45 



2.2.3 



MARINE FOOD CHAINS 



Dr. Lawrence R. Pomeroy 



Department of Zoology 



University of Georgia 



Athens, GA 30602 



Marine foodchain research has undergone a minor revolution over the past decade, evolving from the view that 

 marine foodchains are short and simple to the view that food webs are complex, composed of many frequently 

 changing short chains. In oligotrophic subtropical water, a major part of primary production is by the 

 cyanobacterial genus, Synechococcus and by picoeukaryotes that are from 0.5 to 5 um in maximum dimension. 

 What is most revolutionary is the realization that more than 50% of consumption of primary production is by 

 microorganisms, bacteria and protozoans. Bacteria utilize both dissolved and particulate materials in the sea. 

 They utilize dissolved organic carbon (DOC) released by phytoplankton and DOC excreted by zooplankton, 

 including protozoans. They utilize fecal materials, some of which are compact pellets but much of which are 

 rather diffuse particles. They also utilize senescent phytoplankton and allochthonous inputs from river plumes 

 and continental fallout. Because phytoplankton tend to grow very rapidly when nutrients are available, 

 zooplankton having longer life cycles cannot catch up, and so the phytoplankton deplete the nutrients and become 

 stressed, sick or dead. In this state they are invaded by bacteria and sometimes fungi and thraustochytrids. 

 Protozoans varying from a few um to several cm in size, utilize the picoautotrophs, the bacteria, and some even 

 catch and eat large diatoms and crustacean zooplankton (Reid et al. 1989). So marine foodchains are dominated 

 by microbial metabolism, with bacteria competing with zooplankton and their consumers for particulate material 

 (Pomeroy and Weibe 1988) (Figures 1 and 2). 



Many measurements of photosynthesis have been made, but serious questions remain about our ability to 

 estimate primary production on a regional scale. Few measurements of respiratory rate have been made in the 

 sea and coastal waters, because the technology of measuring very small changes in dissolved oxygen has been 

 challenging. We are now beginning to accomplish rate measurements. On the Southeast Atlantic shelf, Griffith 

 (1990) found respiratory rates exceeded photosynthetic rates: the continental shelf waters are heterotrophic, and 

 nearly all of this respiration is bacterial. This was predicted on theoretical geochemical grounds by Smith and 

 McKenzie (1987) (Figure 3). Thus, we see marine food webs as microbially dominated systems in which 

 zooplankton and fishes generally compete poorly. They compete comparatively well in highly productive systems 

 like the TEXLA shelf waters. 



Will measurements of ecosystem processes, such as photosynthesis and respiration, be useful to managers who 

 are concerned with the effects of potentially toxic materials released as a result of offshore and coastal oil and 

 gas operations? The answer is not a satisfying one. Many studies in both fresh and marine water suggest that 

 the community as a whole tends to be resilient to perturbations unless and until they are extreme. This resilience 

 is the result of the natural species diversity of aquatic and especially marine systems. At any time one or a few 

 species of phytoplankton, zooplankton, and microorganisms will be numerically and metabolically dominant. But 

 many rare species that have different requirements and sensitivities lie in wait. If there is a change in nutrient 

 concentrations, toxin concentrations, temperature, or water transparency, new dominants will emerge rapidly. 

 Rates of photosynthesis and respiration may change little, except for transitory variations, unless the perturbation 

 exceeds the tolerance of all species in that community. Numbers of bacteria were seen to change little at an 

 ocean dump site, but species composition of culturable bacteria changed markedly (Singleton et al. 1984). 

 Other studies suggest that rate processes can indeed be highly sensitive to seemingly minor perturbations. 

 Chavez and Barber (1987) measured photosynthesis by modern, toxin-free techniques in the equatorial Pacific 

 comparing Niskin samplers, a bucket, and Go-Flo samplers. The rates measured in water drawn from the Niskin 

 samplers were < 10% of those measured in water from the bucket or the Go-Flo samplers. We now know that 

 this is because the rubber used inside the Niskin sampler to close the end caps is toxic to phytoplankton, even 

 after months of use in the sea, and even when the exposure is for just a few minutes. 



Boesch (1984) cites a case in which the high variability of biological samples is reason for discontinuing a 

 monitoring program. Perhaps the problem was as much with the questions asked and the expectations for 

 answers as with the monitoring program itself. Marine ecosystems, especially tropical and subtropical ones, are 



