Open Journal of Marine Science, 2011, 1, 18-30
doi:10.4236/ojms.2011.11002 Published Online April 2011 (
Copyright © 2011 SciRes. OJMS
Abiotic Sponge Ecology Conditions, Limski Kanal and
Northern Adriatic Sea, Croatia
Anne Klöppel1, Corinna Messal2, Martin Pfannkuchen1, Jörg Matschullat2, Wolfgang Zucht1,
Bojan Hamer3 , Franz Brümmer1
1Universität Stuttgart, Biologisches Institut, Abteilung Zoologie, Pfaffenwaldring, Stuttgart, Germany
2Technische Universität Bergakademie Freiberg, Interdisciplinary Environmental
Research Centre, Brennhausgasse, Freiberg, Germany
3Ruđer Bošković Institute, Center for Marine Research, Giordano Paliaga, Rovinj, Croatia
Received April 1, 2011; revised April 18, 2011; accepted April 20, 2011
The Limski kanal, a semi-closed inlet (channel-like bay) located on the western coast of Istria (Croatia), is an
extraordinary sponge habitat. Research on the marine ecosystem has been conducted there for more than 100
years. Today, 42 valid Porifera species are described. 139 species are listed for the area around Rovinj and
159 species for the northern Adriatic Sea. While several scientists described the sponge fauna, information
on the abiotic situation or an explanation for the diversity differences is missing. This study interprets phys-
icochemical and ecological parameters including depth profiles (temperature, salinity, pH-value, oxygen,
water current velocity, photosynthetically active radiation [PAR], water transparency and nutrients). Their
variances are linked with the distribution patterns of the prevailing sponge fauna. The channel is character-
ised as predominantly marine habitat. Although a shallow sediment barrier and a headland reduce the impact
of the northern Adriatic Sea, differences between the channel and the open sea seem to be limited. Compared
to the more homogenous water body of the Adriatic sampling locations, the channel shows variations and
gradients of ecological parameters between different locations (e.g., due to freshwater influx: nutrients, tem-
perature, oxygen content, salinity and water current velocity) – this offering habitat diversity. The sponge
fauna changes along those ecological gradients. It is dominated by the photophilic species Aplysina
aerophoba and Chondrilla nucula, but sciaphilic species such as Dysidea avara, Axinella polypoides and
Aplysina cavernicola can be found, too. Some specialised species (e.g., Geodia cydonium, Tethya aurantium)
even populate the muddy bottom of the channel.
Keywords: Sponge Fauna, Porifera, Ecological Parameters, Abiotic Parameters, Habitat Characteristics
1. Introduction
1.1. The Limski Kanal
The Limski kanal (as of now Limski) is an 11 km long,
semi-closed marine inlet of the Adriatic Sea at the west-
ern coast of Istria, 5 km north of Rovinj, Croatia (Figure
1(a)). This inlet constitutes a unique environment due to
its isolated geographic position – supporting the deve-
lopment of endemic species of marine flora and fauna,
e.g., sponges [1-3]. Examples are Tethya limski [4] and
Geodia rovinjensis [5]. Situated along an E-W axis with
two parallel coast lines, the channel reaches its maximum
width of about 650 m and a maximum depth of 32 m at
its western opening (Figures 1(b,c)). While the northern
coast is sun-exposed throughout the year, the steep
southern side is shaded from October through March [6].
1.2. Ecological Characteristics of the Limski and
the Northern Adriatic Sea
Publications on ecological conditions in the Limski are
rather outdated and erratic. Paul [7,8] provided an over-
view of the sediments in both Limski and northern Adri
atic Sea. He described a sediment barrier of a few meters
height, separating the Limski from the open sea [9,10].
The bottom is composed of muddy sediments with in-
creasing grain size towards the mouth. The channel
Copyright © 2011 SciRes. OJMS
Figure 1. (a) Istrian peninsula with location of the Limski
kanal and the city of Rovinj; (b) Limski kanal (www.agen-; (c) Sampling lo-
cations in Limski and northern Adriatic Sea (from Messal
flanks are slightly rising rock walls with a thin detritic
layer. Wave-cut notches (30 - 40 cm wide) commonly
occur on the northern side [6]. Elevations and rises in the
posterior part result from reef-like colonies of Cladocora
caespitosa [11-14]. These C. caespitosa reefs are very
abundant in the western part of the channel and known
from just a few places in the Mediterranean Sea [15].
