Phenology of particle size distributions and primary productivity in the North Pacific subtropical gyre (Station ALOHA)

Abstract

The particle size distribution (PSD) is a critical aspect of the oceanic ecosystem. Local variability in the PSD can be indicative of shifts in microbial community structure and reveal patterns in cell growth and loss. The PSD also plays a central role in particle export by influencing settling speed. Satellite-based models of primary productivity (PP) often rely on aspects of photophysiology that are directly related to community size structure. In an effort to better understand how variability in particle size relates to PP in an oligotrophic ecosystem, we collected laser diffraction-based depth profiles of the PSD and pigment-based classifications of phytoplankton functional types (PFTs) on an approximately monthly basis at the Hawaii Ocean Time-series Station ALOHA, in the North Pacific subtropical gyre. We found a relatively stable PSD in the upper water column. However, clear seasonality is apparent in the vertical distribution of distinct particle size classes. Neither laser diffraction-based estimations of relative particle size nor pigment-based PFTs was found to be significantly related to the rate of ¹⁴C-based PP in the light-saturated upper euphotic zone. This finding indicates that satellite retrievals of particle size, based on particle scattering or ocean color would not improve parameterizations of present-day bio-optical PP models for this region. However, at depths of 100-125 m where irradiance exerts strong control on PP, we do observe a significant linear relationship between PP and the estimated carbon content of 2-20 μm particles.

Keywords: North Pacific; laser diffraction; particle size; phytoplankton; productivity.

Figure 1

The mean PSD derived from a spherical inversion (a), random inversion (c) and n = 1.17 (e). Data are binned to 2 m depth bins, and each sample represents the median of all available LISST profiles at that depth. The black line indicates the mean power law fit. The normalized bias of a power law fit is shown for the spherical (b), random (d) and n = 1.17 (f) inversions as a function of size (x‐axis) and depth (color scale).

Figure 2

(a–c) Monthly mean (±standard error) contributions of picophytoplankton, nanophytoplankton, and microphytoplankton to chlorophyll a as determined by the HPLC‐based algorithm (see equation (1)–(4)) for samples collected within the upper 45 m (circles, including standard sampling depths of 5, 25, and 45 m) and for samples collected between 100 and 125 (squares) over the period of 1988–2013. Anomaly (fraction—monthly mean for each standard sampling depth) of the relative contributions of picophytoplankton, nanophytoplankton, and microphytoplankton relative to paired measures of net in situ primary productivity in the upper 45 m (d–f) and 100–125 m, (g–i) also corrected to remove the monthly climatological mean. Colors correspond to sampling month. No significant statistical relationships were found between the HPLC‐based size composition and primary productivity (t test, p > 0.05).

Figure 3

Seasonal cycle of monthly mean (±standard error) NPP (a) and HPLC chlorophyll a (b) for samples collected within the upper 45 m (circles, including standard sampling depths of 5, 25, and 45 m) and for samples collected between 100 and 125 (squares) over the period of 1988–2013.

Figure 4

Relationship between NPP measured between 25–125m, (mg m⁻³ d⁻¹) and (a) the average particle diameter (D_avg) and (b) the absolute value of exponent of a power law fit (ξ) of the particle size distribution. Colors correspond to sampling depth. The mean depth profile of D_avg and ξ for winter (November–January, blue), spring (February–May, green), summer (June–August, red), and fall (September–October, orange) and shown in Figures 4c and 4d, respectively. While shown, values at less than 20 m are not considered due to the potential influence of bubbles (<20 m). In the 20–175 m strata, the mean ± the standard deviation for ξ was 4.2 ± 0.7, whereas the D_avg was 13.5 ± 5.7 μm (n = 63 casts binned to 2 m resolution for a period spanning September 2009 to April 2014). Larger values of ξ indicate greater contributions by small particles; smaller values of ξ indicate greater contributions by large particles.

Figure 5

Contour plots of particle volume in the following size classes: (a) 1.25–2.0 μm, (b) 2.0–20 μm, and (c) 20–100 μm as measured at approximately monthly intervals between September 2009 and April 2014. Breaks in coverage are a result of gaps in instrument availability or cruise scheduling.

Figure 6

Mean particle volume normalized to the maximum particle volume for each profile in two size classes: (a) 1.25–2 μm and (b) 2–20 μm. Profiles are shown as the mean for each season (months noted in the legend). Note that the x‐axis limits differ. The depth profile of primary productivity measured at HOT standard sampling depths (5, 25, 75, 100, and 125 m) is shown in panel C, where the central mark of each box is the median, the edges of the boxes are the 25th and 75th percentile and whiskers extend to the 95th percentile. Outliers are plotted as crosses.

Figure 7

Spectra of the carbon content of particles of equivalent spherical diameter [Carbon (D)] of 2–20 μm for (a) 25 m and (b) 125 m. At both depth horizons, a peak in carbon content is found at 5 μm; however, the peak is broader at 125 m.

Figure 8

Box and whisker plot of weighted mean particle diameter (D_avg) at the depth horizon of (a) 25 ± 2 m and (b) 125 ± 2 m. The climatological median ± one standard deviation was 10.3 ± 4.7 μm at 25 m and 10.9 ± 7.3 at 125 m. In both plots, the central mark of each box is the median, the edges of the boxes are the 25th and 75th percentile and whiskers extend to the 95th percentile. Outliers are plotted as crosses. Data are derived from 2 to 3 nighttime casts per cruise.

Figure 9

Relationship between the rate of NPP measured in 12 h in situ incubations at Station ALOHA between September 2009 and December 2012 and the carbon content of particles in the following size classes: (a) 1.25–2.0 μm, (b) 2.0–2 0 μm, (c) 20–100 μm as well as (d) particulate carbon collected by HOT program. LISST data are the mean of values measured within ±2m of the depth of NPP incubations. Colors correspond to depth. Total particulate carbon and particles in the 2–20 μm range show a positive relationship to measured productivity rates, however this relationship is only linearly significant at 100 and 125 m depth horizons (Type II linear regression shown in Figure 9b: slope = 0.36 d⁻¹, R² = 0.41; Type II linear regression shown in Figure 9d: slope = 0.13 d⁻¹, R² = 0.43).