Veliko Jezero Research Articles

The coastal ichthyofauna of the Mediterranean coral reef: the case of Mljet National Park (Croatia, southern Adriatic Sea)

The authors studied the structure of the coastal fish assemblage in a temperate coral reef within a marine protected area in order to provide a baseline information on the occurrence and temporal distribution of fish and to highlight the importance of the coral reef to ichthyofauna. The coastal fish assemblage was investigated at two sites in Veliko jezero (Mljet National Park) in the southern Adriatic Sea with a non-destructive SCUBA visual technique in the period from 2013 to 2021. Altogether, 38 fish taxa were recorded on the right bank (coral reef) and 36 species on the left bank. The presence of the coral reef at a depth range between 9 m to 12 m is the main factor differentiating the fish fauna in these two areas, which are otherwise governed by the same environmental factors. At the coral reef a decrease in fish diversity was discovered with a steady regression from 2013 to 2021.

The relationship between the geochemical and mineralogical characteristics of Calcocambisol, colluvium and recent marine lake sediment of the narrow seashore intertidal zone: a case study from the Veliko Jezero (Mljet Island, Croatia)

This study investigates the mineral composition, particle size distribution and geochemical characteristics of Calcocambisol, colluvium and recent marine lake sediment in a narrow intertidal seashore zone of the Veliko Jezero on the Island of Mljet (Croatia). The obtained results show that the fractions of Calcocambisol/colluvium less than 2 mm and 2 µm display similar particle size distribution (PSD) curves compared to marine lake sediments containing larger particles in these fractions. The smallest fractions of the investigated materials that are less than 1 µm show identical PSD curves. The bulk and clay mineral composition of the marine lake sediment show that the non-carbonate fraction is derived from weathering of the surrounding soils and colluvium containing quartz, feldspars and phyllosilicates (illitic material, kaolinites, chlorite, and a mixed-layer clay mineral, MLCM), as well as the authigenic formation of early-diagenetic pyrite, while one part is related to the yield of material by aeolian deposition (amphibole). The observed difference between the phyllosilicate mineral phases in the clay fraction of the Calcocambisol/colluvium and the carbonate-free clay fraction of the marine lake sediment is related to 1) the presence of chlorite in the marine lake sediment and 2) the higher content of MLCM in the Calcocambisol/colluvium. The chlorite in the marine lake sediment was inherited from the Calcocambisol/colluvium as a result of soil erosion prior to its complete destabilization in the soil. High Chemical Index of Alteration (CIA) values in the Calcocambisol and colluvium clearly indicate their intense weathering. Based on the Sm/Nd and Ti/Al ratios, it can be concluded that the aluminosilicates in the Calcocambisol, colluvium and marine lake sediment are of the same provenance. The distribution of each analysed element among the sequential fractions is very similar in both the Calcocambisol and colluvium. The highest concentrations for most of trace elements in the Calcocambisol, colluvium and marine lake sediment was determined in their residual fraction. Mn, Co and Pb show a different partitioning between the Calcocambisol/colluvium and marine lake sediment, respectively.

Past climate variations recorded in needle-like aragonites correlate with organic carbon burial efficiency as revealed by lake sediments in Croatia

The drivers of organic carbon (OC) burial efficiency are still poorly understood despite their key role in reliable projections of future climate trends. Here, we provide insights on this issue by presenting a paleoclimate time series of sediments, including the OC contents, from Lake Veliko jezero, Croatia. The Sr/Ca ratios of the bulk sediment are mainly derived from the strontium (Sr) and calcium (Ca) concentrations of needle-like aragonite in Core M1-A and used as paleotemperature and paleohydrology indicators. Four major and six minor cold and dry events were detected in the interval from 8.3 to 2.6 calibrated kilo anno before present (cal ka BP). The combined assessment of Sr/Ca ratios, OC content, carbon/nitrogen (C/N) ratios, stable carbon isotope (δ13C) ratios, and modeled geochemical proxies for paleoredox conditions and aeolian input revealed that cold and dry climate states promoted anoxic conditions in the lake, thereby enhancing organic matter preservation and increasing the OC burial efficiency. Our study shows that the projected future increase in temperature might play an important role in the OC burial efficiency of meromictic lakes.

