Marine Ecology. ISSN 0173-9565
ORIGINAL ARTICLE
Ecology of an anchialine cave in the Adriatic Sea with
special reference to its thermal regime
Maja Novosel1, Branko Jalžić2, And-elko Novosel3, Mira Pasarić4, Antonieta Požar-Domac1 &
Ivan Radić1
1
2
3
4
Department of Biology, Faculty of Science, University of Zagreb, Zagreb, Croatia
Zoological Department & Croatian Biospeleological Society, Croatian Natural History Museum, Demetrova, Zagreb, Croatia
Nerej Diving Film Tourism Ltd, Vojnovićeva, Kašina, Croatia
Andrija Mohorovičić Geophysical Institute, Faculty of Science, University of Zagreb, Zagreb, Croatia
Keywords
Adriatic Sea; Anchialine cave; hexactinellids;
Hvar Island; Oopsacas minuta; polychaetes;
sea temperature.
Correspondence
Maja Novosel, Department of Biology, Faculty
of Science, University of Zagreb, Rooseveltov
trg 6, 10000 Zagreb, Croatia.
E-mail: [email protected]
Abstract
This study examines the ecology and thermal regime in the Živa Voda anchialine cave on Hvar Island (Adriatic Sea, Croatia). The cave, which has no
direct connections with the open sea, contains a dense population of the deepsea hexactinellid sponge Oopsacas minuta Topsent, 1927. Several species of
polychaetes, molluscs and crustaceans were also recorded. Two-year temperature data recorded simultaneously at two depths within the cave and at a
nearby outside site provide information on thermal conditions in the system.
The temperature oscillations within the cave are strongly dampened – varying
from 14.6 C in winter to 17.9 C in summer – the annual range being four
times smaller than in the adjoining sea. These data suggest that O. minuta can
withstand higher temperatures than previously expected from deep-sea or coldwater cave occurrences, and that other environmental factors must be playing
an important role.
Problem
Anchialine habitats are coastal water bodies flooded with
seawater which fluctuates with the tides, and with no
obvious connection to the sea. Stock et al. (1986) defined
anchialine as bodies of haline waters, usually with a
restricted exposure to open air, always with more or less
extensive subterranean connections to the sea, and showing noticeable marine as well as terrestrial influences.
Along the eastern Adriatic coast, a several anchialine caves
have been recorded. Rad-a (2000) was the first to record
the presence of the hexactinellid sponge Oopsacas minuta
Topsent, 1927, normally known from the deep-sea, in the
Živa Voda anchialine cave. Ozimec & Jalžić (2002) then
provided a description of the same cave, while Kršinić
(2005) described Speleohvarella gamulini, a new genus
and species of calanoid copepod from this cave. Most
other studies on anchialine caves in the Adriatic Sea were
conducted on the surface, i.e. freshwater or brackish parts
of the water body (Sket 1986, 2005). This paper focuses
on the ecology and thermal regime of the Živa Voda anchialine cave on Hvar Island (Adriatic Sea, Croatia), particularly because this is only the eighth locality in the
world where hexactinellid sponges (glass sponges) are so
shallow that they can be reached by SCUBA diving (Leys
& Lauzon 1998; Jalžić et al. 2005; Bakran-Petricioli et al.
2007).
Material and Methods
The Živa Voda anchialine cave is situated on the island of
Hvar, on the border between the central and southern
Adriatic (Fig. 1). As this cave has no direct connections
with the open sea, both speleological and SCUBA diving
methods were necessary for this study. Both the land
and underwater parts of the cave have been mapped by
standard topographical methods used in speleology
(Fig. 2). The land part was mapped using a compass,
Marine Ecology 2007, 28 (Suppl. 1), 3–9 ª 2007 Blackwell Publishing Ltd No claim to original US government works
3
Ecology of an anchialine cave in the Adriatic Sea
Fig. 1. The Eastern Adriatic Sea with a detailed map of the study
location (O ¼ Živa Voda cave).
clinometer and measuring tape, while underwater parts
were surveyed using the compass, the depth gauge and
the measured guideline. The open sea water surface level
and the water surface level in both cave lakes were
measured using a geodetic level.
Novosel, Jalžić, Novosel, Pasarić, Požar-Domac & Radić
Sampling of benthic fauna inside Živa Voda cave was
carried out on 11–12 April 2003 and 2–3 April 2005.
