Acta Botanica 2-2015.indd ACTA BOT. CROAT. 74 (2), 2015 333 Acta Bot. Croat. 74 (2), 333–343, 2015 CODEN: ABCRA 25 ISSN 0365-0588 eISSN 1847-8476 DOI: 10.1515/botcro-2015-0031 Diatoms in an extreme euxinic environment (Rogoznica Lake, eastern Adriatic coast) NIKOLA MALEŠEVIĆ1*, IRENA CIGLENEČKI2, ELVIRA BURA-NAKIĆ2, MARINA CARIĆ3, IRIS DUPČIĆ3, ENIS HRUSTIĆ3, DAMIR VILIČIĆ1, ZRINKA LJUBEŠIĆ1 1 University of Zagreb, Faculty of Science, Division of Biology, Rooseveltov trg 6, 10000 Zagreb, Croatia 2 Ruđer Bošković Institute, Division of Marine and Environmental Research, Bijenička cesta 54, 10000 Zagreb, Croatia 3 University of Dubrovnik, Institute for Marine and Coastal Research, Kneza Damjana Jude 12, 20000 Dubrovnik, Croatia Abstract – The Rogoznica Lake marine system is a small, karstic, naturally eutrophic and euxinic marine environment. Abundance and temporal distribution of phytoplankton was investigated in relation to environmental conditions in the period from 1998 to 2013. The 36 determined diatoms contributed 90% of the total phytoplankton abundance. The diatom composition is characterized by low species diversity and high single species abundance (up to 107 cells L–1). There were, on average, 2.6 diatom species per sample (maximum 14 diatom species per sample) refl ecting extreme environmental conditions. Dominant dia- toms Thalassionema nitzschioides, Cyclotella choctawhatcheeana, Dactyliosolen fragilis- simus and Chaetoceros curvisetus occurred repeatedly and were alternately dominant in the lake during the whole research period. Some diatoms were dominant only in limited period, like Cyclotella choctawhatcheeana (evident since 2001), and Pseudo-nitzschia spp. (evident in the period 2002 to 2009). It appears that the interplay of environmental conditions such as variability in thermohaline and redox conditions, nutrient and reduced sulphur concentration infl uence the phytoplankton development and abundance in the lake. Keywords: diatoms, extreme environmental conditions, marine lake, phytoplankton Introduction An increased level of reduced sulphur species (RSS, including sulphide, polysulphide, elemental sulphur and thio-componds) and seasonal changes between oxic and anoxic con- ditions have long been recognized in the Rogoznica Lake marine system, central Dalmatia (CIGLENEČKI et al. 2005, 2013, KRŠINIĆ et al. 2013). * Corresponding author, e-mail: nikola712@gmail.com MALEŠEVIĆ N., CIGLENEČKI I., BURA-NAKIĆ E., CARIĆ M., DUPČIĆ I., et al. 334 ACTA BOT. CROAT. 74 (2), 2015 The thermohaline conditions in this lake are mainly regulated by solar heating, evapora- tion and rainfall, which infl uence surface salinity variations (CIGLENEČKI et al. 2005, 2013). Surface water could be well oxygenated (oxygen saturation up to 300%), while hypoxia/ anoxia occurs in the bottom water layer (STIPANIČEV and BRANICA 1996, CIGLENEČKI et al. 1998, 2005, 2013). The boundary between oxic and anoxic water layers is usually situated below the depth of 9 m (CIGLENEČKI et al 2005, 2013). Although phytoplankton, zooplank- ton nauplii and larvae of benthic organisms may enter the lake through fi ssures between the rocks (KRŠINIĆ et al. 2000), few species are recorded in the lake. Since the anoxic event in September of 1997, when the presence of hydrogen sulphide in the whole of the water column was detected, investigations of the lake have been inten- sive. This event, followed by massive mortality of all organisms, was taken as a zero point from which ecological recovery of the lake was monitored (BARIĆ et al. 2003, CIGLENEČKI et al. 2005, ŠESTANOVIĆ et al. 2005, SVENSEN et al. 2008, BURIĆ et al. 2009, ŽIC et al. 2010). Before the anoxic event in 1997, the dominant species in the phytoplankton was the diatom Pseudo-nitzschia spp., which accounted for 88% of the phytoplankton community. The mi- croplankton usually was composed of about 30 species, among which the most frequent and abundant were the rare microfl agellates Prorocentrum arcuatum and Hermesinum adri- aticum (VILIČIĆ et al. 1996/1997, BURIĆ et al. 2009) and diatoms (Chaetoceros curvisetus and Eunotia sp.) (CIGLENEČKI et al. 2005, BURIĆ et al. 2009). Recently published papers re- ported that diatoms took over the domination of phytoplankton assemblages after 2001, accounting for on average up to 