Geochemistry of recent aragonite-rich sediments in Mediterranean karstic marine lakes: Trace elements as pollution and palaeoredox proxies and indicators of authigenic mineral formation

This study investigates the geochemical characteristics of recent shallow-water aragonite-rich sediments from the karstic marine lakes located in the pristine environment on the island of Mljet (Adriatic Sea). Different trace elements were used as authigenic mineral formation, palaeoredox and pollution indicators. The distribution and the historical record of trace elements deposition mostly depended on the sedimentological processes associated with the formation of aragonite, early diagenetic processes governed by the prevailing physico-chemical conditions and on the recent anthropogenic activity. This study demonstrated that Sr could be used as a proxy indicating authigenic formation of aragonite in a marine carbonate sedimentological environment. Distribution of the redox sensitive elements Mo, Tl, U and Cd was used to identify changes in redox conditions in the investigated lake system and to determine the geochemical cycle of these elements through environmental changes over the last 100 years. The significant enrichment of these elements and the presence of early formed nanostructured authigenic framboidal pyrite in laminated deeper parts of sediment in Malo Jezero, indicate sporadic events of oxygen-depleted euxinic conditions in the recent past. Concentrations of trace elements were in the range characteristic for non-contaminated marine carbonates. However, the increase in the concentrations of Zn, Cu, Pb, Sn, Bi in the upper-most sediment strata of Veliko Jezero indicates a low level of trace element pollution, resulting from anthropogenic inputs over the last 40 years. The presence of butyltin compounds (BuTs) in the surface sediment of Veliko Jezero additionally indicates the anthropogenic influence in the recent past.

The distribution of benthic foraminifera in Cladocora caespitosa coral banks of the Veliko Jezero sediments (Mljet National Park, eastern Adriatic Sea)

This paper compares distributional data of benthic foraminifera from surface sediments that were collected in 2003 and 2012 from Cladocora caespitosa coral banks in Veliko Jezero (Mljet National Park, eastern Adriatic Sea). Species composition and test preservation of benthic foraminifera provide information on the depositional history of shallow-water sediments, particularly on the source of the displaced foraminiferal tests. Two foraminiferal assemblages were identified. Peneroplids (Peneroplis pertusus and Penereplis planatus) and Elphidium crispum dominate in modern foraminiferal assemblages collected in 2012, whereas samples collected almost ten years earlier are characterized by frequent occurrences of Rosalina bradyi and Quinqueloculina sp. Our quantitative reconstruction revealed that symbiont-bearing peneroplids, foraminifera that are often associated with reef building corals, became dominant constituents over several years in this area. Taphonomic signatures of the assemblages are correlated to taxonomic units and granulometric properties of substrate sediments, indicating that taphonomic processes are crucial in defining the composition of assemblages.

Temporal patterns of the calanoid copepod community in Veliko Jezero, an isolated marine lake (South Adriatic Sea): links to a larger-scale climate changes

We complied and analysed a long-term calanoid community data set (1952–2009) with respect to environmental variations in nearly closed seawater lake Veliko Jezero (South Adriatic Sea). Changes in hydrographic properties were influenced by deepening and broadening the channel between Veliko Jezero and the sea in the early 1960s, which was in particular reflected by increasing salinity. Calanoid abundance displayed high intra- and inter-annual fluctuations but lacked any regular and recurrent pattern. Calanoids in the lake comprised 22 coastal and estuarine species, with five of them present over the entire study period (Calanus helgolandicus (Claus, 1863), Paracalanus parvus (Claus, 1863), Centropages kroyeri Giesbrecht, 1893, Isias clavipes Boeck, Acartia (Acartiura) clausi Giesbrecht, 1889). Four sample groups were identified by cluster analysis, with the highest degree of dissimilarity between samples from the 1950s and those from the 1990s and onwards. Observed changes – low densities in early 1980s; a shift in the dominant species; reduction and, finally, disappearance of Pseudocalanus elongatus (Boeck); increases in Paracalanus parvus and Diaixis pygmaea (Scott T., 1894) – can be associated with synchronous changes across trophic levels that have occurred in other European seas. The results presented here underline the importance of long-term studies of semi-closed marine lakes and lagoons, as these are particularly sensitive to global climatic changes.

