Pico-plankton

An ongoing debate concerns the extent that the current N cycle is close to steady state (see Chapter 1, Gruber, this volume). Recent estimates of denitrification considerably exceed those of N2 fixation (Codispoti et al., 2001), perhaps because of our lack of understanding of the quantitative importance of other sources of N2 fixation such as that occurring in the nano- and pico-plankton and associated with symbionts. Indeed, isotopic constraints suggest that the marine N Cycle is much closer to steady state, and that N2 fixation and denitrification are rightly coupled and in fact provide a N homeostat (Deutsch et al., 2004 Gruber, 2004). 

Suzuki, M., Rappe, M. S., and Giovannoni, S. J. (1998). Kinetic bias in estimates of coastal pico-plankton community structure obtained by measurements of smaU-subunit rRNA gene PCR amplicon length heterogeneity. Appl. Environ. Microbiol. 64, 4522—4529. 

This is because in general, in the open waters the dominant organisms involved in the phytoplankton bloom are nano- and pico-plankton which do not have a siliceous skeleton. There are few siliceous phytoplankton within the winter phytoplankton bloom and even less during the spring-summer period which follows (see Section 4.4.2.2). 

Synechococcus-like unicellular cyanobacteria are the second key component of the pelagic phytoplankton, found in practically all marine environments. Although bigger than the prochlorophytes, the unicellular cyanobacteria are also pico-planktonic measuring less than 2 pm in diameter. In the pelagic waters of the Levantine Basin, they are usually evenly distributed with depth between the surface and the DCM below which their concentrations decline with increasing depth (Li etal., 1993). 

NEM, on the other hand, was characterized by the dominance by pico-plankton, particularly Synechococcus, with lesser contributions from Prochlorococcus and picoeukaryotes (Veldhuis etal., 1997). 

Photoautotrophic flagellates are pico to nanometer-sized plankton, usually with two flagella, whip-like appendages that beat within grooves in the cell wall. They are found in most regions of the ocean. 

The phytoplankton community in the offshore region is dominated by pico- and nano-plankton, with microplankton and eukaryotes only important in coastal regions and upwelling areas associated with mesoscale features such as the Rhodes Gyre. Heterotrophic bacteria are an important component of this nutrient depleted system. Both depth integrated chlorophyll a and primary productivity levels are extremely low, characteristic of ultra-oligotrophic systems. 

Fig. 6.11 Biomasses within different size fractions at comparable stations sampled by the US JGOFS (S2-S15) and ARABESQUE (A1-A6) investigators during (a) SWM and (b) NEM (see insets of Figs 6.4 and 6.6 for station locations). For each station there is a set of three bars showing contributions by pico (filled), nano (hatched) and micro (open) plankton. The bar in the middle is for autotrophs and the other two for heterotrophs. Reproduced from Garrison etal. (2000) with permission from Elsevier Science.

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