Diatom rapid sedimentation

Another type of seasonally driven export event is associated with larger diatoms (>50pm) that grow vmder nutrient- and light-limited conditions at the base of the euphotic zone. These diatoms seem to imdergo a mass settling event, called a fall dump, in response to destratification of the summer thermocline due to seasonal cooling and early winter storms. These diatoms sink rapidly and are relatively well preserved in the sediments. 

The diatom demand occurred mainly during the prestratification period. It is unlikely that external loading provided a major part of the 200 mg of P/m2 utilized during this 2-month period. Thus, the probable sources were either rapid recycling of diatom-P, other components of the particulate P pool, or bottom sediments. 

Residence Times. Phosphorus residence times with respect to major depositional processes (see Tables II and IV) are summarized in Table VI. In comparison, the total-P residence time based on external loading is about 4.5 years. Residence times were calculated for a mean water-column depth of 85 m, and steady state was assumed. Although transport of P to the sediment surface by the combination of diatoms, calcite, and terrigenous material is relatively rapid, the low burial efficiency results in a relatively long residence time for total P (about 5 years). In comparison, the residence time for Pb is about 0.6 years (20). Thus, the response time for P changes with respect to loading should be on the order of 5-15 years. 

The BSi content in ECS sediments was less than 1%, which is similar to those from the Bohai and Yellow Seas, but lower than that in Jiaozhou Bay sediment (Li et al., 2006a). The low concentrations of BSi can be attributed to the high content of SPM and the shallow euphotic zone. It has been reported that iron may stimulate diatom growth and enhance nitrate uptake in coastal areas, but limited increase in silica uptake leads to rapid dissolution of BSi. 

One use of NIRS on sediments is to replace more expensive and slower classical chemical analyses. Successful NIR calibrations for carbon, carbonate, nitrogen, and phosphorus concentrations in a sediment core were recently reported by Malley et al. (1999), which demonstrates that a rapid, non-destructive, simultaneous analysis of these elements can be achieved by the use of NIRS. In this study on the eutrophication history of the lake Arendsee, Germany, an initial attempt was also made to develop calibrations for total diatoms, as well as the relative frequencies of three dominant diatom species. A similar study was also conducted on sediment cores from four Canadian lakes. In this study, the explained variance between results from conventional chemical analysis and NIR-predicted concentrations ranged between 97 and 99% for total C, organic C, and N (Malley et al., 2(XX)). 

Several studies have surveyed diatoms, i.e. preserved sediments, on lake bottoms. Although to date we have only read of studies in Scandinavia and Scotland, these show there has been a trend towards increased acidity. Evidence from Scottish lochs indicates a slow acidification rate up to the 1900 s, which then increases from the 1940 s onwards. Where catchments are forested, the acidification rate is even faster. Evidence from a study of lakes in Sweden shows a gradual pH decline from the earliest sediments analysed in 1949, through to 1964. After 1964, pH declined more rapidly. Figure 8.2(c) shows the decrease in pH found in Lake Cardsjon in S Sweden. Another study of a single lake in Norway showed no reduction in pH over the last two centuries. 

Diatoms can multiply very rapidly and dominate in nutrient-rich marine waters. Their distinguishing feature is a hard mineral shell made by polymerized silicic acid. They float in the water column or attach to surface of sediment. Diatoms are major contributors to production, especially in spring blooms. 

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