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Primary production - a basic factor in the sea

water quality assessment in the middle Adriatic

I. Marasovic & Z. Nincevic

Institute of Oceanography and Fisheries, P.O. Box 500, 21000Split, Croatia

E-mail: marasovic@jadran. izor. hr

Abstract

The results of a long-term study (15 years) are used in this paper. The study includedmonitoring of the coastal waters from Zadar to Dubrovnik (middle Adriatic) withidentification of the eutrophication level in this area. A parallel monitoring program of theopen sea water was also done. These results were compared and used to differentiate betweenthose changes brought about trough natural fluctuations or trough anthropogenous factors.This study deals only with the summer period, during which the sampling was most regular,with added emphasis on the surface layer-where the metabolic activity is highest. With highersummer sewage waste input and the presence of a thermocline, or picnocline, the surface layeris most affected by coastal factors. From the examined data, an increasing trend ofeutrophication is shown in the coastal waters.

1 Introduction

Primary production is the first link in the marine food web, and entire secondaryproduction in the sea is based on this first step. Every imbalance and all changesin an ecosystem is firstly shown at this primary trophic level. Thus, with regularmonitoring of the primary production, it is possible to get a very good insightinto the state of a given ecosystem.

The increase of eutrophication of mid-Adriatic coastal waters at thebeginning of 1980's was recorded through the increase of primary productionand the changes in the structure of phytoplankton community (Pucher-Petkovic& Marasovic, 1980% Pucher-Petkovic et al., 1988 , Marasovic et al, 1988\Marasovic, 1989*, Marasovic & Pucher-Petkovic, 1991% Marasovic et al.,1995 ). Monitoring program for a wider area of the middle Adriatic region wasstarted at that time, running parallel to the already existing, long-termmonitoring of the Kastela Bay. Sampling was conducted during the summerperiod on 5 stations near larger urban zones. The results of these studies are

Transactions on Ecology and the Environment vol 14, © 1997 WIT Press, www.witpress.com, ISSN 1743-3541

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presented through several aspects: the analysis of the biomass of chlorophyll a;density of phytoplankton cells; analysis of relation of different phytoplanktongroups; the process of primary production and the size fraction analysis of thephytoplankton cells. The results show the increase of eutrophication in mostparts of the studied area. The data also enables us to easily differentiate betweeneutrophic, mesotrophic and oligotrophic areas. If we base our conclusions onresults obtained by different methods, it is possible to characterize individualareas with a high level of certainty. This paper presents results of the monitoringof the wider mid-Adriatic coastal area as well as results of the Kastela bay andmid-Adriatic open waters monitoring programs.

2 Material and methods

Phytoplankton samples were collected at seven stations along the eastern coastof the Adriatic Sea and one off shore station of the middle Adriatic (Fig. 1).Stations Zi, Si, Si, PI and DI are located in coastal waters of the towns Zadar,Sibenik, Split, Ploce and Dubrovnik. These samples were collected only duringthe summer for ten years period. Stations 83 and 825 are located in the KastelaBay and station 89 is located in the open Adriatic waters. On these three stationswere sampled monthly during thirty years period. Samples were take by Nansenreversing bottles from the depths Om, 10m, 20m and at near the bottom atnearshore stations, and Om, 10m, 20m, 30m, 50m, 75m and 100m at the opensea station. Phytoplankton samples were preserved in a two percent neutralizedformaldehyde solution. The cell counts were obtained by the invertedmicroscope method (UtermohL, 1958 ). Clorophyll a biomass was estimated bymethod of Strickland and Parsons (1972)* on a Turner 111 fluorometer.Primary production was measured by Carbon-14 method (Steemann-Nielsen,1952*). Size fractination was done for cells < 10 um and for cells < 2 um.

Figure 1: Study area. The locations of the coastal and the open sea station aredenoted by black circles.


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3 Results and discussion

Figures 2, 3, 4, 5 and 6 show results of the 10-years long study ofchlorophyll a biomass on five coastal stations in the middle Adriatic. The densityof phytoplankton cells is shown on figures 7, 8, 9, 10 and 11. The increase ofthe chlorophyll a biomass is very obvious on stations Zi, Si, Si and PI. Onstation DI there was no increase of the biomass. The density of the cells showsimilar results, but not with such a regularity as the chlorophyll a biomass. Onsome stations (Si, Si and PI) there is a certain decrease of the number ofphytoplankton cells during the last period of research, what can be explained bythe changes in the phytoplankton community. This example indicates that justtaking into account the number of phytoplankton cells, without simultaneouslymeasuring the plasmatic volume of the cells is not an objective parameter forobserving eutrophication. Stations Zi and Si are exclusively influenced by urbansewage waters. Stations Si and PI are influenced by urban effluents and freshwater inflows (Krka and Neretva rivers). Station DI is under strong influence ofoffshore Adriatic waters. On this station increase of eutrophication is notobserved.