Impact of bleaching on the coral Cladocora caespitosa in the eastern Adriatic Sea

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 509:193-202 (2014) - DOI: https://doi.org/10.3354/meps10962 Impact of bleaching on the coral Cladocora caespitosa in the eastern Adriatic Sea Petar Kružić1,*, Lovrenc Lipej2, Borut Mavrič2, Petra Rodić3 1Laboratory for Marine Biology, Department of Zoology, Faculty of Science, University of Zagreb, Rooseveltov trg 6, 10000 Zagreb, Croatia 2Marine Biological Station Piran, National Institute of Biology, Fornaˇce 41, 6330 Piran, Slovenia 3State Institute for Nature Protection, Trg Mažurani΄ca 5, 10000 Zagreb, Croatia *Corresponding author: pkruzic@zg.biol.pmf.hr ABSTRACT: Thermal anomalies during the last 15 yr caused bleaching in the zooxanthellate scleractinian coral Cladocora caespitosa (Linnaeus, 1767) in the eastern part of the Adriatic Sea. In this study, we assessed the long-term response of this temperate coral to warming seawater in the Adriatic Sea and described, for the first time, the relationship between recurrent coral bleaching events and local seawater temperature regimes. These were also the first coral bleaching events reported for the Mediterranean Sea. Coral bleaching was recorded in the period from 1997 to 2012 in Veliko Jezero within Mljet National Park (NP), Croatia. The most severe bleaching in Veliko Jezero was observed during the summer of 2003, when C. caespitosa colonies were exposed to temperatures >29°C for 46 d. A total of 426 colonies experienced partial bleaching and 133 colonies died due to complete bleaching. Bleaching results during 2011 were compared with bleaching events in the Piran area (Slovenia) over this same temporal interval. The highest seawater temperatures were recorded in the period from August to October of 2011 (28.8°C in Piran, 30.3°C at Mljet NP). Severe bleaching was observed on 54 (29.3%) colonies within the Mljet coral bank, while 39 colonies with complete bleaching died. In the Piran area, severe bleaching was observed on 26 (34.7%) colonies, while complete bleaching occurred in 8 (10.6%) colonies. Under the present climate-warming trend, the coral bleaching reported here could lead to a severe decline in the abundance of the surveyed populations of C. caespitosa. KEY WORDS: Cladocora caespitosa · Coral bleaching · Coral mortality · Adriatic Sea Full text in pdf format PreviousNextCite this article as: Kružić P, Lipej L, Mavrič B, Rodić P (2014) Impact of bleaching on the coral Cladocora caespitosa in the eastern Adriatic Sea. Mar Ecol Prog Ser 509:193-202. https://doi.org/10.3354/meps10962 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 509. Online publication date: August 27, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

Mesh size effects on mesozooplankton community structure in a semi‐enclosed coastal area and surrounding sea (South Adriatic Sea)

AbstractThis paper presents the results from a study in which two commonly used plankton mesh sizes (125 and 200 μm) were compared. We examined the influence of the mesh size effects on zooplankton community structure by comparing abundance, biomass and seasonal cycles of mesozooplankton. The study was performed at five stations (four in the marine lakes ‘Veliko jezero’ and ‘Malo jezero’, and one in the open sea in front of the lakes) during a 1‐year period (February 2008–February 2009). The estimated differences obtained from the catch results of the different mesh nets were more pronounced in semi‐enclosed coastal areas, such as lakes, than in the oligotrophic conditions prevailing at the station in the front of the lakes. Total zooplankton abundance with the 125‐μm net was significantly greater than with the 200‐μm net. The differences were mainly due to the loss of small‐sized copepods in the samples collected with the 200‐μm net. Excepting copepods, major losses were recorded for Limacina spp. and Bivalvia larvae. Abundance losses through the coarser net reflect the community structure and seasonal abundance pattern, which is most pronounced during the summer. Diversity indices and total dry weight were less influenced by mesh selection. The retention efficiency of the 125‐μm mesh size must be taken into account to provide a reliable description of the mesozooplankton community structure in coastal and especially eutrophicated areas. The 200‐μm mesh net could be recommended for oligotrophic systems due to the catch of larger calanoid species that contribute to overall zooplankton biomass, and because it allows a comparison with historical datasets.