Here, species diversity is high due to the availability of
hard substrate offered by the Cladocora caespitosa
benches [12].
Several ecological and hydrographical data (e.g., cur-
rent velocities at the channel`s opening, temperature,
salinity: 36‰–38‰, pH-value: 8.3, and O2-saturation:
70‰–100%) were reported for the Limski [6,10,16-21].
Small creeks and springs discharge into the posterior part
of the channel. Their influx becomes important during
heavy rainfall periods with resulting changes in visibility
(particulate matter) but do not significantly alter – be-
sides surface runoff – temperature and salinity [7,8,22].
During the formation of a thermocline in spring,
O2-saturation decreases to 70% near the bottom for about
four months [7,8,18].
Light (PAR) seems to be the most influential parame-
ter for the distribution of sponges – apart from a suitable
substrate for larvae settlement – because of the symbiotic
association of sponges with cyanobacteria [23,24]. Müller
& Zahn [4] determined a Secchi disc-visibility [25] of 10
meters in the central section.
High terrigenous nutrient intake was reported, e.g.,
due to freshwater inflow in the rear part of the channel
[11]. Rising DOM levels triggered an increase of plank-
tonic organisms with a positive effect on mussel and
sponge growth rates [22,24,26,27]. Enhanced sedimenta-
tion rates in the posterior part cause an additional in-
crease of food supply and turbidity [28,29].
1.3. Sponge Diversity and Ecology in the Limski
and Northern Adriatic Sea
Sponges are a major component of the benthic fauna in
marine environments with 8,224 valid species known
worldwide [30]. While most species thrive in shallow
coastal waters, some are known to live in depths below
1000 m, e.g., Hexactinellida. Amongst all invertebrates,
sponges provide the largest number of bioactive natural
products which also could be used in human medicine
[31,32]. They play a remarkable role in drug discovery.
An example of successful application is avarol from Dy-
sidea avara which is used in ointments against psoriasis
[33]. This sponge is also abundant in the Limski kanal.
Examinations of the sponge fauna in the waters sur-
rounding Rovinj were pioneered by Graeffe [34], Buc-
cich [35] and Breitfuss [36]; the latter describing the first
sponges from the Limski. Further investigations on the
sponge fauna of the northern Adriatic Sea near Rovinj,
including the Limski, were presented by Vatova [11] and
Rützler [37]. The species list for the area around Rovinj
was upgraded to 139 [2,3,38], and the species list for the
northern Adriatic now contains 159 species [39]. The
most recent study for the Limski describes a total number
of 42 valid sponge species [12].
Distribution and abundance of sponge species are
subject to a combination of abiotic and biotic parameters
[40-47]. Nevertheless, publications about the channel as
sponge habitat are rare, discuss a few parameters only, or
are outdated. This study first summarized known eco-
logical conditions, followed by an overview of current
abiotic parameters in the Limski and the northern Adri-
atic Sea near Rovinj. These are correlated with distribu-
tion patterns of the prevailing sponge fauna. Parameters
like photosynthetically active radiation (PAR), deter-
mined for the first time in the Limski, and recent data
(pH-value, O2-concentration, salinity, temperature, water
current velocity, water transparency, total organic carbon
[TOC], dissolved organic carbon [DOC], particulate or-
ganic carbon [POC]) are presented. The related experi-
Copyright © 2011 SciRes. OJMS
ence can be applied to determine ecological sponge pref-
erences and plays an important role in managing closed
systems for ex situ sponge cultivation to overcome the
supply problem in drug discovery [48,49].
2. Material and Methods
Detailed studies were conducted at seven locations in the
Limski and four sites in the northern Adriatic Sea near
Rovinj (Figure 1(c), Table 1), following a scheme by
Sidri [19] and by Zucht [6]. Four locations in the Limski
were selected at the western opening: L5, L32, L37,
L(M), and three in the central part of the channel: L8,
L35, L36. Sampling was impossible at the eastern end
because of its protected character. Locations in the Adri-
atic Sea were chosen at the northern shore of “St. Gio-
vanni” (A1), between the two “Figarola” islands (A2)
and in the open water (A3,A4). Field campaigns for
physicochemical measurements were performed in May
and September 2005.
