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Distribution and Seasonal Variability of Organic Matter in a Small Eutrophicated Salt Lake
Estuarine, Coastal and Shelf Science (2000) 51, 705–715 doi.10.1006/ecss.2000.0721, available online at http://www.idealibrary.com on
Distribution and Seasonal Variability of Organic Matter in a Small Eutrophicated Salt Lake B. C u osovic´a, I. Ciglenecˇkia, D. Vilicˇic´b and M. Ahela a
Ruder Bosˇkovic´ Institute, Center for Marine and Environmental Research, POB 1016, 10001 Zagreb, Croatia University of Zagreb, Faculty of Science, Department of Botany, Rooseveltov trg 6, 10001 Zagreb, Croatia
b
Received 13 December 1999 and accepted in revised form 5 October 2000 Distribution and seasonal variability of dissolved organic carbon (DOC) and surface active substances (SAS) were studied along the depth profile (15 m) in a small eutrophicated and periodically anoxic sea lake (Rogoznica Lake, Eastern Adriatic coast) in 1996 and 1997. The range of DOC concentrations was characteristic for productive coastal marine ecosystems (60% of samples in the range of 1–2 mg l 1 and 40% between 2 and 3 mg l 1). Distribution of SAS concentrations was uniform and shifted toward higher concentrations in comparison to other coastal areas in the Adriatic Sea. Eutrophication in the lake is generated by nutrient recycling under anaerobic conditions. Systematically higher concentrations of chlorophyll a, DOC and SAS were determined at the chemocline in the bottom layer (10–12 m) than in the upper water layer (0·5–2 m). Seasonal variability of organic matter was discussed regarding distributions of microphytoplankton (cells >20 m) and photosynthetic pigments as well as oxygen and salinity changes along the depth profile. The dissolved oxygen saturation reaching up to 300% in the water layer between 8 m and 10 m depths in May and June 1996, was correlated with enhanced concentrations of phytoplankton biomass (reflected as chl a and b, fucoxanthin, peridinin, zeaxanthin) and increased concentrations of DOC and SAS. 2000 Academic Press Keywords: organic matter; surface active substances; photosynthetic pigments; phytoplankton; eutrophication in sea lake; Adriatic coast
Introduction Dissolved organic carbon is the largest organic carbon pool in the marine environment and it plays a central role in the marine biogeochemical cycle of carbon. The accumulation of DOC during the spring phytoplankton bloom and subsequent vertical mixing in fall and winter are the main processes to explain the field data of DOC seasonal and spatial distributions. A large part of dissolved organic matter in the sea represents organic compounds that possess surface active properties. Surface active substances accumulate at marine interfaces (seawater boundaries with the atmosphere, with living and nonliving dispersed and particulate material and sediment) making up a major part of the surface microlayer and organic coating at mineral particles. There is clear evidence of surfactant production by marine phytoplankton based on laboratory experiments with phytoplankton cultures as well as field measurements in real marine environments (Z { utic´ et al., 1981; Plavsˇic´ et al., 1990; Vojvodic´ et al., 1999). Total surfactants content in culture media generally increased with cell density while marked differences in quantities and kinds of surfactants between different cultures of microphyto0272–7714/00/120705+11 $35.00/0
plankton species were detected (Z { utic´ et al., 1981). The influence of bacteria and cell lysis in the surfactant production of marine phytoplankton make the problem more complex. Due to specific reactivity of SAS at natural phase boundaries, their distribution and fate in the sea could be different in comparison to those of the organic matter pool. For example, in the northern Adriatic the DOC values vary seasonally within a narrow range, approximately twice the value, while at the same time the SAS concentrations changed more than an order of magnitude (Vojvodic´ &C u osovic´, 1996). In order to contribute to a better understanding of the nature and reactivity of organic matter in the marine environments distribution and seasonal variability of dissolved organic carbon and surface active substances were studied along the depth profile (15 m) in a small intensely eutrophicated and periodically anoxic sea lake (Rogoznica Lake, Eastern Adriatic coast) in 1996 and 1997. The lake is selected as an unique ecosystem owing to its isolation and extreme conditions, caused by the absence of oxygen in anoxic bottom layers of the lake, all these influencing the number and kind of biological species. The microphytoplankton cell density and abundance of species in the lake were monitored 2000 Academic Press
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and compared with concentrations of relevant photosynthetic pigments. Materials and methods The Rogoznica Lake is a small (area about 5300 m2, maximum depth of 15 m) meromictic saline lake which periodically becomes anoxic. Seasonal variations of anoxic conditions in the Rogoznica lake at 12 m depth
have been investigated since 1994 onwards (Ciglenecˇ ki et al., 1998a,b; Krsˇ inic´ et al., 2000). The lake has no visible connection with the surrounding sea but lake tides are detectable on cliffs, which indicates permanent water exchange through the porous karst. Geographic position of the lake is presented in Figure 1. It is assumed that the Rogoznica Lake was formed in the early Holocene at the time of the post-glacial rise in the sea level (Buljan, 1956).
Distribution and seasonal variability of organic matter 707
The lake is protected from the effects of the winds, because of the relatively high banks, and thus the stratification of the water column is maintained throughout most of the year. The stratification and mixing of lake waters were greatly affected by rainfall, as shown from decreased salinity in deeper waters (Ciglenecˇ ki et al., 1998a,b). During the thermohaline stratification, the surface water is well oxygenated while the deeper water layers become anoxic and rich in reduced sulphur species (up to 900 M (Ciglenecˇ ki et al., 1996)). Samples of Rogoznica Lake water were collected with 5 l Niskin sampling bottles in April and October 1995 and at approximately monthly intervals between October 1995 and October 1997, along the vertical profile of the Lake. Dark glass bottles were used for storage and transportation of the samples at 0–4 C. Samples for SAS were analysed non-filtered within 24 h. For DOC analyses seawater samples were filtered immediately after sampling (Whatman GF/F filters-pore size 0·7 m), poisoned with HgCl2 solution and stored cold in the dark. Samples for phytoplankton analysis were preserved in 2% (final concentration) neutralized formaldehyde solution. Adsorption effects of SAS at the mercury electrode were measured by A C voltammetry (out of phase mode) as described previously (C u osovic´ & Vojvodic´ , 1982, 1987). For quantitative determination, the calibration curve of the nonionic SAS, Triton-X-100 was used. Electrochemical measurements were performed with a Methrom E-506 polarecord (Methrom, Switzerland) using a hanging mercury drop electrode, and Ag/AgCl as the reference electrode. A high-temperature catalytic oxidation analyser (TOC-500 Model, Shimadzu, Japan) was used for the determination of DOC by direct injection of a filtered sample. Samples for the photosynthetic pigment analyses (0·5–1 l) were filtered onto 47 mm GF/F filters. Filters were immediately stored at 20 C until analysis. The filters were extracted in 4 ml of cold 90% acetone using sonication, centrifuged to clarify the extract, and the chlorophylls and carotenoids separated by reversed-phase HPLC according to Barlow et al. (1993). Briefly, extracts were mixed (1:1, v/v) with 1 ml l 1 ammonium acetate and injected into a HPLC system. The HPLC system consisted of two pumps (LKB, Bromma, Model 2150), gradient controller (Model 2152), injector (Rheodyne, Model 7125), a C18 m Pecosphere column (3·30·45 cm, Perkin Elmer) and serially coupled spectrophotometric and spectrofluorimetric
detectors. A binary linear gradient was used to separate the pigments. Solvent A consisted of 80:20 (v/v) methanol : 1 mol l 1 ammonium acetate, while solvent B contained 60:40 (v/v) methanol:acetone. Chlorophylls and carotenoids were detected by absorbance at 440 nm (Spectra Physics UV 2000). Data collection and reprocessing utilized Varian Star software. Cell counts were obtained by the inverted microscope method (Utermo¨ hl, 1958). Subsamples of 25 and 50 ml were analysed microscopically after sedimentation times of 24 h and 48 h, respectively. Microphytoplankton cells (MICRO, cells longer than 20 m, including those of microheterotrophic species Hermesinum adriaticum), were counted under magnifications of 400(1–2 transects) and 100(transects along the rest of the counting-chamber base plate). Results Dissolved organic matter and surface active substances Determinations of dissolved organic carbon in lake samples collected from different depths (by approximately monthly frequency in the period from February 1996 to July 1997) showed that most of 136 samples (about 60%) contained between 1·0 and 2·0 mg l 1 of DOC [Figure 2(a)]. About 40% of samples contained more than 2·0 mg l 1 while only few samples from deeper water layer of the lake contained more than 5 mg l 1 of dissolved organic carbon. The mean DOC value of 1·75 mg l 1 was determined for Rogoznica Lake samples. Mean values of DOC for open waters of the Adriatic Sea were determined to be 1·2 mg l 1 for the middle Adriatic samples and 1·78 for the north Adriatic Sea samples (C u osovic´ & Vojvodic´ , 1993). When comparing to the Rogoznica Lake, very similar range of DOC values (minimum value of 1·13 mg l 1 and maximum value of 3·06 mg l 1) were found in surface waters in the northern Adriatic Sea in the period from 1989 to 1993 (Vojvodic´ & C u osovic´ , 1996). This similarity is expected since the northern part of the Adriatic Sea is a shallow basin and the most productive part of the Adriatic Sea (Pettine et al., 1998; Gilmartin & Revelante, 1983; Degobbis et al., 1995), known for its anthropogenic eutrophication (Vollenweider et al., 1996). Eutrophication in the lake is strongly influenced by nutrient recycling under anaerobic conditions. In the bottom layer, high concentrations of nutrients were detected such as ammonia up to 200 M, phosphate at about 14 M and silicate at 300 M (Ciglenecˇ ki et al., 1998b;