94%, recognizing Chaetoceros curvisetus and Dactylioso- len fragilissimus as dominant species until the end of 2005 (KRŠINIĆ et al. 2013). C. curvise- tus and dinofl agellate Ceratium furca were found as dominant species in the lake in sum- mer 2004 (SVENSEN et al. 2008). The calanoid copepod Acartia italica is the only metazoan plankton species commonly present in Rogoznica Lake surviving after anoxia episodes and reaching high abundance in the lake (KRŠINIĆ et al. 2000, 2013, SVENSEN et al. 2008). Before the disastrous anoxic event in 1997, top-down control governed the biological processes in the lake. This disappeared with anoxic stress conditions and it took several years for the full recovery of top-down control (KRŠINIĆ et al. 2013). Rogoznica Lake due to its relative isolation, semi-closed nature, environmental charac- teristics and size (10300 m2 and maximum depth 15 m) can serve as natural laboratory for studying extreme environment and certain biogeochemical processes. In this paper changes of phytoplankton composition and low species diversity are studied in relation to physico- chemical conditions in the lake. The special focus is on the diatoms, with the regularity of specifi c species domination being discussed. Materials and methods Study area Rogoznica Lake is situated on Gradina Peninsula (43°32’N, 15°58’E) in the vicinity of the settlement of Rogoznica and the city of Šibenik, on the eastern Adriatic coast (Fig. 1). It is a eutrophic, karstic lake with a surface area 10300 m2 and a maximum depth of 15 m. Lake is round in shape and is surrounded by 4–24 m high porous karst cliffs with under- ground connections with the open sea visible through tides with a certain delay. DIATOMS IN THE EXTREME EUXINIC ENVIRONMENT ACTA BOT. CROAT. 74 (2), 2015 335 Sampling and analyses Investigation of phytoplankton was conducted in the period from December 1998 to September 2013 (Tab. 1). Samples for phytoplankton and nutrient analyses were collected in 5 L Niskin bottles, from the water column in the center of the lake. Samples were taken from various depths from surface to the bottom (0, 2, 5, 7, 8, 9, 10, 11, 12, 13 m). Because Fig 1. Location of Rogoznica Lake (marked by arrow) in the eastern Adriatic coast. Tab. 1. Sampling of phytoplankton conducted in the period from December 1998 to September 2013. Sampling campaigns are marked with »+«. Date Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec 1998 + 1999 2000 + + + + + + 2001 + + 2002 + + + + 2003 + + + 2004 + + + + + + 2005 + + 2006 + + + + 2007 + + + 2008 + + + 2009 + + + + + + 2010 + + + + + 2011 + + + + + + 2012 + + + + + + 2013 + + + MALEŠEVIĆ N., CIGLENEČKI I., BURA-NAKIĆ E., CARIĆ M., DUPČIĆ I., et al. 336 ACTA BOT. CROAT. 74 (2), 2015 of the appearance of anoxic conditions at depths below 9 m, in this research these phyto- plankton data were excluded. The samples were preserved in a 2% (fi nal concentration) neutralized formaldehyde solution. Cell counts were obtained by the inverted microscope method according to UTERMÖHL (1958). Nutrient concentrations were measured by standard methods (STRICKLAND and PARSONS 1972, IVANČIĆ and DEGOBBIS 1984). Software packages Grapher 7 and Statistica 12 were used for statistical and graphical analyses. Results Physico-chemical conditions In the investigated period, seasonal variations in vertical gradients of salinity and tem- perature, together with changes between oxic and anoxic conditions indicated the existence of surface (0–2 m) and bottom water layers (12 m) with higher salinity and temperature values (Fig. 2). Mixing of the water layers (marked by arrows in Fig. 2) ending with the Fig. 2. Seasonal variations of salinity and temperature in the surface (0–2 m) and bottom waters (12 m) of Rogoznica Lake. Mixing of water layers is marked by arrows, months are denoted by abbreviations: J for January, A for April, J for July and O for October. In all these mixing oc- casions, except October 2011, hypoxic conditions (1–3 mg L–1) within the whole water column with elevated concentrations of nitrite (up to 3.5 μM) and ammonium (31 μM) were record- ed. In 2011 anoxia was present for more than 1 week in the whole water column of the lake. DIATOMS IN THE EXTREME EUXINIC ENVIRONMENT ACTA BOT. CROAT. 