Temperature, pH-value, O2-content and conductivity
were gauged from sea surface to seafloor with a
multi-parameter probe (MPP 350, WTW GmbH, Weil-
heim, Germany), equipped with a stirrer and a 25 m ca-
ble. Salinity was calculated using the Practical Salinity
Scale from 1978 [50]. Light (PAR) was quantified with a
quantum sensor connected to a microvolt integrator
(Delta-T Devices Ltd, Cambridge, England). The probe
was operated from a water-proof box (GSI Lexan Utility
Box). The error caused by the Plexiglas plate was cor-
rected by introducing a factor (f = 1.3636) under a de-
fined light source. TOC, DOC and POC (= TOC - DOC)
were analysed with a Shimadzu TOC-5000A analyser
[51]. Current velocities were determined observing tracer
droplets, injected from a syringe in front of a black
benchmark, installed parallel to the main current direc-
tion [52,53]. Timing was done with a waterproof mi-
cro-chronometer. Visibility was determined by Secchi-
disc [25].
At each location in the Limski (western opening: L5,
L32, L37; central part: L8, L35, L36) horizontal belt
transects with a length of 20 meters and a width of two
meters at four different depths (2, 6, 10, 15 m) were used
to record the species and number of sponge specimen. In
case of encrusting sponges (e.g., Chondrilla nucula) as-
sumed clone patches were counted. For exact species
identification by spicula preparation sponge samples
were taken and stored in Ethanol for later determination.
Fisher’s Alpha [54] was chosen for the quantification
of sponge diversity as it is mainly influenced by the fre-
quencies of species of medium abundances. This test is
based on a log-series of the species’ frequencies that we
found at all locations. The results of all line transects per
site were added for these calculations. The scale of
α-diversity is not clearly defined [55]. Multiple Range
Test (MRT) and Fisher’s least significant difference test
(LSD) were applied to detect statistically significant dif-
ferences between the study sites.
3. Results
3.1. Current Ecological Parameters in Limski
and Northern Adriatic Sea
Measurements were taken at L32, L35, L36, L37, L(M),
A3 and A4 during both field campaigns (Figure 1(c)).
Water current velocity, PAR, visibility and TOC, DOC
were determined in the Limski (L5, L8, L35 and L36)
only. For data see Table 2. Temperatures ranged from
23.5 ± 0.4˚C at the surface to 17.9 ± 0.9˚C at 25 m in
August and September 2005, consistently decreasing
with depth (Figure 2(a)). While sampling points showed
statistically significant variability, all data from the
Limski showed thermocline-like discontinuity points at
Table 1. Geographical position (GPS) of the investigated sites in Limski and northern Adriatic Sea.
location GPS – north GPS – east description
L 37 45°08.285 13°37.775 channel entrance, north coast
L 5 45°08.202 13°38.109 channel entrance, north coast
L (M) 45°08.030 13°38.200 channel entrance, centre
L 32 45°07.896 13°38.002 channel entrance, south coast
L 8 45°08.088 13°39.713 central channel, north coast
L 35 45°07.819 13°39.325 central channel, south coast
L 36 45°07.779 13°40.872 central channel, south coast
A 1 45°02.763 13°37.431 Adriatic Sea, San Giovanni island
A 2 45°05.622 13°37.132 Adriatic Sea, Figarola island
A 3 n. a. n. a. Adriatic Sea, open sea
A 4 n. a. n. a. Adriatic Sea, open sea
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Table 2. Comparison of previous and new ecological data for the Limski kanal.
Parameters old data this work
Water temperature () 9 - 12/20 - 25 [1,2] 17.9 ± 0.9 - 23.5 ± 0.4 (June)
Salinity (‰) 35 - 39 [1,2,3] 35.2 ± 0.3 - 36.9 ± 0.1
pH-value 8.3
[1,4] 8.06 ± 0.1 - 8.20 ± 0.01
O2-concentration (mg·L-1) n.a.
