Light driven seasonal patterns of chlorophyll and nitrate in the lower euphotic zone of the North Pacific Subtropical Gyre

Abstract

The euphotic zone below the deep chlorophyll maximum layer (DCML) at Station ALOHA (a long-term oligotrophic habitat assessment; 22Њ45ЈN, 158Њ00ЈW) transects the nearly permanently stratified upper thermocline. Hence, seasonal changes in solar radiation control the balance between photosynthesis and respiration in this lightlimited region. Combining profiles of radiance reflectance, algal pigments, and inorganic nutrients collected between January 1998 and December 2000, we explore the relationships between photosynthetically available radiation (PAR), phytoplankton biomass (chlorophyll a), and the position of the upper nitracline in the lower euphotic zone. Seasonal variations in the water-column PAR attenuation coefficient displace the 1% sea-surface PAR depth from approximately 105 m in winter to 121 m in summer. However, the seasonal depth displacement of isolumes (constant daily integrated photon flux strata) increases to 31 m due to the added effect of changes in sea-surface PAR. This variation induces a significant deepening of the DCML during summertime with a concomitant increase in chlorophyll a and the removal of 36 mmol m Ϫ2 inorganic nitrogen [NO ϩ NO ] in the 90-200-m depth range,equivalent to approximately 34% of the annual flux of particulate nitrogen collected in sediment traps placed at 150 m. We conclude that in this oceanic region the annual light cycle at the base of the euphotic zone induces an increase in the phototrophic biomass analogous to a spring bloom event.

Characterization of the bio-optical anomaly and diurnal variability of particulate matter, as seen from scattering and backscattering coefficients, in ultra-oligotrophic eddies of the Mediterranean Sea

“…These water depths roughly correspond to the depths of the euphotic zone (83 m for A, and 102 m for C) and are slightly deeper than the top of the nitricline which were observed at 72, and 93 m, respectively . These results are in good agreement with previous 3300 H. Loisel et al: Bio-optical anomaly and diurnal variability studies performed in oligotrophic areas (Moutin and Raimbault, 2002;Marty et al, 2002;Letelier et al, 2004;Uitz et al, 2006). The TChl-a values at the DCM are 0.49 and 0.46 mg m −3 which, compared to their surface values, correspond to an increase in TChl-a with depth by a factor of 10.5 and 14, respectively.…”

Predictable and efficient carbon sequestration in the North Pacific Ocean supported by symbiotic nitrogen fixation

“…In marine ecosystems, summertime density stratification of the upper water column can lead to abrupt and large increases in the daily irradiance for cells at a given depth, even with an otherwise constant light field at the ocean's surface. Furthermore, because light penetration is unaffected by the mixing depth and decreases exponentially in the water column, small changes in stratification can lead to large changes in solar photon flux for cells growing in the mixed layer (66). At Station ALOHA, there is a large change in mixed layer depth from April to May; thereafter, it remains relatively stable until September, long after the SEP (Fig.…”

Seasonal Surface Chlorophyll a Variability in the Seychelles–Chagos Thermocline Ridge

“…The surface PAR averaged over WSCTR and ESCTR ranges from ~36 to ~48 Einstein/m 2 /day (Figure 7 i). Latelier et al 36 used surface PAR value less than 32 (Einstein/m 2 /day) to represent the less productive winter light condition in the North Pacific subtropical gyre. The satellite-derived surface PAR values noted in WSCTR and ESCTR were always greater than 32 Einstein/m 2 /day and reached 48 Einstein/m 2 /day.…”