankton biomass as a consequence of increased light intensity in summer. These observations have important implications for phytoplankton dynamics in the subtropical oceans and for remote sensing of phytoplankton biomass.