[5] 6.6 ± 0.2 - 8.8 ± 0.7
Current velocity (cm·sec-1) 0.1 - 11.8 [6] 0.8 - 2.2 ± 0.9
Secchi visibility (m) 6 [1],10 [7] 12.6 ± 0.8
PAR (µmol·m-2·sec-1) n.a. 17.9 - 1,925.1
TOC (mg·L-1 ) n.a. 1.8 ± 0.7 - 2.1 ± 1.5
DOC (mg·L-1) n.a. 1.5 ± 0.9
11 and 21 m depth. The northern Adriatic Sea was
slightly warmer (+ 1.3˚C) than the channel. The tem-
perature barely decreased to the discontinuity layer at
about 18 m depth and showed no second irregularity.
Temperature profiles could show dramatic changes of up
to 2˚C·m–1 within one meter of decreasing depth (L36).
No significant difference in salinity occurred between
the Limski – L32, L35, L36, L37, L(M) – and the north-
ern Adriatic Sea at Rovinj (A3, A4). Salinity was lower
within the first 3 to 5 m depth of the channel compared
to the sampling points of the same depth in the Adriatic
Sea. Practical salinity increased at all sampling places
with depth and ranged from 35.2 ± 0.3 to 36.9 ± 0.1‰
(Figure 2(b)). Salinity of surface waters at L37 was
about 1‰ below average in the Limski.
In both northern Adriatic Sea and Limski, pH-values
remained fairly constant to 10 m depth (about pH 8.2). In
the Limski, pH-values remained relatively stable within
the first 11 m, then decreased to pH 8.06 between the
first and the second discontinuity layer and were stable
again below 21 m (Figure 2(c)). In the Adriatic Sea,
pH-values remained stable to 25 m. The homogeneous
groups obtained by LSD test agreed with the channel
morphology and corresponding benthic fauna [6].
Dissolved oxygen was almost identical (8.8 mg·L–1) in
the upper 5 m of the Limski and the northern Adriatic
Sea (Figure 2(d)). While a slight increase (9.2 mg·L–1)
occurred with depth in the Adriatic Sea, the Limski
showed a considerable decrease from about 8.8 ± 0.7 to
approximately 6.6 ± 0.2 mg·L–1. The most pronounced
changes were located between the two discontinuity
points (at 11 and 21 m). Oxygen content at L36 showed
natural occurring fluctuations and was about 1.6 mg·L–1
above average (data not shown).
Water current values were 2.2 ± 0.9 cm·sec–1 at 5 m
depth and 0.8 ± 0.9 cm·sec–1 at 15 m depth in the Limski
(L5, L8, L35, L36; Figure 3), based on 2 to 6 repetitions
at four locations. Current velocities at 5 m depth were
slightly higher than those in 15 m depth. There was no
significant difference between the locations. Water cur-
rent velocities at L36 were higher than average with 3.5
cm·sec–1 at 5 m depth and 2.1 cm·sec–1 at 15 m.
PAR was measured at the water surface, and at 5, 10
and 15 m depth (L5,L8,L35,L36). Under exposed condi-
tions, PAR values of about 1700 - 1900 µmol·m–2·sec–1
were determined at the surface, about four times higher
than under shaded weather conditions (480 - 550
µmol·m–2·sec–1). PAR decreased to 35% residual light
intensity at 5 m, compared with surface values, and 11%
residual light intensity at 15 m (Figure 3). Water trans-
parency by Secchi disc reached 12.6 ± 0.8 m in the Lim-
ski (L5, L8, L35, L36) and 12.1 m (A1) in the northern
Adriatic Sea.
Organic carbon content was measured at location L5
and L36 (Figure 3). DOC ranged from 0.9 to 2.1 mg·L–1
at both locations and both depths (5 and 15 m), without
significant differences (average Limski: 1.5 ± 0.9 mg·L–1).