708 B. C u osovic´ et al.
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F 2. Distribution of (a) DOC and (b) SAS concentrations, equivalent to Triton-X-100, in the Rogoznica Lake, during the period February 1996 and July 1997 (136 samples), and (c) correlation of SAS and DOC concentrations (r=0·1795). Lines in (c) correspond to different model substances: (1) protein albumin, (2) fulvic acid, (3) polysaccharide dextran and (4) polysaccharide xanthan.
Krsˇ inic´ et al., 2000). Direct freshwater input from local precipitation brings some nutrients to the lake and also increases eutrophication. As shown in Figure 3(b), seasonal variability of DOC values is not well defined neither for surface
water (0·5–2 m depth) nor for the bottom water layer (12 m depth) in the Rogoznica Lake. Generally, in all seasons higher values of dissolved organic carbon were determined in the bottom layer where most of the time anoxia or hypoxia
Distribution and seasonal variability of organic matter 709
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F 3. Seasonal variabilities of (a) chlorophyll a, (b) DOC and (c) SAS concentrations at two different depths: ( ) 0·5 m and ( ) 12 m.
prevailed (Ciglenecˇ ki et al., 1998a). Anaerobic mineralization enhanced supply of nutrients in deeper waters of the lake that resulted in increased phytoplankton biomass as shown from chlorophyll data in Figure 3(a). Concentrations of SAS as determined by the electrochemical method showed a sharp maximum in
the frequency distribution in the range between 0·16 mg l 1 and 0·32 mg l 1 equiv. Triton-X-100 (83% of the total number of 136 samples fall into that concentration range) [Figure 2(b)]. It is characteristic for the lake that low concentrations of surface active substances (in the range between 0·08 and 0·16 mg l 1 equivalent
710 B. C u osovic´ et al.
Triton-X-100) were found in very few samples. None of the sample in the lake had the concentration of SAS below 0·08 mg l 1, which is quite different from the previously reported data for other marine and estuarine environments including the highly eutrophic northern Adriatic Sea (C u osovic´ et al., 1985; Marty et al., 1988), where such low surfactant concentrations were determined in the whole water column in winter months, and sporadically, in deeper water layer in different seasons (Vojvodic´ & C u osovic´ , 1996). Seasonal variabilities of SAS concentrations in the Rogoznica Lake are presented for the surface layer (0·5–2 m depth) and for the bottom layer (12 m depth) in Figure 3(c). As observed for the DOC values, concentrations of SAS in the lake were systematically higher in the bottom layer than in the surface layer. Maximum concentrations of surface active substances were determined in June–July in both years. The correlation of SAS and DOC values, made for all samples collected in the Rogoznica Lake, in the period from February 1996 to July 1997, is presented in Figure 2(c). The correlation coefficient is rather low (r=0·19) indicating a weak but still significant coupling of those two parameters at 95% probability. The variations of the chemical composition of organic matter, i.e. the variable partition of strongly adsorbable organic substances in the dissolved organic matter pool, could be a fair explanation for relatively low correlative pattern between DOC and SAS values in this lake. A comparison of normalized surfactant activities of the samples from Rogoznica Lake was made with different model substances as representative constituents of organic matter in natural samples. The obtained surfactant activity value (SAS eq. T-X-100) for each model substance was normalized for its organic carbon content (C u osovic´ & Vojvodic´ , 1998). The obtained normalized surfactant activity values were as follows: oleic acid (2·7), protein (here albumin) (0·20), fulvic acid (0·17), polysaccharides: dextran T-500 (0·20) and xanthan (0·04). Some of these values were introduced in Figure 2(c) as dashed lines of the corresponding slopes. The normalized surfactant activity value for the lake samples was determined as the ratio between the mean SAS value (0·18 mg l 1 eq. T-X-100) and the mean DOC value (1·75 mg l 1), which gives the mean normalized surfactant activity value of approximately 0·1. In comparison with the north Adriatic samples there is a slight shift towards higher surfactant activity values for the Rogoznica Lake samples. This could be ascribed to the higher influence of sulphur compounds on the adsorption properties of
organic matter in the lake water (Ciglenecˇ ki et al., 2000). Phytoplankton and organic matter In the period between February 1996 and July 1997, the density of microphytoplankton cells at different depths and in different seasons varied between values below 103 cells 1 1 and the maximum value of 7·65106 cells 1 1, that was determined in the surface layer (0·5–2 m) in July 1997. A small number of microphytoplankton species (30) was found during the research activities. Twentyfive species, more abundant than 1000 cells 1 1, dominated in the Rogoznica Lake (Table 1). Diatoms provided 15 species, nine dinoflagellates and only one coccolithophorid. No correlation was found for the Rogoznica Lake samples between the microphytoplankton cell densities and either the DOC values or the SAS concentrations. For example, in a number of seasons rather low cell densities were determined in the surface layer (0·5 m) while, in contrast, the organic matter content was relatively high in the upper water layer. Moreover, taking into consideration 100 pairs of data for chlorophyll a and DOC concentrations the calculated correlation coefficient was rather low (r=0·09) indicating a more complex relationship between these two parameters in the Rogoznica Lake. At the same time a correlation coefficient of r=0·17 for chlorophyll a and surface active substances indicated a weak but significant coupling of those parameters at 90% probability. However, for 95% probability the coupling is not any more significant. In Figure 4(a,b) the variation of the maximum saturation with oxygen along the vertical profile of the lake water was considered as an illustration of the photosynthetic activity in the lake. Comparison is also made with the cell density of the microphytoplankton. Diatoms were found to be the dominant microphytoplankton group in most lake samples, except for May to June 1996, when dinoflagellates were determined by microscopic counting, as denoted in Figure 4(b). It is obvious from Figure 4(a) that the position of the maximum saturation with oxygen as well as the position of the maximum density of microphytoplankton cells were shifted towards deeper layers of the lake in most productive periods of year, May/June/July, in comparison to other seasons. Extremely high oxygen saturation up to 300% was determined from May to June 1996 between 8 m and 10 m depths. Oxygen saturation up to 300% in the water layer between 8 m and 10 m depths prevented the expected development of anoxia in the lake during the spring and summer of
Distribution and seasonal variability of organic matter 711 T 1. List of dominant microphytoplankton species, with frequency of finding (F) d2% and maximum cell abundance d1000 cells l 1, in the Rogoznica Lake, in the period from 1995 to 1998. Number of samples=278
Taxa Caliptrosphaera oblonga Cerataulina pelagica Ceratium furca Chaetoceros affinis Chaetoceros convolutus Chaetoceros curvisetus Chaetoceros danicus Chaetoceros decipiens Cyclotella sp. Diplopsalis ‘ complex ’ Eunotia sp. Glenodinium spp. Gymnodinoid cells Gyrodinium spp. Hermesinum adriaticum Leptocylindrus danicus Nitzschia longissima Prorocentrum arcuatum Pseliodinium vaubanii Pseudonitzschia spp. Rhizosolenia fragilissima Scripsiella spp. Synedra spp. Thalasiossira decipiens Thalassionema nitzschioides
Higher taxonomic group
F
COCC BACI DINO BACI BACI BACI BACI BACI BACI DINO BACI DINO DINO DINO DINO BACI BACI DINO DINO BACI BACI DINO BACI BACI BACI
10 10 14 16 10 189 27 18 17 10 40 11 65 8 85 6 25 114 3 43 55 16 33 19 54
F(%)
Maximum cell abundance (cells l 1)
Date of recorded maximum
3·6 3·6 5·0 5·8 3·6 68·0 9·7 6·5 6·1 3·6 14·4 4·0 23·4 2·9 30·6 2·2 9·0 41·0 1·1 15·5 19·8 5·8 11·9 6·8 19·4
4000 205 400 3600 12 900 2400 9 031 000 169 800 21 600 146 000 2400 7 432 000 3600 45 800 220 000 203 800 3200 4 780 000 467 000 1000 6 520 000 3 890 000 30 530 101 700 754 700 815 000
27 Oct 1995 13 Jun 1996 18 Feb 1997 22 Feb 1996 1 Oct 1997 2 Apr 1998 7 Jul 1997 6 Oct 1998 27 Oct 1995 17 Jun 1998 21 Aug 1996 29 Apr 1997 29 Apr 1997 13 Jun 1996 17 Jun 1998 25 Aug 1998 13 Jun 1996 1 May 1996 18 Feb 1997 5 Jun 1997 13 Jun 1996 29 Apr 1997 18 Feb 1997 18 Nov 1997 30 Sep 1996
BACI=diatoms; DINO=dinoflagellates; COCC=coccolithophorids.
