The Seasonal Variability and Sedimentation Rate of Suspended Matter in Photic Layer of the Black Sea

Monitoring of the annual cycle of main components of ecosystem of the Black Sea was carried out in the economic area of the ex-USSR during 1992 – 1993. As a part of the programme, suspended matter content, organic fractions and sedimentation rates in photic zone of the sea were investigated with the involvement of research vessels of the Ukrainian Research Centre of Ecology of Sea (UkrRCME). The investigation has shown that at known prevailing of particulate organic matter over abundance of structural elements of seston, seasonal changes in its content in the open sea agreed with seasonal changes in the abundance of plankton organisms; sedimentation rate of organic particles in photic zone was high and is driven by the hydrodynamic activity.


Introduction
Scientific evidence about total suspended matter (TSM), its organic fraction (suspended organic matter, SOM) and their sedimentation rates can be of use in assessment of productivity and understanding of bioproduction generation and sedimentation processes in diverse water bodies [2,17,20].
Considering this aspect, coastal zones of the Black Sea were carefully and repeatedly studied. Most of the investigations were related to geology and assessed horizontal (from the shore towards the sea) and vertical fluxes of sedimentation material [14].
In recent decades, insight into suspended organic matter (SOM) of the Black Sea was formed due to some regional [25][26][27][28] and regular investigations launched by Institute of Biology of the Southern Seas (IBSS) and Marine Hydrophysical Institute (MHI) [4]. These works have clarified the distribution and composition of SOM on the shelf and in an active layer of the deep-water zone of the Black Sea in different seasons of a year.
Little is known about SOM sedimentation in the deep-water zone of the sea. No doubt, direct measurements of sinking particulate matter using sediment traps [1,11,12,16,24] provide valuable information thereby permitting one to assess sedimentation extent; however, this technique is of little use if the researcher wants to gain insight into the early stage of sedimentation carryover of non-transformed particulate matter going in active layer of the open sea and to compare this removal and the abundance of seston in photic zone. Interestingly, some researchers [10] used 234 Th as a tracer when detecting the transfer of particulate substances from the surface layer of the Black Sea during regular surveys made in the western cyclonic gyre in different seasons of 1992 -1993. This experience is of special interest with regard to methodology, too: exactly at that time we measured SOM content and sedimentation rate by means of sediment traps (ST) positioned in different locations of the sea and in different seasons. These measurements which were within the framework of regular multidisciplinary investigations of ecosystem of the Black Sea initiated by the Ukrainian Research Centre of Marine Ecology (UkrRCME), Odessa [13,15] allow comparing the data on SOM having been obtained by different methods (see below).

Materials and Methods
According to Agreement about Scientific Cooperation between IBSS and MHI that involved investigations of annual cycle of main elements of ecosystem of the northern Black Sea, material for our investigation was collected during expeditions on the RVs of the URCME during 1992 -1993. Scientific scenario of the experiment was worked out and implemented under the supervision of V.I. Medinets. Phyto-, nano-, meso-, zoo-and bacterioplankton, TSM and SOM and their sedimentation rates in photic layer were studied by the scientific team headed by E.Z. Samyshev  For studying the particulate substances and phyto-, bacterio-and nanoheterotrophic plankton in seston, seawater samples were synchronously taken from a 30-l plastic water sampler. Samples of deposited particulate matter were collected at daily stations by means of a floating stretching vertical set of three detritus traps [5]. The device was adjusted so that the upper and the middle traps were above and below thermocline, correspondingly; and the lower was at the depth of 65 m (st. 55 and 96) and 40 m (st. 23). Exposition time varied from 12 to 23 hours. The samples were preliminary handled and the suspended matter filtered onto preliminary weighed ashless membrane filters (0.45 µm pore size) using routine procedure. SOM content was determined by combustion on the filter at 450 о С. The portion of SOM in TSM was derived from the difference between TSM and ash fraction. Weighted averages of tested seston components were calculated using planimeter.