TOC-variability was higher, with values between 1.0 and
3.1 mg·L–1 at 5 and 15 m depth (average Limski: 1.95 ±
1.1 mg·L-1). Differences in TOC and DOC with depth
were negligible. Additional measurements at L36 in 25
m depth showed an increase of TOC and DOC from 2.3
to 10.1 mg·L–1 and from 2.0 to 4.7 mg·L–1. Data from the
northern Adriatic Sea were generally much more homo-
geneous than those measured in the Limski (data not
3.2. Sponge Biodiversity and Abundance
in the Limski
Sponge diversity was higher on the southern than the
northern side of the channel and reached its maximum in
the central part around L35 (Figure 4(a)). Biodiversity
increased with depth on the southern and northern side of
[1]Sidri [19], measurements taken in winter and summer at one depth;
[2]Gillet [20]; [3]von Daniels [18]; [4]Paul [7, 8]; [5]only data in percent-
age of dissolved oxygen available: 70%-100%, e.g. Sidri [19], Gillet
[20], Paul [7, 8]; [6]Kuzmanovic [21]; [7]Müller & Zahn [4]; n.a.: not
Copyright © 2011 SciRes. OJMS
Figure 2. Parameter-depth profiles in the Limski and Adriatic Sea; a) Water temperature; b) Practical salinity, calculated
from conductivity determination; c) pH-values; d) Oxygen-concentrations. Ranges depict individual values from each site at
every depth.
Figure 3. Depth-related parameter changes in the Limski. a)
Current velocities (2-6 measurements per site); b) Relative
PAR intensities, Limski 4 determinations; c) TOC (= POC
+ DOC), Limski 2 determinations.
the Limski (Figure 4b). At a depth of 2 m it was slightly
higher on the northern side due to the presence of shad-
Figure 4. (a) α-diversity (Fisher’s alpha) along the Limski
kanal, from the western opening (W) to the eastern end (E)
on the northern and southern side. Diversity was highest in
the central part at the northern coast; (b) α-diversity in-
creased from the surface down on both the northern and
the southern side. Shading rocky protrusions close to the
surface are frequent on the northern side leading to higher
diversity values at a depth of 2 m.
ing projections at the water surface. The most common
species were Aplysina aerophoba, Chondrilla nucula and
Chondrosia reniformis. Since the channel bottom is mud
covered, sponges could only be found either loose with-
Copyright © 2011 SciRes. OJMS
out substrate or on stones emerging from the mud. Large
specimen of Geodia cydonium and Ircinia variabilis
were common. Species usually restricted to depths > 20
m or caves like Axinella polypoides, Axinella cannabina
and Aplysina cavernicola could be found in the Limski
kanal in shallow waters as well.
Distribution patterns of the most frequent species are
shown in Figures 5 and 6. They are divided into photo-
philic and sciaphilic species according to Rützler [56].
The fauna was dominated by photophilic species. Among
these, Aplysina aerophoba and Chondrilla nucula were
the most common species. Their abundance decreased
continuously with depth. Only Cliona viridis specimen
reached their maximal distribution at 10 m depth (Figure
5(a)). In shallow waters (2-6 m), the sponge community
was dominated by Aplysina aerophoba and Chondrilla
nucula in comparable numbers whereas further photo-
philic species already decreased. Several boring species
(Cliona celata, C. nigricans and C. viridis) were amongst
these. The number of sciaphilic specimen increased with
depth (Figure 5(b)). There were species without related
trend, e.g., Hemimycale columella however, an inverted
trend, e.g., Chondrosia reniformis (facultative associated
with cyanobacteria) that decreased with depth.
All species – especially photophilic ones – showed a
decrease towards the eastern end of the channel (Figure
6). The maximum distribution was observed in the first
third (predominately photophilic species, Figure 6a) and
the central part (predominantely sciaphilic species, Fi-
gure 6b) of the Limski. Only Antho involvens and Hemi-
Figure 5. Number of specimen in the Limski of (a) most abundant photophilic sponge species according to depth; (b) most
abundant sciaphilic sponge species according to depth. Sciaphilic species are less frequent than photophilic species. Therefore,
the scale is halved in comparison to Figure 4a. Horizontal belt transects, length 20 m, width 2 m, four different depths (2, 6,
10, 15 m), were used to record the species and number of sponge specimen at defined measurement points (L5, L32, L37, L8,
L35, L36) from the eastern to the western side).