1996, except in layers below 13 m (Ciglenecˇ ki et al., 1998a). The oxygen saturation maximum is in good agreement with the maximum chlorophyll a value and with the maximum of SAS concentrations in both the upper water layer and the bottom (12 m) water layer as presented in Figure 3(a,c). The DOC maximum in Figure 3(b) for June 1996 was less pronounced than for the SAS maximum. In the time period of the detected maximum oxygen saturations between 200 and 300% the cell densities of microphytoplankton as determined by microscopic counting were low and, except for 12 June, dinoflagellates were the dominant species [Figure 4(b)]. Relatively high concentrations of chlorophyll a indicated that the majority of the phytoplankton cells at the time of sampling would be smaller than 20 m. In Table 2 mean concentration values are presented of selected photosynthetic pigments determined in the time period May/June 1996, in the same water layer, between 8 and 10 m, characterized by high oxygen saturations. In addition to chlorophyll a, we have selected fucoxanthin as a marker for diatoms,
peridinin for dinoflagellates, chlorophyll b for green algae and zeaxanthin for cyanobacteria. We have denoted also the maximum concentrations of pigments for the whole water column (values in brackets). Thus, high concentrations of peridinin (2443 and 2850 ng l 1) are well correlated with dinoflagellate bloom registered on 1 May 1996. On 12 June 1996 the determined concentrations of fucoxanthin are in accordance with predominance of diatoms as determined microscopically. On 26 June 1996 the microphytoplankton density was very low (mean value below 5000 cells 1 1 in that layer) and only dinoflagellates were detected microscopically despite the fact that the levels of fucoxanthin (125 ng l 1 between 8 m and 10 m depths and 980 ng l 1 at 13 m depth) are supporting the presence of fucoxanthincontaining dinoflagellates (Jeffrey et al., 1975) and probably some epiplanktonic diatoms colonized on zooplankton as reported for the lake by Krsˇ inic´ et al. (2000). In all three cases presented in Table 2 marked abundance of green algae (chl b) and cyanobacteria (zea) can also be noted.
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712 B. C u osovic´ et al.
F 4. (a) Variations of the position of maximum oxygen saturation ( ) and maximum cell-density ( ) of microphytoplankton along the depth profile in the Rogoznica Lake: (b) Maximum oxygen saturation ( ) in different sampling situations and cell-density ( , ) of microphytoplankton in the same water layer. Open circles denote predominantly diatoms and squares dinoflagellates. T 2. Mean concentrations values of the selected pigments in the water layer between 8 m and 10 m depths in the Rogoznica Lake in May and June 1996. In brackets are given maximum concentrations of pigments on the depth profile
Date 1 May 1996 12 June 1996 26 June 1996
Dissolved oxygen saturation (%)
chl a (ng l 1)
Fuco (ng l 1)
Per (ng l 1)
chl b (ng l 1)
Zea (ng l 1)
223 270 190
3830 (4680) 2293 (7550) 1110 (6750)
120 (140) 906 (1660) 125 (980)
2443 (2850) 136 (2090) 53 (550)
330 (830) 153 (2360) 210 (3450)
310 (780) 103 (1760) 360 (3530)
chl a=chlorophyll a; Fuco=fucoxanthin; Per=peridinin, chl b=chlorophyll b; Zea=zeaxanthin.
From February to July 1997 the maximum oxygen saturation in the lake was determined to be between 100 and 130%, and the position of the maximum changed from 0·5 m in February 1997 towards 12 m depth in June 1997. The increase of the microphyto-
plankton cell density predominantly diatoms, from February 1997 to July 1997 [Figure 4(b)] was in a good correlation with the increase of concentrations of surface active substances [Figure 3(c)]. Extremely high DOC values were determined in the bottom layer
Distribution and seasonal variability of organic matter 713
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F 5. Annual variation of salinity at three different depths in the Rogoznica Lake: ( ) 0·5 m, ( ) 2 m and ( ) 12 m.
in May 1997 [Figure 3(b)], following a diatom bloom in April 1997. Discussion Considering the variations of temperature, salinity and concentrations of dissolved oxygen and reduced sulphur species in the Rogoznica Lake over several years (Ciglenecˇ ki et al., 1996, 1998a,b), the period from February 1996 to July 1997 was characterized by development of anoxic conditions only in the bottom layer, i.e. between 13 m and 15 m depths. In contrast, in years 1994 and 1995 anoxic conditions were detected in much shallower layers (below 9 m). Despite these differences we have not observed marked differences in distributions of DOC and SAS concentrations for the two periods. Spatial distribution of DOC and SAS in the water column indicate that higher DOC and SAS concentrations were determined in deeper layers as a result of increased phytoplankton activity on the boundary between the oxic and anoxic layers due to recycling of nutrients. The possible sources of DOC and SAS are excretion of organic matter and decomposition of particulate organic carbon. The increased biomass in the bottom layer is evidenced from higher chlorophyll a concentrations in comparison to those in the upper water layer. The range of DOC concentrations in the lake is characteristic for productive coastal marine ecosystems (about 60% of the samples in the range
between 1 and 2 mg l 1 and about 40% between 2 and 3 mg l 1) (Sharp et al., 1993; Vojvodic´ & C u osovic´ , 1996; Pettine et al., 1999). The distributions of DOC and SAS concentrations in the lake [Figure 2(a,b)] were found to be more uniform than those in other open and coastal marine systems (Marty et al., 1988; Vojvodic´ & C u osovic´ , 1996). For example, in the Rogoznica Lake, 83% of samples had concentrations of SAS in the range between 0·16 mg l 1 and 0·32 mg l 1 eq. T-X-100 compared to only 10% of samples in open waters of the northern Adriatic Sea (1979–1986) (Marty et al., 1988). In the Adriatic Sea the distribution of SAS was spread in a wider concentration range, i.e. from very low concentrations (0·02– 0·04 mg l 1 eq. T-X-100) to high concentrations (between 0·64 mg l 1 and 1·28 mg l 1 for very few samples. The most frequent values (about 40% of samples in the Adriatic Sea) were in the range from 0·08 to 0·16 mg l 1 eq. T-X-100. Concentrations of SAS below 0·08 mg eq. T-X-100 were never determined in the Rogoznica Lake, which means that vertical mixing in the lake during fall and winter is bringing up organic matter-rich deeper waters, which is very different from what usually occurs in the Adriatic Sea. Taking into consideration all the data collected on microphytoplankton cell-densities, we did not find a correlation with corresponding values of DOC and SAS concentrations for the same samples. The microphytoplankton cell density was not statistically correlated with chlorophyll a due to a significant but highly