74 (2), 2015 337 formation of a chemically homogeneous water column, characterized by hypoxic or anoxic conditions, usually take place in autumn. All recorded mixing events in the investigated period except of the ones in October 2011, were characterized by hypoxic conditions (1–3 mg L–1) within the whole water column accompanied with elevated concentrations of nitrite (up to 3.5 μM) and ammonium (up to 31 μM). Anoxic holomixis was present in the whole lake for more than a week in October 2011. The maximum concentration of nitrate (44.21 μmol L–1) was detected in January 2005 in the surface layer (0–1 m). Phosphate concentration varied from 0.01 μmol L–1 to 33.1 μmol L–1 with the minimum recorded in May 2004 in the surface layer. Silicate concentra- tion varied from 0.34 μmol L–1 to 679 μmol L–1 (Tab. 3). Statistical analyses among nutri- ents showed a signifi cant positive correlation between nitrite and: nitrate (r = 0.15; p = 0.04), ammonium (r = 0.92; p = 0.00) and phosphate (r = 0.31; p = 0.00). Phytoplankton composition The phytoplankton composition in the lake is characterized by low species diversity and high single species abundance (up to 107 cells L–1) (Tab. 2). During the entire research pe- riod diatoms were dominant, accounting, on average, for 90% of the whole phytoplankton (Fig. 3, Tab. 3). In all, 36 diatoms were recorded in the lake with a low average diversity of 2.6 diatom species per sample (maximal number 14 diatom species per sample). Dominant diatoms were: Thalassionema nitzschioides, Cyclotella choctawhatcheeana, Dactyliosolen fragilissimus and Chaetoceros curvisetus (Tab. 2). Those diatoms occurred repeatedly in the lake, and were alternately dominant in the phytoplankton assemblage over the research period (Fig. 3). The diatom Cyclotella choctawhatcheeana was found in the lake for the fi rst time in May 2004, and was a permanent constituent of the phytoplankton assemblage until the end of the investigated period. Pseudonitzschia spp. was occasionally the domi- nant species from September 2002 to November 2009, and after that was not detected in lake (Fig. 3). In the fi rst half of 2000, when stratifi cation of the water column was very weak, and concentration of RSS increased in the upper layers the abundance of diatom spe- cies C. curvisetus decreased in the following period. In the summer of 2003, when thermo- haline conditions in the lake water weakened again, the abundance of C. curvisetus and D. fragilissimus decreased, while the abundance of T. nitzschioides slightly increased. The next mixing of the water layers occurred in winter 2005 and probably triggered the de- crease of abundance of all dominant species except of C. choctawhatcheeana. In the winter of 2007 mixing occurred again and consequently the abundance of the two dominant spe- cies D. fragilissimus and T. nitzschioides increased. In winter 2008 mixing resulted in a de- creased abundance of the two dominant species C. curvisetus and T. nitzschioides, while the abundance of D. fragilissimus and of C. choctawhatcheeana increased. In autumn 2010 during isothermy in the lake, abundance of C. curvisetus and T. nitzschioides stayed almost the same, while the abundance of other two dominant species decreased (Fig. 2, Fig. 3). Statistical analyses revealed a signifi cant positive correlation of D. fragilissimus with ni- trite (r = 0.15; p = 0.03) and with ammonium concentration (r = 0.19; p = 0.01). There was no signifi cant correlation between the development of other dominant diatoms and nutrient concentration distribution. Diatom abundance and development responded to mixing as shown in Fig. 3. Every mixing event had as consequence a change in the diatom develop- ment. Furthermore, D. fragilissimus had positive correlation with the species Pseudonitzschia spp. (r = 0.26; p = 0.00) and C. choctawhatcheeana with T. nitzschioides (r = 0.17; p = 0.02). MALEŠEVIĆ N., CIGLENEČKI I., BURA-NAKIĆ E., CARIĆ M., DUPČIĆ I., et al. 338 ACTA BOT. CROAT. 