Copyright © 2011 SciRes. OJMS
Figure 6. Number of specimen in the Limski of (a) most frequent photophilic sponge species along the Limski kanal from the
western opening (W) to the eastern end (E); (b) most frequent sciaphilic sponge species along the Limski kanal from the west-
ern opening (W) to the eastern end (E). Sciaphilic species are less frequent than photophilic species. Therefore, the scale is
halved in comparison to Figure 5a. Horizontal belt transects, length 20 m, width 2 m, four different depths (2, 6, 10, 15 m),
were used to record the species and number of sponge specimen at defined measurement points (L5, L32, L37, L8, L35, L36)
from the Eastern to the Western side).
mycale collumela were more frequent in the eastern sec-
4. Discussion
Ecological and distribution differences between locations
in the Limski were rather low. The Bray-Curtis similarity
was around 0.45 [57]. The homogenous groups obtained
through LSD test show accordance to the morphology of
the channel. The Limski can be divided into three parts:
the entrance [L5, L32, L37, L(M)] the central part [L8,
L35, L36], and the end [begins with L36]. Differences
between locations at the entrance and the end were small
compared to those in the central part. The channel’s end
Copyright © 2011 SciRes. OJMS
is only represented by a single measurement point due to
its protective status and the local mariculture. With this
study we show that the most influential ecological pa-
rameters on sponge distribution patterns were depth
(change of PAR), sedimentation (especially in the rear
part) and type of substrate, which confirms with experi-
ence described in the literature [e.g., 24,27,29].
Objectives against the application of random field
models are that it is difficult to judge whether the models
are well related to the spatial distribution they are sup-
posed to describe. Also the bias caused by spatial distri-
butions that cannot adequately be taken to be a normal
realization of the used random field model is difficult to
However, the fact that these methods allow subjective
selected samples has lead many scientists to choose these
methods. See the book by Rivoirard et al. (2000) and
references therein.
4.1. Water Column Changes along the E-W Axis
Despite the strongest impact of the open sea at the chan-
nel’s mouth, the general ecological situation does not
significantly change to the eastern end. Temperature,
salinity, pH–value and oxygen as well as visibility, PAR
and organic carbon content remained similar along the
E–W axis of the entire channel. The channel’s opening
was more affected by winds than the posterior part which
is sheltered by the mountains. Wind did not exceed a
light breeze (average maximum: 2.7 Bft) during the first
field campaign, triggering small waves only (wave
height ca. 0.6 m). The influence of wind decreased rap-
idly with depth and current velocities were not affected a
few meters below surface. Still, current velocities in the
western part were higher compared to the central part
due to the impact of the open sea. Sedimentation rates
increased considerably from the northward bend of the
channel (indicated in Figure 1c).
4.2. Water Column Changes along the N-S Axis
Temperature, salinity, pH-values and oxygen, visibility,
water current velocity and organic carbon did not change
from north to south, except for PAR at 5 and 15 m (data
not shown). While the northern side is sun-exposed
throughout the year, the southern coastline is mostly
shaded from the surrounding mountains. Since light, es-
pecially PAR, is highly important for many symbionts
living with sponges, a difference in the faunal assem-
blage should be noticeable between the northern and
southern coastlines (our observations, see 4.6).
4.3. Water Column Changes with Depth
All parameters showed changes with depth (Figure 2, 3).
Two thermocline-like discontinuity points at 11 and 21
m were formed with increasing surface water tempera-
tures in spring and were nearly stable over the summer
months (Figure 2(a)). This stable situation during sum-
mer had significant effects on oxygen concentrations,
which remained nearly stable to 15 m, followed by a
strong decline below. Beneath 20 m depth, oxygen con-
centrations stayed constant (Figure 2(d)). The Limski
yielded high oxygen amounts throughout the year with
minimum saturation in summer. The homoeotherme
situation during winter causes vertical mixing and even
the channel`s bottom is oxygenated [7,8,18].
Salinity started with 35‰ in surface waters and in-
creased continuously to 37‰ at 20 to 25 m, suggesting a
limited water exchange at those depths (Figure 2(b)).
pH–values increased to 8.2 at 11 m and decreased to 8.05
at 25 m depth. This behaviour differed from the situation
prevailing at the sampling points in the Adriatic Sea and
may be related to cold freshwater intrusions (see below;
Figure 2(c)).