714 B. C u osovic´ et al.
variable contribution of smaller phytoplankton cells to the total biomass and due to the contribution of some heterotrophs to cell-density counts (see Table 1 and Vilicˇ ic´ et al., 1996/97). DOC concentrations were not correlated with chlorophyll a data, while weak but significant correlation of chl a with SAS concentrations supported the evidence of surfactant production by marine phytoplankton (Z { utic´ et al., 1981). The observed differences in distributions of organic matter (DOC and SAS) and of microphytoplankton in 1996 and 1997 could be explained by different hydrographic situations (Figure 5), especially by variability of inputs of nutrient rich fresh waters. It should be stressed that the observed decrease of salinity in the lake exceeds the direct freshwater input from local precipitation and very probably includes also the freshwater inflow through the porous karst from a larger area. As shown in Figure 5, there is an apparent difference in salinity changes in different layers in the two subsequent years. In 1996 the freshwater input had a pronounced effect on salinity values in the whole water column. In May and June 1996, in the upper layer (0·5–2 m depth) salinity values were below 30, compared with a salinity of about 35 in the deeper layer. This resulted in a very stable thermohaline stratification in spring/summer 1996 with an enhanced phytoplankton bloom in deeper water layers of the lake. Increased concentrations of relevant photosynthetic pigments chlorophyll b and zeaxanthin, concomitant with the increased concentrations of chlorophyll a, indicated a marked contribution of nanoplanktonic green algae and picoplanktonic cyanobacteria to the oxygen saturation (between 200 and 300%) in deeper lake waters. In 1997 the input of freshwater was lower than in 1996 and the differences of salinity values along the depth profile of the lake from May 1997 to October 1997 were low. The maximum chlorophyll a values were also lower in 1997 while the maximum oxygen saturation values were between 100 and 130%. Besides the fact that phytoplankton is an important source of authochtoneous DOC and SAS in the Rogoznica Lake, demonstrated by coincidence of maximum oxygen saturation and chlorophyll a concentrations along the vertical profile in the lake in different seasons, it should be anticipated that other physicochemical (e.g. adsorption, association and aggregation, and degradation) and biological processes (e.g. bacterial degradation and heterotrophic consumption) influence the distribution of organic matter and determine its residence time in the lake. Rather uniform distributions of DOC and SAS concentrations in the lake are leading us to the
conclusion that abiotic and biotic processes are well balanced in the investigated eutrophicated sea lake. Conclusion The variability of dissolved organic matter and surface active substances was studied monthly during the period from February 1996 and July 1997 in the eutrophic, periodically anoxic sea lake. The range of DOC concentrations was found to be characteristic of productive coastal marine ecosystems (60% of samples were in the range of 1–2 mg l 1 and 40% between 2 and 3 mg l 1). Distribution of SAS was very uniform and at higher concentrations in comparison to the North Adriatic samples, while the concentrations of SAS below 0·08 mg eq. T-X-100 were not determined in the lake. Systematically higher concentrations of organic matter were determined at the chemocline in the bottom layer (10–12 m) than in the upper water layer (0·5–2 m). The increased biomass in the bottom layer is evidenced from higher chlorophyll a concentrations in comparison to those in the upper water layer. The vertical mixing in the lake during fall and winter brings up deeper water which is rich with organic matter. This does not occur in the Adriatic Sea. The microphytoplankton cell density was not statistically correlated with the DOC and SAS concentrations. A weak but significant correlation of chlorophyll a with SAS concentrations supported the evidence of surfactant production by marine phytoplankton. Phytoplankton cells smaller than 20 µm very probably represented majority of phytoplankton cells. The eutrophication in the lake is strongly influenced by nutrient recycling under anaerobic conditions prevailing in the bottom layer below 12 m depth. Direct freshwater input from local precipitation brings nutrients to the lake and increases eutrophication as well. Acknowledgement Financial support from the Ministry of Science of the Republic of Croatia is gratefully acknowledged. References Barlow, R. G., Mantoura, R. F. C., Gough, M. A. & Fileman, T. W. 1993 Pigment signatures of the phytoplankton composition in the northeastern Atlantic during the 1990 spring bloom. Deep-Sea Research II 40, 459–477. Buljan, M. 1956 Prvi nalazi sumporovodika (H2S) u vodi Jadrana. Mornaricˇki glasnik (Split) 2, 207–214. Ciglenecˇ ki, I., Kodba, Z. & C u osovic´ , B. 1996 Sulfur species in Rogoznica Lake. Marine Chemistry 53, 101–110.
Distribution and seasonal variability of organic matter 715 Ciglenecˇ ki, I., Kodba, Z., Vilicˇ ic´ , D. & C u osovic´ , B. 1998a Seasonal variation of anoxic conditions in the Rogoznica Lake. Croatica Chemica Acta 71, 217–232. Ciglenecˇ ki, I., Kodba, Z., Vilicˇ ic´ , D., Caric´ , M. & C u osovic´ , B 1998b Development of anoxia in the small coastal sea lake, Rogoznica Lake (eastern Adriatic Coast).. Rapport Commission Internationale pour l’Exploration Scientifique de la Mer Mediterrane´ 35, 242–243. Ciglenecˇ ki, I., C u osovic´ , B., Vojvodic´ , V., Plavsˇ ic´ , M., Furic´ , K., Minacci, A. & Baldi, F. 2000 The role of reduced sulfur species in the coalescence of polysaccharides in the Adriatic Sea. Marine Chemistry 71, 233–249. C u osovic´ , B. & Vojvodic´ , V. 1982 The application of ac polarography to the determination of surface-active substances in seawater. Limnology Oceanography 27, 361–369. C u osovic´ , B., Z { utic´ , V., Vojvodic´ , V. & Plesˇ e, T. 1985 Determination of surfactant activity and anionic detergents in seawater and sea surface microlayer in the Mediterranean. Marine Chemistry 17, 127–139. C u osovic´ , B. & Vojvodic´ , V. 1987 Direct determination of surface active substances in natural waters. Marine Chemistry 22, 363–373. C u osovic´ , B. & Vojvodic´ , V. 1993 Surface active substances, dissolved organic carbon and anionic detergents. Discussion on the result of the Italian-Yugoslav bilateral collaboration on the pollution protection of the Adriatic Sea (ASCOP), Rovinj, pp. 59–66. C u osovic´ , B. & Vojvodic´ , V. 1998 Voltammetric analysis of surface active substances in natural seawater. Electroanalysis 10, 429–435. Degobbis, D., Fonda-Umani, S., Franco, P., Malej, A., Precali, R. & Smodlaka, N. 1995 Changes in the northern Adriatic ecosystem and the hypertrophic appearance of gelatinous aggregates. Science of the Total Environment 165, 43–58. Gilmartin, M. & Revelante, N. 1983 The phytoplankton of the Adriatic Sea: standing crop and primary production. Thalassia Jugoslavica 19, 173–188. Jeffrey, S. W., Sielicki, M. & Haxo, F. T. 1975 Chloroplast pigment patterns in dinophlagellates. Journal of Phycology 11, 374–384. Krsˇ inic´ , F., Caric´ , M., Vilicˇ ic´ , D. & Ciglenecˇ ki-Jusˇ ic´ , I. 2000 Calanoid copepod Acartia italica Steuer, 1910 phenomenon in