Sampling was undertaken on the cave walls and on
bottom rocks and sediment. Samples were taken individually or by scraping from the walls, and were preserved in
70% ethyl alcohol.
Temperature conditions within the cave and at a
nearby open sea site were surveyed during 2 years using
small Onset StowAway TidbiT data loggers fixed on
rocks. The data were measured from 12 April 2003 to 16
February 2005, with ±0.2 C accuracy, and a 30-min
sampling interval. Three loggers were deployed at 12 and
24 m depths in the right channel of the cave and at 22 m
depth in the nearby open sea (Fig. 1). The obtained
2-year time series of 30-min temperature values were analysed together with corresponding smoothed time series,
formed respectively by 24-hour and 30-day moving averages (e.g. Emery & Thomson 1998). In order to examine
in more detail the seasonal temperature cycle, harmonic
analysis was performed by fitting the smoothed time series to a sum of an annual and semi-annual sine function
(e.g. Emery & Thomson 1998). The analysis of amplitudes
and phases reveals the relationship between heating and
cooling within the cave and in the nearby sea.
Fig. 2. Profile and ground plan with sections of the Živa Voda cave.
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Marine Ecology 2007, 28 (Suppl. 1), 3–9 ª 2007 Blackwell Publishing Ltd No claim to original US government works
Novosel, Jalžić, Novosel, Pasarić, Požar-Domac & Radić
Ecology of an anchialine cave in the Adriatic Sea
Results
Geologic and hydrographic setting
Živa Voda cave is located on the E-SE slope of Kozja Cove,
on the southern coast of the island of Hvar (Fig. 1). The
cave entrance is 31 m above sea level. Morphologically,
Živa Voda cave is a ramified system developed by corrosion
and erosion following bedding planes within Upper Cretaceous (Senonian) beds (Borović et al. 1977). Cave channels
clearly follow inclination of layers, from 45 to 50, and dip
towards the S-SE. These layers are the southern part of an
anticline whose axis extends along the island of Hvar in an
E-W direction. Along Kozja Cove numerous breccias were
observed, implying the presence of multiple transversal
faults on the eastern part of the island (Fig. 2).
The entrance passage is lens-shaped in profile and descends at an angle of 45 towards a large chamber and to
the left (east) towards a narrower channel. The large chamber bends towards the west and has a small pool of
1 · 1.5 m at its lowest point. Underwater, this right channel reaches a maximum depth of 38 m. In the left channel,
after 30 m of dry passage, there is a 5 · 3 m pool with submerged passage extending to a maximum depth of 26 m.
Both channels have a thick layer of mud on the floor.
At the surface of both pools, there is a brackish layer
approximately 3 m deep. The water surface in both lakes
is on average 50 cm above sea level, probably due to the
lagging capillary water exchange between the open sea
and the cave as well as the constant input of freshwater
into the cave. Although the limestone bedrock has a high
secondary permeability, there is no (or only extremely
weak) circulation between the left and right channels. The
left channel contained no macrofauna, whose absence
here may be due to the configuration of the cave – the
left channel extends under the island, while the right
channel trends towards the sea.
Fig. 3. Time series of water temperature, recorded with a 30-minute
sampling interval at two depths in Živa Voda cave (top) and at a nearby
open-sea location (bottom), between April 2003 and 16 February 2005.
temperatures varied from 12 C in winter to around
22 C in early autumn (Fig. 3). The abrupt warming of
seawater by more than 5 C within several days in early
Temperature profiles
Water temperatures within the right channel at 12 and
24 m depth were very steady, equalling 16.6 ± 1.0 and
16.3 ± 0.8 C, respectively. Winter values did not fall
below 14.6 C, while maximum values at the same depths
were 17.9 and 17.5 C, respectively, in late summer and
early autumn (Fig. 3). During the warm part of the year
the water column was weakly stratified – the temperature
difference at the two depths never exceeded 2 C. On the
daily time scale, the cave temperature was extremely
stable (Fig. 4), all deviations being within the precision of
the instrument.
These temperature conditions are remarkably different
from those in the nearby open sea, where (at 22 m depth)
Fig. 4. Departures of 30-min temperature values from corresponding
daily means.
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Ecology of an anchialine cave in the Adriatic Sea
September – which resulted in increased sea temperatures
throughout autumn – was completely absent in the cave.