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Figure 11: Five-year running averages of phytoplankton density (No cellsdm^) at the station DI

Figure 12 shows the increase of primary production in Kastela Bay during atwelve years period. It is important to note that over this period the number ofresidents along the coasts of the Bay has increased five times (city of Split).

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Major part of urban sewage waters runs into the Bay, causing the inevitableincrease of eutrophication. This is most notable during the summer, when thewater dynamics are very weak, and closed circulation forms in the inner part ofthe Bay (Zore-Armanda, 1980 ). Since 1980, red tide of Gonyaulax polyedrahas been regularly occurring in that area every summer. This state is well shownin figures 13, which give ratios of the number of diatoms and dinoflagellates insummer periods of 1975 and 1984. While dinoflagellates were not abundant inthe Bay at the beginning of 1970's, at the beginning of 1980's their share in thegeneral community has been over 90% (Marasovic and Pucher-Petkovic,1991").

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Figure 13: Relations between diatoms and dinoflagellates during the summer1975 and 1984 in the Kastela Bay (S,)

Figure 14 shows the different phytoplankton size categories present in thebiomass of the area studied. On station 9 (open waters) the contribution of picofraction is much higher than in eutrophised part of the Kastela Bay, where theamount of pico fraction is almost negligible. At the same time, microplanktonshare is highest at this station, while the nanoplankton category is most abundantin the outer part of the Kastela Bay.


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75%

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Figure 14: Contribution of pico-, nano- and microplankton in the chlorophyll abiomass in the coastal and open waters of the middle Adriatic.

Similar results have been obtained through the analysis of amount of differentphytoplankton size components in primary production of this area (Fig. 15).The ratio between different size categories of phytoplankton (pico-, nano- andmicroplankton) is an important parameter for eutrophication assessment(Vilicic,1989).


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Station 3

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Figure 15: Contribution of pico-, nano- and microplankton in the primaryproduction in the coastal and open waters of the middle Adriatic.


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4 Conclusion

By measuring different parameters within primary production (number ofphytoplankton cells, chlorophyll a biomass, ratio of phytoplankton groups,primary production in situ, size fractionation in chlorophyll biomass and inprimary production) we have established the increase of eutrophication in thecoastal waters of the middle Adriatic (Split, Sibenik, Zadar and Ploce). At thesame time, the changes have not been recorded in the coastal waters aroundDubrovnik, which are under strong influence of the open Adriatic waters.

5 References

1. Pucher-Petkovic, T. & Marasovic, I. Developpement des populationphytoplanctonique characteristique pour en milieu eutrophise (baie deKastela), /Wa Adriatica, 1980, 21, 79-93.

2. Pucher-Petkovic, T., Marasovic, I, Vukadin, I. & Stojanoski, L Timeseries of productivity of parameters indicating eutrophication of the MiddleAdriatic waters, in Fifth technical consultation of stock assessment in theAdriatic/ 88 ( ed. J. F Caddy & M. Savini), pp. 41-50, Ban, Italy, 1987,GFCM, Rome, 1988.

3. Marasovic, L, Pucher-Petkovic, T. & Alegria-Hernandez, V Phytoplanktonproductivity of the Adriatic Sea in relation to pelagic fisheries, Biljeske-Notes, 1988,72,1-8

4. Marasovic, I Phytoplankton and eutrophication of the Adriatic Sea, in LoStato di Salute dell'Adriatico, pp. 36-46, Urbino, Italy, 1989, Sogesta,Urbino, 1989.

5. Marasovic, I. & Pucher-Petkovic, T. Eutrophication impact on the speciescomposition in a natural phytoplankton community, Acta Adhatica, 1991,32, 719-729.

6. Marasovic, L, Grbec, B. & Morovic, M. Long term production changes inthe Adriatic, Netherlands Journal of Sea Research, 1995, 34, 267-273.

7. Utermohl, H. Zur Vervollkommnung der quantitativen PhytoplanktonMethodik, Mitt int. Ver. Limnoi, 1958, 9, 1-38.

8. Strickland, J.D.H. & Parsons, T.R. A practical handbook of seawateranalysis, Bulletin of Fisheries Research Board of Canada, 1972, 167,1-310.

9. Steemann-Nielsen, E. Theuse of radioactive carbon for measuring organicproduction in the sea, J. Cons. Int. Explor. Mer, 18, 117-140.

10. Zore-Armanda, M. Some dynamic and hydrographic properties of theKastela Bay, Acta Adriatica, 1980, 21, 35-74.

11. Vilicic, D. Phytoplankton population density and volume as indicators ofeutrophication in the eastern part of the Adriatic Sea, Hydrobiologia, 1989,174,117-132.


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