the small saline lake Rogoznica near S { ibenik, Croatia. Journal of Plankton Research 22, 1441–1464. Marty, J. C., Z { utic´ , V., Precali, R., Saliot, A., C u osovic´ , B, Smodlaka, N. & Cauwet, G. 1988 Organic matter characterization in the northern Adriatic Sea with special reference to the sea surface microlayer. Marine Chemistry 25, 243–263. Pettine, M., Patrolecco, L., Camusso, M. & Crescenzio, S. 1998 Transport of carbon and nitrogen to the northern Adriatic Sea by the Po River. Estuarine, Coastal and Shelf Science 46, 127–142. Pettine, M., Patrolecco, L., Manganelli, M., Capri, S. & Farrace, M. G. 1999 Seasonal variations of dissolved organic matter in the northern Adriatic Sea. Marine Chemistry 64, 153–169. Plavsˇ ic´ , M., Vojvodic´ & C u osovic´ , B. 1990 Characterization of surface active substances during a semi-field experiment on a phytoplankton bloom. Analytica Chimica Acta 232, 131–140. Sharp, J. H., Suzuki, Y. & Munday, W. L. 1993 A comparison of dissolved organic carbon in North Atlantic Ocean nearshore waters by high temperature combustion and wet chemical oxidation. Marine Chemistry 41, 253–261. Utermo¨ hl, H. 1958 Zur Vervollkommung der quantitativen Phytoplankton—Methodik. Mitt. Int. Ver. Limnol. 9, 1–39. Vilicˇ ic´ , D., Marasovic´ , I. & Kusˇ pilic´ , G. 1996/97 The heterotrophic ebridian microflagellate Hermesinum adriaticum Zach. In the Adriatic Sea. Archiv fu¨ r Protistenkunde 147, 373–379. Vojvodic´ , V. & C u osovic´ , B. 1996 Fractionation of surface active substances on the XAD-8 resin: Adriatic Sea samples and phytoplankton culture media. Marine Chemistry 54, 119–133. Vojvodic´ , V., C u osovic´ , B., Cauwet, G., Gasˇ parovic´ , B. & Kodba, Z. 1999 Variability of surface active organic material in the northern Adriatic. In Proceedings of the Portonova Conference (April 23–27, 1996), EC/Environment, Bruxelles, EUR 18834, pp. 281–295. Vollenweider, R. A., Rinaldi, A., Viviani, R. & Todini, E. 1996 Assessment of the state of eutrophication in the Mediterranean Sea. MAP Technical Reports Series No. 106, UNEP Athens, 455 pp. Z { utic´ , V., C u osovic´ , B., Marcˇ enko, E., Bihari, N. & Krsˇ inic´ , F. 1981 Surfactant production by marine phytoplankton. Marine Chemistry 10, 505–520.
Distribution and Seasonal Variability of Organic Matter in a Small Eutrophicated Salt Lake B. C u osovic´a, I. Ciglenecˇkia, D. Vilicˇic´b and M. Ahela a
Ruder Bosˇkovic´ Institute, Center for Marine and Environmental Research, POB 1016, 10001 Zagreb, Croatia University of Zagreb, Faculty of Science, Department of Botany, Rooseveltov trg 6, 10001 Zagreb, Croatia
b
Received 13 December 1999 and accepted in revised form 5 October 2000 Distribution and seasonal variability of dissolved organic carbon (DOC) and surface active substances (SAS) were studied along the depth profile (15 m) in a small eutrophicated and periodically anoxic sea lake (Rogoznica Lake, Eastern Adriatic coast) in 1996 and 1997. The range of DOC concentrations was characteristic for productive coastal marine ecosystems (60% of samples in the range of 1–2 mg l 1 and 40% between 2 and 3 mg l 1). Distribution of SAS concentrations was uniform and shifted toward higher concentrations in comparison to other coastal areas in the Adriatic Sea. Eutrophication in the lake is generated by nutrient recycling under anaerobic conditions. Systematically higher concentrations of chlorophyll a, DOC and SAS were determined at the chemocline in the bottom layer (10–12 m) than in the upper water layer (0·5–2 m). Seasonal variability of organic matter was discussed regarding distributions of microphytoplankton (cells >20 m) and photosynthetic pigments as well as oxygen and salinity changes along the depth profile. The dissolved oxygen saturation reaching up to 300% in the water layer between 8 m and 10 m depths in May and June 1996, was correlated with enhanced concentrations of phytoplankton biomass (reflected as chl a and b, fucoxanthin, peridinin, zeaxanthin) and increased concentrations of DOC and SAS. 2000 Academic Press Keywords: organic matter; surface active substances; photosynthetic pigments; phytoplankton; eutrophication in sea lake; Adriatic coast
Introduction Dissolved organic carbon is the largest organic carbon pool in the marine environment and it plays a central role in the marine biogeochemical cycle of carbon. The accumulation of DOC during the spring phytoplankton bloom and subsequent vertical mixing in fall and winter are the main processes to explain the field data of DOC seasonal and spatial distributions. A large part of dissolved organic matter in the sea represents organic compounds that possess surface active properties. Surface active substances accumulate at marine interfaces (seawater boundaries with the atmosphere, with living and nonliving dispersed and particulate material and sediment) making up a major part of the surface microlayer and organic coating at mineral particles. There is clear evidence of surfactant production by marine phytoplankton based on laboratory experiments with phytoplankton cultures as well as field measurements in real marine environments (Z { utic´ et al., 1981; Plavsˇic´ et al., 1990; Vojvodic´ et al., 1999). Total surfactants content in culture media generally increased with cell density while marked differences in quantities and kinds of surfactants between different cultures of microphyto0272–7714/00/120705+11 $35.00/0
plankton species were detected (Z { utic´ et al., 1981). The influence of bacteria and cell lysis in the surfactant production of marine phytoplankton make the problem more complex. Due to specific reactivity of SAS at natural phase boundaries, their distribution and fate in the sea could be different in comparison to those of the organic matter pool. For example, in the northern Adriatic the DOC values vary seasonally within a narrow range, approximately twice the value, while at the same time the SAS concentrations changed more than an order of magnitude (Vojvodic´ &C u osovic´, 1996). In order to contribute to a better understanding of the nature and reactivity of organic matter in the marine environments distribution and seasonal variability of dissolved organic carbon and surface active substances were studied along the depth profile (15 m) in a small intensely eutrophicated and periodically anoxic sea lake (Rogoznica Lake, Eastern Adriatic coast) in 1996 and 1997. The lake is selected as an unique ecosystem owing to its isolation and extreme conditions, caused by the absence of oxygen in anoxic bottom layers of the lake, all these influencing the number and kind of biological species. The microphytoplankton cell density and abundance of species in the lake were monitored 2000 Academic Press
706 B. C u osovic´ et al.
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and compared with concentrations of relevant photosynthetic pigments. Materials and methods The Rogoznica Lake is a small (area about 5300 m2, maximum depth of 15 m) meromictic saline lake which periodically becomes anoxic. Seasonal variations of anoxic conditions in the Rogoznica lake at 12 m depth
have been investigated since 1994 onwards (Ciglenecˇ ki et al., 1998a,b; Krsˇ inic´ et al., 2000). The lake has no visible connection with the surrounding sea but lake tides are detectable on cliffs, which indicates permanent water exchange through the porous karst. Geographic position of the lake is presented in Figure 1. It is assumed that the Rogoznica Lake was formed in the early Holocene at the time of the post-glacial rise in the sea level (Buljan, 1956).