Results and Discussion
Being aware of current terminological discrepancies, we should explain that, in our interpretation, suspended organic matter (SOM) is the organic fraction incorporated into total suspended matter (TSM). The latter, in our case, is the net fraction of seston which remained after separation of the large-size fraction using sieve № 68 (76 µm). Therefore, SOM is organic detritus (trypton), and phyto-, bacterio-, nano-and microzooplankton. In sea water, organic detritus usually contributes 80 -99% to SOM [21][22][23]. In the Black Sea, however, "suspended particles are mainly pelite (from 70 to 90%)" and "as depth increases from 8 to 15 m, content of suspended matter decrease. From 30 -35 m depth and deeper, sedimentation acquires pelagic character" [14]. The trace amounts found in the open sea are, probably, due to spatial dispersion of terrigene sediments, and seasonal cycles going in pelagic biocenosis develop against this background. Figures 2,3,4,6 and Table show that seasonal content of SOM agrees with seasonal succession of all components of living plankton. In photic layer for the period from May to November 1992, synchronously with the declined abundance of phyto-, bacterio-and nanoheterotrophic plankton, SOM has decreased three-fold from the initial estimate of 2.03 mg dry weight·l -1 , and in February 1993 -four-fold. During the first half of 1992, the estimates of SOM and other microplankton have been high until September: for instance, the average derived for phytoplankton in March -April was 2900 mg·l -1 . The depression that has stricken the biocenosis after was due to circulation transformation related to atmospheric processes. Concurrently, the observations have detected drastic spatial heterogeneity and variability of oceanographic parameters [6,13] (Fig. 5) and biological characteristics practically all over the Black Sea. At the same time, in most cases SOM values ranged from 40 to 60% of TSM, whereas the acknowledged limits are from 20 to 92% (Fig. 4).
In comparison with estimates derived for the integral Global Ocean and individually for the Pacific Ocean [17,19], our data suggest that in the Black Sea the content of SOM is an order of magnitude greater. Moreover, these estimates evidence nearly a three-fold increase of eutrophication since the 1960s [7].   PhytoPl -phytoplankton (by [3]), BactPl -bacterioplankton (by [18]), MesoZPl -mesozooplankton (by [9]), HNPl -heterothrophic nanoplankton (by [18])  As the Table shows, sedimentation rates of TSM and SOM and TSM/SOM ratios determined for different locations of the Black Sea conform to the amount of these substances in these seawater areas and, at least partially, depend on the local hydrodynamics. In May and July intensive TSM and SOM sedimentation was registered at all stations where the set of sedimentation traps was applied, and in September at st. 23, too; the rate of sedimentation fluctuated depending on depth. According to the records gained at stations, averages of the TSM and SOM flows were evaluated ca. 1.0 and ca. 0.5g . m -2. day -1 , correspondingly; increase of the sedimentation rate in July at st. 55 can be due to a local cyclonic gyre. Thermohaline disturbances were evident in the 0 -200 m layer and only enhanced in the 0 -500 m depth thereby evidencing considerable vertical extent of the phenomenon. At st. 96 estimates of sedimentation rate increased with depth in May, keeping high at 65 m depth in July; it is, probably, related to anticyclonic circulation that persisted in the area over the period of the investigation [6,8].
In September, as the dynamics in the central part of the sea (st. 55) settled down to classical pattern [4,6], sedimentation rates of TSM and SOM decreased to minimal throughout the sampled water column. In the eastern part of the sea the protracted anticyclonic circulation sustained sedimentation rates relatively high.
In conclusion, I would again emphasize the fact that the results we have obtained by direct measuring of SOM and TSM well agree with the relevant measurements made with the involvement of 234 Th in the western halistaze of the Black Sea during the same period of time [10]. Compared to the 1960s, in the 1990s SOM content in photic layer of the Black Sea has increased thrice that evidences a fast increase of eutrophication and an imbalanced trophic dynamics in the ecosystem. The seasonal fluctuations in SOM estimates which were noticed during the investigation concurred with quantitative changes in other seston components.

Conclusions
Direct measurements have shown that in the three inspected areas of the sea TSM and SOM sedimentation rates were relatively high due to the abundance and high quantitative variability of these substances, and also due to hydrodynamic conditions particular to the seawater localities.
The similarity of sediment carryout estimates which were nearly synchronously obtained by direct measurements and by means of thorium proves their reliability. of the annual cycle of basic ecosystem elements of the northern Black Sea during 1992-1993. The authors are grateful to V.T. Georgiev, S.A. Gerasimov and Yu.I. Popov (UkrRCME) who offered hydrological data. In the data analysis we also used results obtained from the hydrobiological investigations of micro-and mesozooplankton and some oceanographic surveys conducted during the expeditions and the studies of phytoplankton performed in March -April 1992 by V.N. Vasileva and L.A. Vinogradova (UkrRCME) who have offered these data in accordance with the scientific exchange procedure implied by the Agreement.