Beyond this wide annual range in temperature, the open
sea exhibited pronounced short-period oscillations during
the warm season. On occasions, the temperature changed
by more than 6 C within several hours (Fig. 4). Such
oscillations are related to internal waves induced in the
region of the thermocline and have been observed in
other parts of the Adriatic (Novosel et al. 2004).
Seasonal temperature variations are represented by time
series of 30-day moving averages (Fig. 5). The ‘monthly’
values of temperature in the cave formed a symmetric
annual oscillation. A sine function with a period of 1 year
described 92–94% of the temperature variability in the cave
at 12–24 m depth, respectively. The seasonal cycle in the
open sea, however, is much more asymmetric and is more
adequately described (92%) by a sum of the annual and
semi-annual harmonics. The amplitudes and phases of the
harmonic constituents reveal the relationship between the
seasonal temperature changes in the cave and in the open
sea. The temperature ranges in the cave (2.7 and 2.1 C) at
12 and 24 m, respectively, are quite similar, but are significantly lower than the annual range in the nearby open sea
(8.1 C). The temperature minimum in the cave is reached
in early March at 12 m, but only 1 month later (8 April) at
Novosel, Jalžić, Novosel, Pasarić, Požar-Domac & Radić
24 m depth, while the minimum in the open sea occurs
earlier (19 February). Similarly, the maximum temperature
in the cave at 24 m (2 October) displays a 1-month lag
compared with the peak at 12 m (9 September), but occurs
almost simultaneously with the maximum at the same
depth in the open sea (12 October).
Benthos features
The cave contained a dense population of the hexactinellid sponge Oopsacas minuta Topsent, 1927 along the cave
roof and vertical and overhanging walls (Fig. 6). Their
highest abundance was between 15 and 35 m depth,
where approximately 15 individuals grew per m2. At
shallower depths (6–15 m) the abundance was 1–2
individual s ⁄ m2, while deeper (35–37 m) the abundance
was low again (2–3 individual) s ⁄ m2.
The second most abundant organisms inside the cave
were polychaetes. Three species of serpulid polychaetes
were found. Live specimens of Metavermilia multicristata
(Philippi, 1844) and Semivermilia crenata (O.G. Costa,
1861) were identified. Metavermilia multicristata, observed
growing on the cave walls, was the most numerous of the
three. The discovery of S. crenata in this cave represents
the second record for the species in the Adriatic Sea. The
third species, Protula sp. sensu (Bianchi 1981), was represented only by fragile, relatively large (12 mm in diameter)
empty tubes. Tubes were collected from the cave wall and
sediment on the bottom of the cave. Furthermore, skeletal
remains of two bivalves, the mussel Pododesmus patelliformis (Linné, 1761) and the shipworm Teredo navalis
(Linné, 1758), and of one decapod, the spider crab Herbstia condyliata (Fabricius, 1787), were found as well.
Discussion
Thermal conditions inside the Živa Voda cave
Fig. 5. Time series of water temperature, smoothed with a 30-day
moving average (black line). Also shown are the fitted annual harmonic (grey full line) and the sum of an annual and semi-annual harmonic (grey dashed line).
6
The cave is characterized by a very stable water temperature of around 16.5 C (which is equal to mean annual
air temperature at Hvar; Penzar et al. 2001), with all temperature variability being related to a small-amplitude,
symmetric annual cycle. Such conditions reflect the fact
that air and water within the cave are sheltered from
direct solar radiation and mechanical perturbations by
wind, and that there are noticeable water currents on a
macroscopic scale. The water in the cave is very still with
no visible motion. Therefore, a number of processes that
control the distribution of temperature at sea are absent
here. In general, seawater is heated by the absorption of
solar radiation and by the conduction of sensible heat
from the atmosphere; it is cooled by back radiation,
conduction of sensible heat to the atmosphere and by
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Novosel, Jalžić, Novosel, Pasarić, Požar-Domac & Radić
Ecology of an anchialine cave in the Adriatic Sea
Fig. 6. Hexactinellid Oopsacas minuta on a wall in the Živa Voda cave.
