Water column phytoplankton

Bianchi, T.S., Engelhaupt, E., McKee, B.A., Miles, S., Elmgren, R., Hajdu, S., Savage, C., and Baskaran, M. (2002a) Do sediments from coastal sites accurately reflect time trends in water column phytoplankton A test from Himmerfjarden Bay (Baltic Sea proper). Limnol. Oceanogr. 47, 1537-1544. 

Comparison with other Studies. How do the results of our investigation compare with similar studies Our results corroborate the data provided in a similar study of the effect of UV-B on primary productivity in the southeastern Pacific Ocean (35). In the latter study, it was noted that enhanced UV-B radiation caused significant decreases in the productivity of surface and deep samples. Compared to ambient, primary productivity decreased with increasing doses of UV-B. In another study in which in situ experiments using natural Antarctic phytoplankton populations, it was noted that incident solar radiation significantly depressed photosynthetic rates in the upper 10-15 meters of the water column (36). It was also found that the spectral region between 305 and 350 nm was responsible for approximately 75 percent of the overall inhibitory effect. 

Oceanic surface waters are efficiently stripped of nutrients by phytoplankton. If phytoplankton biomass was not reconverted into simple dissolved nutrients, the entire marine water column would be depleted in nutrients and growth would stop. But as we saw from the carbon balance presented earlier, more than 90% of the primary productivity is released back to the water column as a reverse RKR equation. This reverse reaction is called remineralization and is due to respiration. An important point is that while production via photosynthesis can only occur in surface waters, the remineralization by heterotrophic organisms can occur over the entire water column and in the underlying sediments. 

It is important to note however that Equation (10) assumes steady state in the Th distribution so that production truly is balanced by decay and export. It is easy to imagine a scenario after a phytoplankton bloom, when the export of POC (and " Th) has decreased or even ceased, such that the water column " Th profile would still show a deficit with respect to caused by prior high flux events. This relief deficit will disappear as " Th grows into equilibrium with on a time scale set by the " Th half-life. The magnitude by which the Th flux is over- or under-estimated depends on whether deficits are increasing or decreasing and at what rate. 

Differences in integration time scales may also affect our perception of key derived parameters such as the ThE ratio (Cochran et al. 2000). This ratio (see above) compares the POC flux derived from water column " Th profiles (and thus integrating into the past) with present primary production. As classically measured using incubation techniques, primary production is an instantaneous measurement representing the phytoplankton community as sampled at a single time. Under bloom conditions, the export of POC may lag the production of fresh organic matter and ThE ratios calculated late in a bloom may be overestimates. 

In addition to the dissolved elements and compounds in the oceanic water column, a wide variety of water column chemicals are found in marine organisms and organic detritus. For example, a milliliter of surface seawater can contain on the order of 10 million viruses, 1 million bacteria, 100,000 phytoplankton, and 10,000 zooplankton [9]. With the advent of soft ionization processes for mass spectrometry systems, scientists have been able to study these marine organisms at molecular level. The use of electrospray ionization (ESI see Section 2.1.15), atmospheric pressure chemical ionization... 

Other chemically non-characterised UV-absorbing compounds have been reported in the water column near phytoplankton blooms such as the so-called P380, which exhibits abroad absorbance between 300 and 470 nm with a maximum at about 380 nm (Llewellyn and Mantoura 1997). 

Redfield (1934), who analyzed the major elemental content of many samples of mixed plankton (phytoplankton and zooplankton) caught in nets towed through the surface ocean. They compared the carbon, nitrogen, and phosphorus composition of these collections to concentration profiles of dissolved inorganic carbon (DIC), NOs, and P04 throughout the water column. This pioneering research demonstrated that these three elements are continually redistributed in the ocean by selective removal into plankton cells and their remains (i.e., fecal pellets), which are then efficiently respired as they sink through the marine water column. 

This process is commonly referred to as assimilatory nitrogen (nitrate or nitrite) reduction. The electrons for these reductions are supplied by half-cell oxidations involving NADPH/NADP" and NADH/NAD" (Table 7.11). All of these reactions and membrane transport processes are mediated by enzymes that are specific to the DIN species. Considerable variation exists among the phytoplankton species in their ability to produce the necessary enzymes. Since marine phytoplankton are often nitrogen limited, the quantity and type of DIN available in the water column can greatly influence overall phytoplankton abundance and species diversity. 

The interplay of physical controls is less complicated in the Polar and Trade (tropical) domains. As shown in Figure 24.11a, only one phytoplankton bloom occurs in the Polar domain, but is larger in amplitude than at mid-latitudes (Westerlies). Phytoplankton growth in the subpolar region is prolific because uniformly cold atmospheric temperatures suppress density stratification of the water column. Abundant winds ensure that... 

Figure 14. Water column analysis involving pump profiling system. In the system pictured, conductivity (an index of salinity) and temperature are profiled. The fluorometer measures ij vivo fluorescence from chlorophyll (from all phytoplankton) and discrete samples are taken for cell counts and nutrient chemistry. Photodetectors can be employed for the measurement of bioluminescence. Light measurements range from Secchi disc readings to more sophisticated transmissometry and spectral radiometry instruments.

This scenario should be considered as a maximum effect of the hypolim-netic contribution because it does not include regeneration of 15N in the hypolimnion from sinking phytoplankton and the subsequent transport of this 15N back into the upper water column. Clearly, this kind of dilution is insufficient to account for the slow labeling of POM at the start of the experiment. 

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