Abstract
Global and regional ocean primary production estimates are highly dependent on assumptions concerning the photosynthetic potential of the resident phytoplankton communities. Little is known, however, about global patterns in the distribution of photosynthetic potential and their causes. Here, we review existing literature reporting photosynthetic characteristics of natural populations. From this, we formulate hypotheses regarding abiotic and biotic factors of potential importance in determining photosynthetic performance. These hypotheses are then tested using data we have compiled from nearly all major ocean basins on the maximum rate of photosynthesis, PBmax, and the slope of the photosynthesis vs. light curve, αB (both parameters normalised to chlorophyll) as well as standard environmental variables, size fractioned chlorophyll, taxonomic data (to group), size and biovolume data for pico-, nano-, and micro-phytoplankton. In terms of abiotic variables, depth of sampling, temperature and nutrient availability all can be related to photosynthetic parameters. The most important biotic variable influencing photosynthetic performance was found to be community size distribution and the small component (i.e. the proportion of the phytoplankton community passing through a 10 µm filter) is shown to have both higher PBmax and αB than the larger phytoplankton component. A simple model was used to derive best fit values for PBmax (1.53/2.50 µgC l-1 h-1) and αB (0.025/0.040) for the large/small groups in the subset of the data where taxonomic data were available (both surface and sub-surface samples) using fractioned chlorophyll data and bulk community photosynthetic parameters. Non-metric multidimensional scaling (NMDS) was used to relate the distribution of photosynthetic parameters and dominant (by biovolume) phytoplankton groups. High PBmax was recorded in communities dominated by dinoflagellates, small flagellates and, in warmer waters, picoeukaryotes and Synecococcus. Diatom dominated communities exhibited lower PBmax and were associated with high inorganic nutrients and colder temperatures. That photosynthetic parameters appear closely related to community size distributions and taxonomic group provides some hope for improving the parameterization of photosynthetic performance in global ocean primary production estimates as both of these parameters can be made from remotely sensed optical characteristics of surface waters.
Highlights
Focus has in recent years increasingly been moving toward developing a more complete understanding of the functioning of the Earth System (ES) and how it may be changing in response to human activities
As background for this study, we surveyed the literature for reports of PBmax determined on natural phytoplankton communities (Table 1)
A number of the studies found had identified specific abiotic and/or biotic variables as being correlated with photosynthetic performance and we examine here all of the variables identified in these studies as potentially being related to photosynthetic characteristics in relation to characteristics of the P vs. E curves constructed from incubations of samples from the surface and the deep chlorophyll maximum in samples collected from most major ocean basins
Summary
Focus has in recent years increasingly been moving toward developing a more complete understanding of the functioning of the Earth System (ES) and how it may be changing in response to human activities. All of the algorithms used to estimate global ocean PP today from data collected by remote sensing include a component describing the photosynthetic potential of the community (Behrenfeld and Falkowski, 1997a and several studies e.g., Behrenfeld and Falkowski, 1997b; Carr et al, 2006) have identified this model component as being important in driving model results They have argued that improvement in the estimation of ocean PP will require a better understanding and parameterization of the factors influencing photosynthetic potential. In what is probably the most commonly applied algorithm for estimating ocean PP from remotely sensed surface optical characteristics, the Vertically Generalized Production Model (Behrenfeld and Falkowski, 1997b), phytoplankton photosynthetic potential, i.e., the maximum rate of photosynthesis normalized to chlorophyll, PBmax, is described as a function of temperature, where the relationship to temperature is derived from empirical data collected in two different geographic regions. As this survey identified community size distribution and taxonomic characteristics as being potentially important biotic factors in controlling photosynthetic characteristics of a given community, a particular focus of this study is on the elaboration of the potential influence of these two parameters on community photosynthetic characteristics
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