evaporation. This exchange is limited to a very thin surface
layer. Heat is transported to deeper layers by small-scale
turbulent eddies (i.e. eddy diffusivity) – the process being
several orders of magnitude stronger than molecular diffusion – by horizontal and vertical fluid flow, and by
wind-induced mechanical mixing (Bowden 1983). In the
Živa Voda cave, however, heating, cooling and the transport of heat are all greatly diminished. First, only the
scattered part of solar radiation is available for absorption. Secondly, evaporation and sensible heat flux strongly
depend on wind velocity and thus are also significantly
reduced. Thirdly, in the absence of turbulent motion, the
diffusion of heat across the air/sea interface and within
the water is restricted to molecular motion, with molecular diffusivity being several orders of magnitude smaller
than eddy diffusivity (Bowden 1983). During summer,
heat slowly diffuses to deeper layers. However, during
winter cooling, a layer of denser water is formed at the
top and starts to sink, setting up thermal convective currents. This asymmetry in heating/cooling is readily
observed in our data in spring as a 2-month lag between
temperatures at the two depths, and in autumn as an
almost simultaneous temperature drop (Fig. 3).
The most pronounced feature observed in our series of
open sea temperatures is the abrupt warming in early
September. This reflects the first episode of strong wind
after a long heating season, usually the gusty NE Bora
wind, when intense mechanical mixing drives warm surface water to deeper layers. However, the fact that this
long-lasting temperature increase is not registered in the
cave leads us to speculate on how the cave is connected
to the open sea. There are two possibilities: either the
connection is at a much greater depth where wind mixing
plays no role, or, more likely, the seawater penetrates the
cave very slowly, through a system of small crevices, filtering out all variability at time scales shorter than several
months. In any case, the aquatic system within the cave is
apparently very isolated from the nearby sea. Note also
that, if this is the case, the convective overturning set up
by autumn cooling could be the most important process
for renewing oxygen in the deeper layers of the cave.
Benthos
Anchialine taxa include organisms with widely disjunct distributions and some with affinities to bathyal species (Iliffe
1992). Anchialine stygobites are almost exclusively of marine origin and consist primarily of crustaceans. The reason
why crustaceans make up 80–90% of anchialine stygobites
is not clear. Perhaps they can better adapt to such limiting
factors in the anchialine environment as lack of light, low
levels of dissolved oxygen, and limited food supply. Only
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Ecology of an anchialine cave in the Adriatic Sea
four strictly anchialine species of sponges and 10 species of
annelids have been recorded so far (Iliffe 2005).
Hexactinellid sponges typically live in deep oceans (500–
3000 m) world wide. Exceptionally, they have been found
at such shallow depths that they can be reached by SCUBA
diving. These eight places in the world include: two submarine caves off southern France, three island caves in the
Adriatic Sea, along the coast of British Columbia and
Alaska, in Antarctica and southern New Zealand (Leys &
Lauzon 1998; Jalžić et al. 2005; Bakran-Petricioli et al.
2007). Based on the assumption that propagules of O. minuta have a short planktonic life-span, Bakran-Petricioli
et al. (2007) hypothesize that suitable ‘stepping-stone’ habitats (still to be located) exist between the southern Adriatic
deep basin and the distant colonized island caves.
Oopsacas minuta is the only hexactinellid from which
live larvae have been observed thanks to its occurrence in
shallow-water caves (Boury-Esnault & Vacelet 1994).
Although it reproduces throughout the year, not much is
known about its recruitment or population dynamics
because of the difficulties encountered in accessing its cave
habitat (Boury-Esnault et al. 1999). The feeding strategy
of hexactinellids differs notably from that of other sponges. These sponges rely on the great volume of their aquiferous cavities coupled with an extreme thinness of the
living tissue. Furthermore, anucleate choanocytes of the
hexactinellid are mostly used to circulate water and do not
trap and digest particles, unlike other groups of sponges
such as Demospongiae and Calcarea (Vacelet 1996).
Until now, O. minuta – also known from the deep sea
(Vacelet 1996; Bakran-Petricioli et al. 2007) – was found in
environments where the sea temperature is always colder
than that recorded in Živa Voda cave. Vacelet (1996)
reported temperatures between 13.0 and 14.7 C in ‘3 PP
cave’ on the French coast. The temperature in Živa Voda
cave ranges from 14.6 C in winter to 17.9 C in summer,
which is significantly higher than the 13 C homothermy
of the deep Mediterranean. O. minuta therefore apparently
withstands dampened seasonal temperature fluctuations in
still water and completely dark environments.