Coral tissue mortality of the coral Cladocora caespitosa caused by gastropod Coralliophila meyendorffi in the Mljet National Park (eastern Adriatic Sea)

The corallivorous gastropod Coralliophila meyendorffi (family Coralliophilidae) is a well-known predator of the coral Cladocora caespitosa, particularly in the eastern Adriatic Sea, where population outbreaks can drastically reduce coral cover. Coralliophila meyendorffi ranges in size from 5 to 40 mm in shell total length, and smaller specimens are often found living with the coral C. caespitosa. Specimens of C. meyendorffi feed exclusively on live coral tissue, stripping it from the calcium carbonate skeleton. Recent outbreaks of the gastropod C. meyendorffi have caused considerable damage on a C. caespitosa bank in Veliko Jezero (the Mljet National Park, eastern Adriatic Sea). The bank occurs at depths between 6 and 18 m, covering a 65 m2 area. During summer of 2010, a substantial outbreak affected about 5%t of the coral colonies. The gastropods were highly aggregated in sheltered areas of the coral bank (up to 41 specimens/dm2) with smaller clusters (from 8 to 12 specimens/dm2) on coral colonies that were influenced by strong bottom currents (up to 1.23 ms−1).

Predation patterns and prey quality of medusae in a semi-enclosed marine lake: implications for food web energy transfer in coastal marine ecosystems

Veliko Jezero (Mljet, Croatia) is a nearly enclosed karstic depression filled with saltwater, where jellyfish and prey exchange with the Adriatic Sea is negligible, making this small ecosystem ideal for the controlled study of medusae in food webs. Based on the analysis of their gut contents, medusae appeared to ingest less carbon than expected on the basis of their carbon content. To accurately define carbon ingestion by medusae, we determined the diet of Aurelia sp. 5 (Dawson and Jacobs, 2001) (Scyphozoa: Semaeostomeae) from Veliko Jezero by combining gut content and stable isotope analyses. During daytime, gut contents identified a mix of small copepods as the dominant prey (62%). In contrast, feeding models based on stable isotope values of medusae and their potential prey analyzed using Stable Isotope Analysis in R indicated appendicularians could have made the greatest contribution to Aurelia sp. 5 diet (14–78%), followed by calanoid copepods (0–50%) and fish larvae (0–43%). Because appendicularians and fish larvae are abundant with medusae at night near the bottom of the “lake” and contain more carbon than small copepods, we suggest diel movements of Aurelia sp. 5 and carbon content of prey determine the carbon assimilated by medusae, with night-time prey making a

Did the alien calycophoran Muggiaea atlantica outcompete its native congeneric M. kochi in the marine lakes of Mljet Island (Croatia)?

AbstractThe calycophoran siphonophore Muggiaea atlantica was recorded for the first time in the marine lakes of Mljet Island (South Adriatic) in 2001. The presence of M. atlantica in such semi‐enclosed environments provided a unique opportunity to describe its life cycle based on in situ observations of the annual variability of its asexual nectophore stage, sexual eudoxid stage (produces gonophores) and calyconula larvae. Muggiaea atlantica was present the whole year in the Malo jezero (Small Lake, SL), where it was actively reproducing between May and September 2001, with a maximum density of gonophores in May (141 gonophores m−3) and nectophores and larvae in June (37 nectophores m−3 and 80 larvae m−3). Higher temperatures below the thermocline (average 14.5–15.6 °C), and the availability of prey (e.g. small copepods, chaetognaths and bivalve larvae) were the main environmental factors related to successful reproduction of M. atlantica in SL. In the Veliko jezero (Great Lake, GL) nectophores and gonophores of M. atlantica were found in low numbers, mainly in the upper 20 m, between September and November 2001 and no larvae were recorded. In 2001, Muggiaea kochi, the sole calycophoran siphonophore previously reported in the GL, was not recorded in the Mljet lakes. We hypothesize that the cold (<13 °C) conditions over winter of 2000/2001 were not suitable for the warm‐temperate M. kochi and may have favoured its cold‐temperate congener, M. atlantica, which has progressively colonized the southern Adriatic since the mid‐1990s.