Distribution and seasonal variability of organic matter 707
The lake is protected from the effects of the winds, because of the relatively high banks, and thus the stratification of the water column is maintained throughout most of the year. The stratification and mixing of lake waters were greatly affected by rainfall, as shown from decreased salinity in deeper waters (Ciglenecˇ ki et al., 1998a,b). During the thermohaline stratification, the surface water is well oxygenated while the deeper water layers become anoxic and rich in reduced sulphur species (up to 900 M (Ciglenecˇ ki et al., 1996)). Samples of Rogoznica Lake water were collected with 5 l Niskin sampling bottles in April and October 1995 and at approximately monthly intervals between October 1995 and October 1997, along the vertical profile of the Lake. Dark glass bottles were used for storage and transportation of the samples at 0–4 C. Samples for SAS were analysed non-filtered within 24 h. For DOC analyses seawater samples were filtered immediately after sampling (Whatman GF/F filters-pore size 0·7 m), poisoned with HgCl2 solution and stored cold in the dark. Samples for phytoplankton analysis were preserved in 2% (final concentration) neutralized formaldehyde solution. Adsorption effects of SAS at the mercury electrode were measured by A C voltammetry (out of phase mode) as described previously (C u osovic´ & Vojvodic´ , 1982, 1987). For quantitative determination, the calibration curve of the nonionic SAS, Triton-X-100 was used. Electrochemical measurements were performed with a Methrom E-506 polarecord (Methrom, Switzerland) using a hanging mercury drop electrode, and Ag/AgCl as the reference electrode. A high-temperature catalytic oxidation analyser (TOC-500 Model, Shimadzu, Japan) was used for the determination of DOC by direct injection of a filtered sample. Samples for the photosynthetic pigment analyses (0·5–1 l) were filtered onto 47 mm GF/F filters. Filters were immediately stored at 20 C until analysis. The filters were extracted in 4 ml of cold 90% acetone using sonication, centrifuged to clarify the extract, and the chlorophylls and carotenoids separated by reversed-phase HPLC according to Barlow et al. (1993). Briefly, extracts were mixed (1:1, v/v) with 1 ml l 1 ammonium acetate and injected into a HPLC system. The HPLC system consisted of two pumps (LKB, Bromma, Model 2150), gradient controller (Model 2152), injector (Rheodyne, Model 7125), a C18 m Pecosphere column (3·30·45 cm, Perkin Elmer) and serially coupled spectrophotometric and spectrofluorimetric
detectors. A binary linear gradient was used to separate the pigments. Solvent A consisted of 80:20 (v/v) methanol : 1 mol l 1 ammonium acetate, while solvent B contained 60:40 (v/v) methanol:acetone. Chlorophylls and carotenoids were detected by absorbance at 440 nm (Spectra Physics UV 2000). Data collection and reprocessing utilized Varian Star software. Cell counts were obtained by the inverted microscope method (Utermo¨ hl, 1958). Subsamples of 25 and 50 ml were analysed microscopically after sedimentation times of 24 h and 48 h, respectively. Microphytoplankton cells (MICRO, cells longer than 20 m, including those of microheterotrophic species Hermesinum adriaticum), were counted under magnifications of 400(1–2 transects) and 100(transects along the rest of the counting-chamber base plate). Results Dissolved organic matter and surface active substances Determinations of dissolved organic carbon in lake samples collected from different depths (by approximately monthly frequency in the period from February 1996 to July 1997) showed that most of 136 samples (about 60%) contained between 1·0 and 2·0 mg l 1 of DOC [Figure 2(a)]. About 40% of samples contained more than 2·0 mg l 1 while only few samples from deeper water layer of the lake contained more than 5 mg l 1 of dissolved organic carbon. The mean DOC value of 1·75 mg l 1 was determined for Rogoznica Lake samples. Mean values of DOC for open waters of the Adriatic Sea were determined to be 1·2 mg l 1 for the middle Adriatic samples and 1·78 for the north Adriatic Sea samples (C u osovic´ & Vojvodic´ , 1993). When comparing to the Rogoznica Lake, very similar range of DOC values (minimum value of 1·13 mg l 1 and maximum value of 3·06 mg l 1) were found in surface waters in the northern Adriatic Sea in the period from 1989 to 1993 (Vojvodic´ & C u osovic´ , 1996). This similarity is expected since the northern part of the Adriatic Sea is a shallow basin and the most productive part of the Adriatic Sea (Pettine et al., 1998; Gilmartin & Revelante, 1983; Degobbis et al., 1995), known for its anthropogenic eutrophication (Vollenweider et al., 1996). Eutrophication in the lake is strongly influenced by nutrient recycling under anaerobic conditions. In the bottom layer, high concentrations of nutrients were detected such as ammonia up to 200 M, phosphate at about 14 M and silicate at 300 M (Ciglenecˇ ki et al., 1998b;
708 B. C u osovic´ et al.
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F 2. Distribution of (a) DOC and (b) SAS concentrations, equivalent to Triton-X-100, in the Rogoznica Lake, during the period February 1996 and July 1997 (136 samples), and (c) correlation of SAS and DOC concentrations (r=0·1795). Lines in (c) correspond to different model substances: (1) protein albumin, (2) fulvic acid, (3) polysaccharide dextran and (4) polysaccharide xanthan.
Krsˇ inic´ et al., 2000). Direct freshwater input from local precipitation brings some nutrients to the lake and also increases eutrophication. As shown in Figure 3(b), seasonal variability of DOC values is not well defined neither for surface
water (0·5–2 m depth) nor for the bottom water layer (12 m depth) in the Rogoznica Lake. Generally, in all seasons higher values of dissolved organic carbon were determined in the bottom layer where most of the time anoxia or hypoxia
Distribution and seasonal variability of organic matter 709
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F 3. Seasonal variabilities of (a) chlorophyll a, (b) DOC and (c) SAS concentrations at two different depths: ( ) 0·5 m and ( ) 12 m.
prevailed (Ciglenecˇ ki et al., 1998a). Anaerobic mineralization enhanced supply of nutrients in deeper waters of the lake that resulted in increased phytoplankton biomass as shown from chlorophyll data in Figure 3(a). Concentrations of SAS as determined by the electrochemical method showed a sharp maximum in
the frequency distribution in the range between 0·16 mg l 1 and 0·32 mg l 1 equiv. Triton-X-100 (83% of the total number of 136 samples fall into that concentration range) [Figure 2(b)]. It is characteristic for the lake that low concentrations of surface active substances (in the range between 0·08 and 0·16 mg l 1 equivalent