None of the polychaete species found in Živa Voda cave
are strictly anchialine: all have been reported either in the
bathyal zone, in marine caves or sites with reduced salinity,
and have an Atlantic–Mediterranean or cosmopolitan distribution. Serpulid polychaetes dominate the cave in terms of
species diversity. Metavermilia multicristata is common in
the bathyal zone and one of the most abundant serpulid
worms at depths down to 1000 m (Bianchi 1981). This species was also reported to settle in some marine caves subjected to freshwater input (Zibrowius 1968). Semivermilia
crenata is common in the bathyal and in the dark parts of
submarine caves. It is rare on infralittoral and circalittoral
concretions and on detritic bottoms (Bianchi 1981). Both
8
Novosel, Jalžić, Novosel, Pasarić, Požar-Domac & Radić
species were found near shallow-water hydrothermal vents
in the Aegean Sea (Bianchi & Morri 2000). Protula sp. (sensu
Bianchi) has a worldwide distribution. It is common in the
coralligenous biocoenosis, Posidonia oceanica meadows, and
the biocoenosis of coastal detritic bottoms, but also in marine caves and in the bathyal down to 900 m (Bianchi 1981).
Only the skeletal remains of two bivalves and one decapod species were found inside the cave. One left valve of
the mussel Pododesmus patelliformis was found in the
examined material. Pododesmus patelliformis is a relatively
rare species with wide distribution found attached on different hard substrata. It was reported to a depth of 1400 m
and has an Atlantic–Mediterranean distribution (Parenzan
1974). The skeletal remains of several valves and numerous
tubes found on the muddy bottom recorded the presence
of the bivalve shipworm Teredo navalis. The decapod spider
crab Herbstia condyliata was identified based on one carapace. This species is generally found in dark places, almost
exclusively in the innermost parts of caves, where it is active
even during the day. Herbstia condyliata is distributed
along the eastern coast of the Atlantic and throughout the
Mediterranean (Türkay 2001).
As Humphreys et al. (1999) demonstrated, conventional open circuit diving methods significantly influence
environmental conditions in anchialine habitats. Exhalent
bubbles from conventional SCUBA disrupt the physicochemical environment and destroy the fragile anchialine
fauna. Individuals of O. minuta are weakly attached to
substrata. Therefore, conventional SCUBA divers dislodge
many individuals from the cave roof with their air bubbles. Even a careless diver’s fin thrust can easily detach
them from their substrata. Consequently, for future
exploration of Živa Voda and all other anchialine caves,
only rebreathers should be used.
Summary
Temperature conditions within the Živa Voda anchialine
cave and at a nearby open sea site were surveyed during
2 years, from 12 April 2003 to 16 February 2005, at a 30min sampling interval. Water temperatures within the 12
and 24 m depth were very steady, equalling 16.6 ± 1.0 and
16.3 ± 0.8 C, respectively. Winter values never fell below
14.6 C, while maximum values at the same depths
were 17.9 and 17.5 C, respectively, in late summer and
early autumn. Inside the cave, a dense population of the
‘deep-sea’ hexactinellid sponge Oopsacas minuta was
found. Their highest abundance was between 15 and 35 m
depth, where approximately 15 individuals grew per m2.
The second most abundant organisms observed inside the
cave were polychaetes. The aquatic system within the cave
is very isolated from the nearby sea. Until now, O. minuta
has been found in environments where the sea tempera-
Marine Ecology 2007, 28 (Suppl. 1), 3–9 ª 2007 Blackwell Publishing Ltd No claim to original US government works
Novosel, Jalžić, Novosel, Pasarić, Požar-Domac & Radić
ture is always colder than that recorded in Živa Voda cave.
Oopsacas minuta, therefore, withstands dampened seasonal
temperature fluctuations in still water and complete darkness. As conventional SCUBA can destroy fragile individuals of O. minuta, for future exploration of Živa Voda we
suggest that only rebreathers should be used.
Acknowledgements
We wish to thank V. Jalžić and D. Lovretić for their help
during the speleological and diving work. T. Šiletić
assisted with the bivalve and B. Kljajo with the decapod
identification. We are very grateful to Dr Frank K.
McKinney and two anonymous reviewers for their useful
suggestions on the manuscript. The support of the Ministry of Science, Education and Sport of the Republic of
Croatia (Project No. 0119125) is acknowledged.
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