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In this report measurements of photosynthetic rates per unit of phytoplankton volume …


Biology Articles » Hydrobiology » Photosynthesis in naturally reared aquatic communities » Discussion

Discussion
- Photosynthesis in naturally reared aquatic communities

The method of measuring photosynthesis described above has three advantages which recommend it to students of aquatic photosynthesis: (1) it does not require pretreatment of the water, or elaborate precautions to avoid oxygen contamination, as methods of measuring oxygen evolution do, (2) the measuring technique leaves the samples unchanged so repeated observations can be made on the same sample, and (3) it is a simple, rapid method which requires no elaborate equipment. Although artificial light was used in the present study the method can readily be adapted to experiments under natural light.

The data presented above represent an attempt to measure the photosynthetic activity of the communities arising naturally in western Lake Erie. Other methods have been devised to obtain such information, notably the clear and blackened bottle technique used by RILEY (3). (He has also applied the technique widely in studies of marine production.) The present report affords a marked contrast, in two aspects, to his 02-production values obtained in Linsley Pond: (1) the high correlation between C02-absorption and phytoplankton volume reported here as compared to a statistically insignificant correlation between standing crop and 02-production in Linsley Pond, and (2) the high rate of apparent photosynthesis (86%o of total photosynthesis) reported here, as compared to a negative value for apparent photosynthesis (02-consumption exceeded 02-production) in Linsley Pond. It is obvious that Riley's experiments differ in several respects from those reported here. Both methods, however, are attempts to isolate the primary producers under approximately natural conditions. Whether concentrating the populations and subjecting them to artificial light for a few hours is a greater departure from the natural than confining unconcentrated populations in clear and blackened bottles for seven days is a debatable question. It seems logical, however, to expect that a rather good correlation exists between natural photosynthesis and phytoplankton volume, and that apparent photosynthesis is usually positive in natural waters. If the latter were not true there would be no organic matter deposited on the bottom. The high correlation evident in figure 2 suggests that the major components of the communities which are adapted to a particular aquatic habitat exhibit closely similar photosynthetic rates per unit. of plant volume. This observation has an important bearing on aquatic production. Many students of productivity consider the standing crop of phytoplankton an unreliable index of production. This concept contains the tacit assumption that photosynthetic rates per unit volume vary widely with varying conditions and different organisms. The present study suggests that such variations may not be so erratic as has been assumed, and that the standing crop, since it represents the photosynthetic equipment on which production is based, may be worthy of reconsideration as an index of productivity. The average annual standing crop, at least, when based on a sufficient sampling frequency and expressed in volumetric phytoplankton units, may provide a fairly satisfactory criterion for estimating the base of the production pyramid.


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