710 B. C u osovic´ et al.
Triton-X-100) were found in very few samples. None of the sample in the lake had the concentration of SAS below 0·08 mg l 1, which is quite different from the previously reported data for other marine and estuarine environments including the highly eutrophic northern Adriatic Sea (C u osovic´ et al., 1985; Marty et al., 1988), where such low surfactant concentrations were determined in the whole water column in winter months, and sporadically, in deeper water layer in different seasons (Vojvodic´ & C u osovic´ , 1996). Seasonal variabilities of SAS concentrations in the Rogoznica Lake are presented for the surface layer (0·5–2 m depth) and for the bottom layer (12 m depth) in Figure 3(c). As observed for the DOC values, concentrations of SAS in the lake were systematically higher in the bottom layer than in the surface layer. Maximum concentrations of surface active substances were determined in June–July in both years. The correlation of SAS and DOC values, made for all samples collected in the Rogoznica Lake, in the period from February 1996 to July 1997, is presented in Figure 2(c). The correlation coefficient is rather low (r=0·19) indicating a weak but still significant coupling of those two parameters at 95% probability. The variations of the chemical composition of organic matter, i.e. the variable partition of strongly adsorbable organic substances in the dissolved organic matter pool, could be a fair explanation for relatively low correlative pattern between DOC and SAS values in this lake. A comparison of normalized surfactant activities of the samples from Rogoznica Lake was made with different model substances as representative constituents of organic matter in natural samples. The obtained surfactant activity value (SAS eq. T-X-100) for each model substance was normalized for its organic carbon content (C u osovic´ & Vojvodic´ , 1998). The obtained normalized surfactant activity values were as follows: oleic acid (2·7), protein (here albumin) (0·20), fulvic acid (0·17), polysaccharides: dextran T-500 (0·20) and xanthan (0·04). Some of these values were introduced in Figure 2(c) as dashed lines of the corresponding slopes. The normalized surfactant activity value for the lake samples was determined as the ratio between the mean SAS value (0·18 mg l 1 eq. T-X-100) and the mean DOC value (1·75 mg l 1), which gives the mean normalized surfactant activity value of approximately 0·1. In comparison with the north Adriatic samples there is a slight shift towards higher surfactant activity values for the Rogoznica Lake samples. This could be ascribed to the higher influence of sulphur compounds on the adsorption properties of
organic matter in the lake water (Ciglenecˇ ki et al., 2000). Phytoplankton and organic matter In the period between February 1996 and July 1997, the density of microphytoplankton cells at different depths and in different seasons varied between values below 103 cells 1 1 and the maximum value of 7·65106 cells 1 1, that was determined in the surface layer (0·5–2 m) in July 1997. A small number of microphytoplankton species (30) was found during the research activities. Twentyfive species, more abundant than 1000 cells 1 1, dominated in the Rogoznica Lake (Table 1). Diatoms provided 15 species, nine dinoflagellates and only one coccolithophorid. No correlation was found for the Rogoznica Lake samples between the microphytoplankton cell densities and either the DOC values or the SAS concentrations. For example, in a number of seasons rather low cell densities were determined in the surface layer (0·5 m) while, in contrast, the organic matter content was relatively high in the upper water layer. Moreover, taking into consideration 100 pairs of data for chlorophyll a and DOC concentrations the calculated correlation coefficient was rather low (r=0·09) indicating a more complex relationship between these two parameters in the Rogoznica Lake. At the same time a correlation coefficient of r=0·17 for chlorophyll a and surface active substances indicated a weak but significant coupling of those parameters at 90% probability. However, for 95% probability the coupling is not any more significant. In Figure 4(a,b) the variation of the maximum saturation with oxygen along the vertical profile of the lake water was considered as an illustration of the photosynthetic activity in the lake. Comparison is also made with the cell density of the microphytoplankton. Diatoms were found to be the dominant microphytoplankton group in most lake samples, except for May to June 1996, when dinoflagellates were determined by microscopic counting, as denoted in Figure 4(b). It is obvious from Figure 4(a) that the position of the maximum saturation with oxygen as well as the position of the maximum density of microphytoplankton cells were shifted towards deeper layers of the lake in most productive periods of year, May/June/July, in comparison to other seasons. Extremely high oxygen saturation up to 300% was determined from May to June 1996 between 8 m and 10 m depths. Oxygen saturation up to 300% in the water layer between 8 m and 10 m depths prevented the expected development of anoxia in the lake during the spring and summer of
Distribution and seasonal variability of organic matter 711 T 1. List of dominant microphytoplankton species, with frequency of finding (F) d2% and maximum cell abundance d1000 cells l 1, in the Rogoznica Lake, in the period from 1995 to 1998. Number of samples=278
Taxa Caliptrosphaera oblonga Cerataulina pelagica Ceratium furca Chaetoceros affinis Chaetoceros convolutus Chaetoceros curvisetus Chaetoceros danicus Chaetoceros decipiens Cyclotella sp. Diplopsalis ‘ complex ’ Eunotia sp. Glenodinium spp. Gymnodinoid cells Gyrodinium spp. Hermesinum adriaticum Leptocylindrus danicus Nitzschia longissima Prorocentrum arcuatum Pseliodinium vaubanii Pseudonitzschia spp. Rhizosolenia fragilissima Scripsiella spp. Synedra spp. Thalasiossira decipiens Thalassionema nitzschioides
Higher taxonomic group
F
COCC BACI DINO BACI BACI BACI BACI BACI BACI DINO BACI DINO DINO DINO DINO BACI BACI DINO DINO BACI BACI DINO BACI BACI BACI
10 10 14 16 10 189 27 18 17 10 40 11 65 8 85 6 25 114 3 43 55 16 33 19 54
F(%)
Maximum cell abundance (cells l 1)
Date of recorded maximum
3·6 3·6 5·0 5·8 3·6 68·0 9·7 6·5 6·1 3·6 14·4 4·0 23·4 2·9 30·6 2·2 9·0 41·0 1·1 15·5 19·8 5·8 11·9 6·8 19·4
4000 205 400 3600 12 900 2400 9 031 000 169 800 21 600 146 000 2400 7 432 000 3600 45 800 220 000 203 800 3200 4 780 000 467 000 1000 6 520 000 3 890 000 30 530 101 700 754 700 815 000
27 Oct 1995 13 Jun 1996 18 Feb 1997 22 Feb 1996 1 Oct 1997 2 Apr 1998 7 Jul 1997 6 Oct 1998 27 Oct 1995 17 Jun 1998 21 Aug 1996 29 Apr 1997 29 Apr 1997 13 Jun 1996 17 Jun 1998 25 Aug 1998 13 Jun 1996 1 May 1996 18 Feb 1997 5 Jun 1997 13 Jun 1996 29 Apr 1997 18 Feb 1997 18 Nov 1997 30 Sep 1996
BACI=diatoms; DINO=dinoflagellates; COCC=coccolithophorids.
1996, except in layers below 13 m (Ciglenecˇ ki et al., 1998a). The oxygen saturation maximum is in good agreement with the maximum chlorophyll a value and with the maximum of SAS concentrations in both the upper water layer and the bottom (12 m) water layer as presented in Figure 3(a,c). The DOC maximum in Figure 3(b) for June 1996 was less pronounced than for the SAS maximum. In the time period of the detected maximum oxygen saturations between 200 and 300% the cell densities of microphytoplankton as determined by microscopic counting were low and, except for 12 June, dinoflagellates were the dominant species [Figure 4(b)]. Relatively high concentrations of chlorophyll a indicated that the majority of the phytoplankton cells at the time of sampling would be smaller than 20 m. In Table 2 mean concentration values are presented of selected photosynthetic pigments determined in the time period May/June 1996, in the same water layer, between 8 and 10 m, characterized by high oxygen saturations. In addition to chlorophyll a, we have selected fucoxanthin as a marker for diatoms,
peridinin for dinoflagellates, chlorophyll b for green algae and zeaxanthin for cyanobacteria. We have denoted also the maximum concentrations of pigments for the whole water column (values in brackets). Thus, high concentrations of peridinin (2443 and 2850 ng l 1) are well correlated with dinoflagellate bloom registered on 1 May 1996. On 12 June 1996 the determined concentrations of fucoxanthin are in accordance with predominance of diatoms as determined microscopically. On 26 June 1996 the microphytoplankton density was very low (mean value below 5000 cells 1 1 in that layer) and only dinoflagellates were detected microscopically despite the fact that the levels of fucoxanthin (125 ng l 1 between 8 m and 10 m depths and 980 ng l 1 at 13 m depth) are supporting the presence of fucoxanthincontaining dinoflagellates (Jeffrey et al., 1975) and probably some epiplanktonic diatoms colonized on zooplankton as reported for the lake by Krsˇ inic´ et al. (2000). In all three cases presented in Table 2 marked abundance of green algae (chl b) and cyanobacteria (zea) can also be noted.
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104
50
103 J FMAM J J A S OND J FMAM J J A S OND J FMAM J J A S OND 1995 1996 1997
Cell density (l–1)
Depth of max. saturation with oxygen (m)
712 B. C u osovic´ et al.
F 4. (a) Variations of the position of maximum oxygen saturation ( ) and maximum cell-density ( ) of microphytoplankton along the depth profile in the Rogoznica Lake: (b) Maximum oxygen saturation ( ) in different sampling situations and cell-density ( , ) of microphytoplankton in the same water layer. Open circles denote predominantly diatoms and squares dinoflagellates. T 2. Mean concentrations values of the selected pigments in the water layer between 8 m and 10 m depths in the Rogoznica Lake in May and June 1996. In brackets are given maximum concentrations of pigments on the depth profile
Date 1 May 1996 12 June 1996 26 June 1996
Dissolved oxygen saturation (%)
chl a (ng l 1)
Fuco (ng l 1)
Per (ng l 1)
chl b (ng l 1)
Zea (ng l 1)
223 270 190
3830 (4680) 2293 (7550) 1110 (6750)
120 (140) 906 (1660) 125 (980)
2443 (2850) 136 (2090) 53 (550)
330 (830) 153 (2360) 210 (3450)
310 (780) 103 (1760) 360 (3530)
chl a=chlorophyll a; Fuco=fucoxanthin; Per=peridinin, chl b=chlorophyll b; Zea=zeaxanthin.