74 (2), 2015 Tab. 2. List of diatoms taxa found in the phytoplankton during the research period. MAX is for maxi- mum abundance of cells per L and Fr is frequency of appearance (382 samples is 100%). Taxa Fr (%) Max Cerataulina pelagica (Cleve) Hendey 2 137960 Chaetoceros affi nis Lauder 4 13300 Chaetoceros compressus Lauder 4 1122000 Chaetoceros curvisetus Cleve 51 1079090 Chaetoceros danicus Cleve 6 112530 Chaetoceros decipiens Cleve 2 28120 Chaetoceros diversus Cleve 2 6400 Chaetoceros lauderi Ralfs 2 1600 Chaetoceros perpusilus Cleve 2 560 Chaetoceros rostratus Lauder 2 800 Chaetoceros spp. 11 1165510 Cyclotella choctawhatcheeana Prasad 30 7926200 Cylindrotheca closterium (Ehrenberg) Reimann & J. C. Lewin 1 760 Dactyliosolen fragilissimus (Bergon) Hasle 30 2582210 Eunotia sp. 3 170808 Guinardia fl accida (Castracane) H.Peragallo 1 272790 Guinardia striata (Stolter.) Hasle 6 109730 Hemiaulus hauckii Grunow ex Van Heurck 1 1520 Hemiaulus sinensis Greville 1 380 Leptocylindrus danicus Cleve 6 313480 Leptocylindrus minimus Gran 2 7090 Licmophora sp. 3 1140 Microtabella interrupta (Ehrenberg) Round 2 760 Nitzschia longissima (Brébisson) Ralfs 10 63080 Pleurosygma sp. 6 380 Proboscia alata (Brightwell) Sundström 1 181060 Pseudo-nitzschia delicatissima (Cleve) Heiden 1 2974060 Pseudonitzschia spp. 6 622945 Rhizosolenia calcar avis Schultze 1 1890 Rhizosolenia delicatula Cleve 1 6400 Rhizosolenia fragilissima Bergon 1 4000 Rhizosolenia imbricata Brightwell 1 1140 Striatella unipunctata (Lyngbye) C. Agardh 3 22800 Thalassionema nitzschioides (Grunow) Mereschkowsky 30 1667885 Thalassiosira sp. 10 100475 unidentifi ed pennate diatoms 25 812600 DIATOMS IN THE EXTREME EUXINIC ENVIRONMENT ACTA BOT. CROAT. 74 (2), 2015 339 Tab. 3. Maximum (MAX), minimum (MIN) and average (AVG) of values of phytoplankton (during 1998–2013) and nutrients (during 2000–2008) in Rogoznica Lake. N – number of samples. Parametar AVG MAX MIN N Total microphytoplankton (cells L–1) 461 400 8 138 480 0 382 Diatoms (cells L–1) 451 360 7 926 200 0 382 Dinofl agelates (cells L–1) 10 040 840 910 0 382 NO3 (μmol L–1) 2.03 44.21 0 476 NO2 (μmol L–1) 0.26 3.7 0 478 NH4 (μmol L–1) 14.84 315 0.23 457 Total inorganic nitrogen (μmol L–1) 17.21 315.83 0.31 440 PO4 (μmol L–1) 1.98 33.1 0.01 496 SiO4 (μmol L–1) 5.5 679 0.34 497 Fig 3. Temporal distribution of: A) diatoms, total phytoplankton and Cyclotella choctawhatcheea- na; B) Chaetoceros curvisetus, Dactyliosolen fragilissimus and Tthalassionema. nitzschioi- des in Rogoznica Lake for the period 1998–2013. Diatom abundance is referring to average of 9 m water column. Mixing events are marked by arrows. MALEŠEVIĆ N., CIGLENEČKI I., BURA-NAKIĆ E., CARIĆ M., DUPČIĆ I., et al. 340 ACTA BOT. CROAT. 74 (2), 2015 Discussion The diatoms Thalassionema nitzschioides, Cyclotella choctawhatcheeana, Dactylioso- len fragilissimus, and Chaetoceros curvisetus, were found to be the most frequent and abundant phytoplankton species, and appeared recurrently during annual plankton succes- sion in the Rogoznica Lake. The maxima of species abundances in the lake are in the same order of magnitude as maxima of species abundances in other coastal regions of the eastern Adriatic Sea (CETINIĆ et al. 2006, VILIČIĆ et al. 2009, BOSAK et al. 2009, BUŽANČIĆ et al. 2012, MARIĆ et al. 2012, VIDJAK et al. 2012, GODRIJAN et al. 2013). The extreme environmental conditions (high sea- sonal variations in salinity and temperature; redox conditions and nutrients; periodically high RSS concentration in the whole water column) that characterize Rogoznica Lake gov- ern the development of the above mentioned species that exist in the lake (CIGLENEČKI et al. 2005, 2013, KRŠINIĆ et al. 2013). During the research period (1998 – 2013) only 36 diatom taxa were recorded in the lake, as a signifi cant difference from other coastal and semi en- closed basins along the eastern Adriatic coast; e.g. during one year of phytoplankton sam- pling in Šibenik Bay and Kaštela Bay in the middle Adriatic, 61 and 80 diatom taxa were recorded, (BUŽANČIĆ et al. 2012) and 100 diatom taxa were found in the northern Adriatic fjord-like Lim Bay (BOSAK et al. 2009). In comparison with other coastal regions of the Adriatic Sea, Rogoznica Lake is charac- terized by a high concentration of nutrients and their high variability within the water col- umn (Ciglenečki unpublished data). In potentially eutrophic coastal areas like the Krka Es- tuary and Kotor Bay, the recorded maximal concentrations of total inorganic nitrogen and silicates were up to 17.70 and 44.1 μmol L–1 (BOSAK et al. 2012; ŠUPRAHA et al. 2014), while in the Rogoznica Lake these numbers are several times higher, 44.21 