Geochemical conditions for the preservation of recent aragonite-rich sediments in Mediterranean karstic marine lakes (Mljet Island, Adriatic Sea, Croatia)

Conditions for the preservation of recent aragonite-rich sediments during early diagenesis in two semi-enclosed Mediterranean karstic seawater lakes on the island of Mljet (Adriatic Sea) were examined. The concentrations and stable isotope compositions of carbonate and sedimentary organic matter, as well as the geochemical parameters in pore water were measured. It was found that the smaller lake (Malo Jezero) receives considerably more terrestrial detritus than the larger lake (Veliko Jezero). A decrease in carbonate δ13C values with depth indicated a rather intensive transfer of organically derived C into the carbonate pool by diagenetic recrystallisation, masking the changes in carbonate δ13C caused by increasing amounts of aragonite. Dissolution of calcite as a result of CO2 released from the decomposition of organic debris and the upward diffusive flux of dissolved inorganic carbon were together responsible for up to 24% of the dissolved inorganic carbon added to the pore water. This indicated locally occurring carbonate dissolution, irrespective of its saturation state in the bulk sediment. Despite the larger input of terrigenous material into Malo Jezero, the carbonate content in the sediment was much higher than in Veliko Jezero, indicating greater authigenic aragonite production. As magnesium calcite accounted for most of the carbonate dissolution, aragonite preservation in the sediment is favoured.

Natural and anthropogenic sources of Hg, Cd, Pb, Cu and Zn in seawater and sediment of Mljet National Park, Croatia

Distributions of Hg, Cd, Pb, Cu and Zn in seawater and sediment from Mljet National Park, Adriatic Sea are presented for the first time. Natural and anthropogenic factors play an important role in determining resultant trace metals' concentrations in the region. We place particular emphasis on the saline “lakes” of Malo Jezero and Veliko Jezero, which have restricted flows of seawater. In Malo Jezero lake, fresh karstic spring water generated by flooding, and weathering of dolomites are the main sources of naturally elevated Cd, Pb and Zn concentrations (20.7 ± 1.6, 289 ± 19, 1260 ± 0.08 ng L −1, respectively); anthropogenic input is negligible. In Veliko Jezero lake enhanced Cu and Zn contents originate from anthropogenic input (tourism and agriculture). Distributions of the Pb and Zn in the water columns of both lakes are influenced by natural aragonite precipitation and sedimentation. Exceptionally high total Hg concentrations of 24.2 and 33.7 ng L −1 in the water column of Malo Jezero, sampled during periods of high rainfall associated with strong eastern winds, suggest an airborne input. Total Hg concentrations in waters of both lakes are elevated because of inefficient mixing. Two different metal distribution patterns exist in the sediment columns. First, Hg, Pb, Cu and Zn show elevated concentrations in recent sediments due to anthropogenic input. Second, Cd content increases with depth due to reprecipitation via a downward redox boundary shift. Described natural processes, as well as anthropogenic influence, enhance levels of trace metals. Careful study followed by suitable interpretation based on geochemical data were necessary to distinguish natural from anthropogenic sources.

