Primary Producers 221 



the upper 2 to 3 mm of sediment. This small consumption represents about 

 0.5% to 2% of the mid-summer biomass of "surface" epipelic algae and 

 only 1% of their daily net productivity. 



Sediment Disturbance 



Animals living in the pond sediments exert a major control on epipelic 

 productivity by physical disturbance of the surface sediments. They are 

 largely responsible for the transfer of "surface" epipelic algae to "buried" 

 epipelic algae, a transfer which has the same effect as grazing in that it 

 removes a part of the biomass of photosynthesizing algae. Chironomid 

 larvae and the tadpole shrimp, Lepidurus arcticus, create the most 

 disturbance by their feeding activities in the surface sediments. Water 

 currents and the release of gas bubbles from the sediments appear to be of 

 secondary importance. The result is a very unstable substratum for the 

 epipelic algae. Moss (1968), after comparing data from different benthic 

 habitats, concluded that the relative stability of sediment, rock, and plant 

 substrata is a key factor influencing the biomass and hence productivity of 

 the benthic algae. Indeed, in mathematical simulations of epipelic algal 

 productivity in the tundra ponds (Chapter 10 and Stanley 1976b), it was 

 found that without the burial the algae at the sediment surface rapidly 

 increased to a much higher biomass than is ever observed in the ponds. 



Control of Species Composition 



> 



The phytoplankton species composition in the Barrow ponds changes 

 dramatically with season in a manner which appears to be consistent from 

 year to year. Although we can understand the factors controlling carbon 

 fixation by considering the algae as a homogeneous group, it is possible 

 that the seasonal changes in overall response are caused by changing 

 species rather than by adaptation. For example, bloom species in the 

 ponds might die out as phosphorus becomes less available. The change in 

 the ratio of photosynthesis to biomass in the spring bloom might be a 

 result of a change to species capable of tolerating reduced PO4 rather than 

 of starvation of an existing population. 



In order to answer these questions about the plankton, we have 

 developed ^^C track autoradiography techniques to study photosynthesis 

 of individual species in situ. Preliminary results show different times of 

 day for peak photosynthesis for Rhodomonas and Chromulina; this 

 implies that these algae have different V „ax and /o 5 values (Figure 5-8). 

 Whether these differences only play a part in niche separation or whether 

 they can control overall rates is unknown. An example of the response we 