and 679 μmol L–1, re- spectively. However, registered mean values in the lake for total inorganic nitrogen, sili- cates and phosphate were 2.09 μmol L−1, 55.5 μmol L−1 and 1.98 μmol L−1, respectively. According to nutrient values Rogoznica Lake can be regarded as a highly eutrophic system. Such a high concentration of phosphate was registered only in the most eutrophic environ- ments along the eastern Adriatic, e.g. in the Krka Estuary and Kaštela Bay (VIDJAK et al. 2012). During stratifi cation in the lake, the surface waters are poor in nutrients and phos- phate limited (CIGLENEČKI unpublished data). Mixing events in the lake bring nutrient-rich bottom waters to the surface layer, which, accompanied by decomposition of dead organ- isms due to the anoxic stress, drastically change the concentration of nutrients and their ra- tios. It has a potential infl uence on the phytoplankton composition, abundance and diversi- ty. However, it appears that the nutrients are not the main factor regulating the phytoplankton development in the lake. The impacts of redox conditions and light penetration might be crucial along with the mixing, concentration of nutrients and thermohaline conditions. Changes in the thermohaline conditions in favor of lower salinity recorded after 2001 in the surface and middle water layers (Fig. 2) (CIGLENEČKI et al. 2013) coincide with a change in phytoplankton composition. After 2001 diatoms took over the domination with four domi- nant species that were alternately dominant. Since D. fragilissimus abundance had a signifi - cantly positive correlation with nitrite and ammonium, it could be speculated that D. fra- gilissiumus could be a possible indicator of the mixing in the lake. Also, two dominant species C. choctawhatcheeana and T. nitzschioides had a positive correlation in the occur- rence in the lake. DIATOMS IN THE EXTREME EUXINIC ENVIRONMENT ACTA BOT. CROAT. 74 (2), 2015 341 Mixing of the water layers depends on meteorological conditions, which highly infl u- ence the intensity of the process (CIGLENEČKI unpublished data). Due to the oxidation of a relatively high concentration of sulphide, which is rapidly transported to the surface, a sud- den consumption of oxygen and the occurrence of anoxic conditions in the whole water column may be seen. Then the presence of RSS (sulphide, polysulphide and colloidal sul- phur formed by oxidation) can be found through the entire water column (CIGLENEČKI et al. 2013). On the other hand, if the mixing is slow, oxygenated surface waters diffuse to the bottom of the lake, or the bottom waters, with plenty of sulphide and nutrients, diffuse to the surface layer of the lake (as was recorded in September of 2003 and November of 2006); the mixing process can last up to 7 days, while the surface layer remains rich in oxy- gen the whole time (CIGLENEČKI unpublished data). In most cases when RSS, i.e. sulphide, enters in the upper water layer, the abundance of phytoplankton species decreases because of the toxic effect which sulphur species could have on the phytoplankton community. On the other hand, those mixing events result in nutrient enrichment of the surface layer, stimu- lating phytoplankton development. Variability in the stratifi cation conditions and the inten- sity of the mixing infl uence the phytoplankton development and domination of certain phy- toplankton species. C. curvisetus usually reaches the highest abundance in spring during weak stratifi cation. After the mixing in autumn, D. fragilissimus takes over the dominant position, probably due to the absence of C. curvisetus which prefers lower temperatures and nutrient concentrations (BURIĆ et al. 2009). Moderate phytoplankton abundance, extreme environmental conditions including high seasonal variations in salinity, temperature, mixing, periodically high RSS and nutrient concentrations characterize Rogoznica Lake as highly eutrophic system which can be re- garded as natural laboratory for monitoring of the selected biogeochemical processes. The changes in the phytoplankton composition and variability coincide with changes of the thermohaline conditions in the lake; only species with high adaptability to stress using en- cystment can survive in such an extreme environment. 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