Acoustic survey of a jellyfish-dominated ecosystem (Mljet Island, Croatia)

Acoustic techniques have been proposed as a new tool to assess jellyfish populations. However, the presence of mixed echoes from jellyfish and other organisms that share their distribution often prevent accurate estimates of their abundance and distribution being obtained. The isolated population of Aurelia inhabiting the Veliko Jezero (Big Lake-BL) of Mljet Island, in the South Adriatic Sea, offered a good opportunity to employ acoustic techniques to assess an entire jellyfish population. During October 2–5, 2006, combined video and acoustic methods were used in BL to determine the vertical distribution of medusae. Two synoptic acoustic surveys were performed during the day and night. In the daylight echograms, medusae were clearly discernible from the acoustic data, and their presence verified by video camera images, as forming a layer of varying density at and below the thermocline (15–30 m). The depth of the jellyfish layer also coincided with the depth of maximum dissolved oxygen concentration. The echointegration of these daylight data enabled quantification the Aurelia population, at a frequency of 120 kHz. In the night echograms, the acoustic signals of Aurelia were at least partially masked by pelagic and demersal fish, which disaggregated from schools and formed a layer associated with a strong thermocline at 15 m. An average target strength (TS) of —76.4 dB was obtained in situ corresponding to a mean length of 10.8 cm and a mean wet weight of 134 g measured from sampled medusae. These results were combined with echo-integration values to provide an estimate of 4,238,602 individuals and a biomass of 568 tons of Aurelia in BL. This study provided a synoptic view of Mljet Lake and illustrated the potential of acoustic surveys of jellyfish populations to contribute to ecosystems studies.

Bioconstructional features of the coral Cladocora caespitosa (Anthozoa, Scleractinia) in the Adriatic Sea (Croatia)

AbstractThe Mediterranean endemic Cladocora caespitosa (Linnaeus, 1767) is a colonial scleractinian coral belonging to the family Faviidae and the only zooxanthellate coral from Mediterranean whose colonies may fuse in reef‐like structures (hermatypic). Recent surveys are focused on three locations where banks occur in the Adriatic Sea (Croatia): near Prvić Island in the northern Adriatic, near Pag Island in the central Adriatic and in Veliko jezero (Mljet National Park) in the southern Adriatic. The C. caespitosa bank in Veliko jezero covers an area more than 650 m2 and is thus the largest bank of C. caespitosa found to date. The strong sea currents, which occur as a result of tidal exchange in the channel, appear to favour the growth of the bank. The goal of the study was to present the influence of major environmental factors upon the build‐up process of the coral bank. Biometrical parameters in the C. caespitosa colonies like diameter of the calyces, polyp ash free dry weight (AFDW), corallite linear growth rate and index of sphericity were investigated and compared from these three locations. The morphology of coral banks from the Adriatic Sea and the disposition of the biometrical values are affected by the sea currents, temperature and sedimentation.

Spawning of the colonial coral Cladocora caespitosa (Anthozoa, Scleractinia) in the Southern Adriatic Sea

Data on sexual reproduction of scleractinian coral species living in temperate zones, particularly in the Mediterranean Sea, are quite scarce. This paper describes sexual reproduction of the colonial coral Cladocora caespitosa from Veliko jezero (Mljet Island) in the Adriatic Sea. Spawned orange eggs and white sperm bundles were observed on the coral bank of C. caespitosa two nights before the full moon (20 June 2005) coinciding with increasing water temperature and correlated with the lunar cycle. Spawning was observed during five nights, involving about 30% of the colonies from the coral bank. Different colonies on the bank released only one type of gamete during the reproductive period. The diameter of the sperm bundles ranged from 100 to 200 μm (average 163 μm; SD = 47.08), while the female gametes diameter ranged from 300 to 500 μm (average 416 μm; SD = 73.12).

Mesaiokeras hurei n. sp. (Copepoda, Calanoida, Mesaiokeratidae) from the Adriatic Sea

A new species of Mesaiokeras is described from the hyperbenthos of Veliko Jezero, a marine lake on the island of Mljet, Croatia. This is the first species of this genus and family (Mesaiokeratidae) reported from the Adriatic and Mediterranean seas. It is characterized by small body length (an average 361 μm for females and 337 μm for males), the low number of free segments (21) and the armature of the antennules, very long spine (a 2 ) of exopodite I of female right leg 4, the presence of long processes on the ventroposterior area of the female genital double-somite, the form and surface ornamentation of the genital area, the form and ornamentation of male and female leg 5, and the form and armature of caudal rami. This species was found in shallow water with very stable hydrographic conditions and at a population density from I to 26 ind. m -3 .