From February to July 1997 the maximum oxygen saturation in the lake was determined to be between 100 and 130%, and the position of the maximum changed from 0·5 m in February 1997 towards 12 m depth in June 1997. The increase of the microphyto-
plankton cell density predominantly diatoms, from February 1997 to July 1997 [Figure 4(b)] was in a good correlation with the increase of concentrations of surface active substances [Figure 3(c)]. Extremely high DOC values were determined in the bottom layer
Distribution and seasonal variability of organic matter 713
38
(b)
(a)
36 34
Salinity
32 30 28 26 24 22
J
F
M
A
M
J J 1996
A
S
O
N
D
F
M
A
M
J J 1997
A
S
O
N
D
F 5. Annual variation of salinity at three different depths in the Rogoznica Lake: ( ) 0·5 m, ( ) 2 m and ( ) 12 m.
in May 1997 [Figure 3(b)], following a diatom bloom in April 1997. Discussion Considering the variations of temperature, salinity and concentrations of dissolved oxygen and reduced sulphur species in the Rogoznica Lake over several years (Ciglenecˇ ki et al., 1996, 1998a,b), the period from February 1996 to July 1997 was characterized by development of anoxic conditions only in the bottom layer, i.e. between 13 m and 15 m depths. In contrast, in years 1994 and 1995 anoxic conditions were detected in much shallower layers (below 9 m). Despite these differences we have not observed marked differences in distributions of DOC and SAS concentrations for the two periods. Spatial distribution of DOC and SAS in the water column indicate that higher DOC and SAS concentrations were determined in deeper layers as a result of increased phytoplankton activity on the boundary between the oxic and anoxic layers due to recycling of nutrients. The possible sources of DOC and SAS are excretion of organic matter and decomposition of particulate organic carbon. The increased biomass in the bottom layer is evidenced from higher chlorophyll a concentrations in comparison to those in the upper water layer. The range of DOC concentrations in the lake is characteristic for productive coastal marine ecosystems (about 60% of the samples in the range
between 1 and 2 mg l 1 and about 40% between 2 and 3 mg l 1) (Sharp et al., 1993; Vojvodic´ & C u osovic´ , 1996; Pettine et al., 1999). The distributions of DOC and SAS concentrations in the lake [Figure 2(a,b)] were found to be more uniform than those in other open and coastal marine systems (Marty et al., 1988; Vojvodic´ & C u osovic´ , 1996). For example, in the Rogoznica Lake, 83% of samples had concentrations of SAS in the range between 0·16 mg l 1 and 0·32 mg l 1 eq. T-X-100 compared to only 10% of samples in open waters of the northern Adriatic Sea (1979–1986) (Marty et al., 1988). In the Adriatic Sea the distribution of SAS was spread in a wider concentration range, i.e. from very low concentrations (0·02– 0·04 mg l 1 eq. T-X-100) to high concentrations (between 0·64 mg l 1 and 1·28 mg l 1 for very few samples. The most frequent values (about 40% of samples in the Adriatic Sea) were in the range from 0·08 to 0·16 mg l 1 eq. T-X-100. Concentrations of SAS below 0·08 mg eq. T-X-100 were never determined in the Rogoznica Lake, which means that vertical mixing in the lake during fall and winter is bringing up organic matter-rich deeper waters, which is very different from what usually occurs in the Adriatic Sea. Taking into consideration all the data collected on microphytoplankton cell-densities, we did not find a correlation with corresponding values of DOC and SAS concentrations for the same samples. The microphytoplankton cell density was not statistically correlated with chlorophyll a due to a significant but highly
714 B. C u osovic´ et al.
variable contribution of smaller phytoplankton cells to the total biomass and due to the contribution of some heterotrophs to cell-density counts (see Table 1 and Vilicˇ ic´ et al., 1996/97). DOC concentrations were not correlated with chlorophyll a data, while weak but significant correlation of chl a with SAS concentrations supported the evidence of surfactant production by marine phytoplankton (Z { utic´ et al., 1981). The observed differences in distributions of organic matter (DOC and SAS) and of microphytoplankton in 1996 and 1997 could be explained by different hydrographic situations (Figure 5), especially by variability of inputs of nutrient rich fresh waters. It should be stressed that the observed decrease of salinity in the lake exceeds the direct freshwater input from local precipitation and very probably includes also the freshwater inflow through the porous karst from a larger area. As shown in Figure 5, there is an apparent difference in salinity changes in different layers in the two subsequent years. In 1996 the freshwater input had a pronounced effect on salinity values in the whole water column. In May and June 1996, in the upper layer (0·5–2 m depth) salinity values were below 30, compared with a salinity of about 35 in the deeper layer. This resulted in a very stable thermohaline stratification in spring/summer 1996 with an enhanced phytoplankton bloom in deeper water layers of the lake. Increased concentrations of relevant photosynthetic pigments chlorophyll b and zeaxanthin, concomitant with the increased concentrations of chlorophyll a, indicated a marked contribution of nanoplanktonic green algae and picoplanktonic cyanobacteria to the oxygen saturation (between 200 and 300%) in deeper lake waters. In 1997 the input of freshwater was lower than in 1996 and the differences of salinity values along the depth profile of the lake from May 1997 to October 1997 were low. The maximum chlorophyll a values were also lower in 1997 while the maximum oxygen saturation values were between 100 and 130%. Besides the fact that phytoplankton is an important source of authochtoneous DOC and SAS in the Rogoznica Lake, demonstrated by coincidence of maximum oxygen saturation and chlorophyll a concentrations along the vertical profile in the lake in different seasons, it should be anticipated that other physicochemical (e.g. adsorption, association and aggregation, and degradation) and biological processes (e.g. bacterial degradation and heterotrophic consumption) influence the distribution of organic matter and determine its residence time in the lake. Rather uniform distributions of DOC and SAS concentrations in the lake are leading us to the
conclusion that abiotic and biotic processes are well balanced in the investigated eutrophicated sea lake. Conclusion The variability of dissolved organic matter and surface active substances was studied monthly during the period from February 1996 and July 1997 in the eutrophic, periodically anoxic sea lake. The range of DOC concentrations was found to be characteristic of productive coastal marine ecosystems (60% of samples were in the range of 1–2 mg l 1 and 40% between 2 and 3 mg l 1). Distribution of SAS was very uniform and at higher concentrations in comparison to the North Adriatic samples, while the concentrations of SAS below 0·08 mg eq. T-X-100 were not determined in the lake. Systematically higher concentrations of organic matter were determined at the chemocline in the bottom layer (10–12 m) than in the upper water layer (0·5–2 m). The increased biomass in the bottom layer is evidenced from higher chlorophyll a concentrations in comparison to those in the upper water layer. The vertical mixing in the lake during fall and winter brings up deeper water which is rich with organic matter. This does not occur in the Adriatic Sea. The microphytoplankton cell density was not statistically correlated with the DOC and SAS concentrations. A weak but significant correlation of chlorophyll a with SAS concentrations supported the evidence of surfactant production by marine phytoplankton. Phytoplankton cells smaller than 20 µm very probably represented majority of phytoplankton cells. The eutrophication in the lake is strongly influenced by nutrient recycling under anaerobic conditions prevailing in the bottom layer below 12 m depth. Direct freshwater input from local precipitation brings nutrients to the lake and increases eutrophication as well. Acknowledgement Financial support from the Ministry of Science of the Republic of Croatia is gratefully acknowledged. References Barlow, R. G., Mantoura, R. F. C., Gough, M. A. & Fileman, T. W. 1993 Pigment signatures of the phytoplankton composition in the northeastern Atlantic during the 1990 spring bloom. Deep-Sea Research II 40, 459–477. Buljan, M. 1956 Prvi nalazi sumporovodika (H2S) u vodi Jadrana. Mornaricˇki glasnik (Split) 2, 207–214. Ciglenecˇ ki, I., Kodba, Z. & C u osovic´ , B. 1996 Sulfur species in Rogoznica Lake. Marine Chemistry 53, 101–110.
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