Population study of the fan shell <i>Pinna nobilis</i> L. in Malo and Veliko Jezero of the Mljet National Park (Adriatic Sea)

SUMMARY: Although previous studies have noted the presence of the bivalve Pinna nobilis Linnaeus, 1758 in Mljet National Park, there is a lack of basic knowledge of this endangered species. The present study was initiated as part of a long-term investigation of P. nobilis density and growth with the aim of gaining a better understanding of this species’ population dynamics, as well as evaluating the efficiency of protection measures of P. nobilis in Mljet National Park. Density, population structure, growth of tagged individuals and epibionts on P. nobilis were studied using SCUBA diving in 1998 and 2000 along four transects in Mljet inlets, Adriatic Sea. The density of P. nobilis ranged from 0.20 ind/m 2 on transect MLJ-9 in Malo Jezero in 1998 to 0.02 ind/m2 on transect MLJ-5 in Veliko Jezero in 2000. Individuals measured in 1998 were significantly smaller than those measured in 2000. Growth curves were constructed from data collected by tagging and re-measuring 47 individuals. A decrease in density along some of the transects and the disappearance of 24 tagged individuals in 2000 is suggestive of poaching. The results of the present study indicate recovery of the P. nobilis population in Mljet lakes, but also provide evidence for the illegal removal of this protected species.

Benthic Foraminiferal Assemblages in a Restricted Environment - An Example from the Mljet Lakes (Adriatic Sea, Croatia)

Benthic foraminiferal assemblages from a peculiar restricted marine environment, the Mljet Lakes (Mljet Island, Adriatic Sea, Croatia) have been studied. These lakes are drowned karst dolines, which are connected with the Adriatic Sea through a narrow, shallow channel. Occasional stagnant conditions in the marine lakes cause hypoxic and anoxic conditions in the bottom waters. Such stressed conditions are reflected in oligospecific benthic foraminiferal assemblages with a Shannon-Wiener species diversity index (H) ranging from 0.8 to 1.0 and equitability index (E) ranging from 0.18 to 0.26, identified in samples from each marine lake. In the more dysoxic Malo Jezero, Haynesina depressula dominates an assemblage of 12 benthic foraminiferal species. In the less (and less frequently) hypoxic Veliko Jezero, we found an Asterigerinata mamilla a s s e m b l a g e with 18 foraminiferal species. A more diverse assemblage containing 55 different benthic foraminiferal species occupies an adjacent opensea station. Long-term salinity measurements indicate that H. depressula tolerates higher salinity than formerly presumed (up to 38 ‰), and is well adapted to stressed hypoxic conditions.

Holocene lake development of two Dalmatian lagoons (Malo and Veliko Jezero, Isle of Mljet) in respect to changes in Adriatic sea level and climate

Within the Austrian IGBP/PAGES project “Palaeolimnology of Adriatic–Alpine Lakes” (PAAL), geochemical and diatom analyses were performed on sediment cores of Malo and Veliko Jezero, two karstic coastal lakes on the island Mljet, Croatia. Combined with diatom calibration, they provide evidence of remarkable environmental changes during the Holocene. The formation of the lakes in the early Holocene was triggered by a rise in the Adriatic sea level from approximately −50 to −30 m, which resulted in the transmission of seawater through karst, and by a temporary increase in precipitation. The onset of this pluvial period was dated in Malo Jezero at 8.4 kyr B.P. and had its maximum approximately 7–6 kyr B.P. It was briefly interrupted by a dryer episode from about 7.2 until 7.1 kyr B.P. Between 6 and 5.5 kyr B.P., a transition to the present Mediterranean climate occurred. The marine ingression into Veliko Jezero through the Soline channel, with a present depth of 2.5 m, took place about 5 kyr B.P. From 5 to 4 kyr B.P., the oscillating sea level gradually approached the present value. A tephra layer at 7.3 kyr B.P. in both lakes was related to sudden environmental changes that were probably caused by tectonic movements accompanying Italian volcanism.