Light is the most limiting factor for the abundance of
photophilic sponges at greater depths. PAR lost about
65% of its intensity at 5 m depth, mainly related to “red”
radiation. At 15 m depth, PAR was almost negligible
(Figure 3). Water current velocities were lower at 15 m
compared to 5 m depth, reflecting a characteristic of the
local current system (Figure 3).
TOC and DOC were hardly affected by depth. No dif-
ference in nutrient concentrations occurred between 5
and 15 m, indicating a relative stable distribution (Figure
3). Obtained values (DOC: 1.5 mg·L–1 in the Limski and
the northern Adriatic) were comparable with literature
data; Puddu et al. [58] found DOC contents between 0.3
and 2.0 mg·L–1 in shallow marine systems of the Adriatic
4.4. Special Ecological Observations
for the Limski
Some inconsistencies occurred concerning the water
column in the Limski (Figures 2a-d), leading to more
pronounced standard deviations of the related data as
compared with corresponding data from the locations in
the Adriatic Sea. Location L36 was situated near a fish
and mussel farm with a research platform (BIO-
TECmarin). Ecological conditions at L36 differed re-
markably from those at the sampling sites in the open
Adriatic Sea and from the other sampling points in the
Limski. Temperature fluctuations of up to 2˚C·m–1 were
detected and temperature variations of 8˚C within one
day [59], due to a displacement of the water body by
currents. No significant differences compared with other
sampling stations were detected for salinity and pH-val-
ues. The oxygen content strongly increased between 1 to
Copyright © 2011 SciRes. OJMS
10 m depth at L36 (10.4 mg·L-1). Water cur- rent veloci-
ties were also highest at L36 at 15 m depth (3.5 cm·sec-1).
This location was likely to be influenced by the above
mentioned mussel and fish farm and thus by high organic
input (anthropogenic impact), as shown by the TOC/
DOC measurements [60]. Further analyses at L36 and 25
m depth showed an increased TOC (10.1 mg·L–1; average
Limski 1.95 mg·L–1) and DOC content (4.7 mg·L–1; av-
erage Limski 1.5 mg·L–1). This might have a positive
effect on sponges. Thus, TOC increases nearby and does
not decrease towards the bottom (Figure 3(c)), as men-
tioned by Steuer [61]. Photosynthesis and the influx of
organic material through rivers and streams are addi-
tional nutrient sources [58].
Subterranean freshwater influx will be responsible for
the enrichment with oxygen, some of the nutrients, and
for the temperature anomalies since the temperature of
these small creeks is always lower than the sea surface
temperatures. Paul [7], Uffenorde [22] and Kuzmanovic
[21] also mentioned the river and spring discharge in the
posterior part of the channel. In contrast to their conclu-
sions, the water body is affected by these influences. The
decreased salinity in surface waters (Figure 2(b)) com-
pared with the sampling points in the Adriatic Sea sup-
ports the influence of surface runoff.
Location L36 is characterised by a very diverse and
abundant sponge fauna. Due to the karst formations of
the Croatian coast, freshwater enters the ocean through-
out the year, following rainfall events. Sponges seem to
tolerate rapid changes of the physicochemical parameters,
because they even dominate areas (“unique habitats”)
under freshwater influx [6].
Except for light exposition and water transparency,
which were largely similar at all locations, the Limski
appears to be a heterogeneous habitat. Fluctuations of
abiotic factors were highest variations in the central part
(L36) where sponge diversity was largest) while the w-
estern part was more homogenous.
4.5. Limski Versus Adriatic Sea
The observation points in the Adriatic Sea were more
exposed and consequently more affected by winds (e.g.,
Bora) than those in the Limski kanal, since no natural
barriers moderate wind force [22]. The wave movement
in the Limski is much smaller than in the Adriatic Sea
[28]. Thus, the water body is well mixed and more ho-
mogenous than the Limski [62]. This can be seen in the
continuous constancy of all parameters in the Adriatic
from 0 to 25 m depth (Figure 2a-d). If considered as two
self contained systems, local influences (e.g., freshwater
influx) on the water column were more distinctive in the
Limski. Not all sponge species described for the northern
Adriatic Sea could be found in the Limski (e.g., Asbesto-
pluma sp., Mycale tunicata), probably due to a limited
variety in adequate substrates and special habitats (e.g.,
caves, soft bottom). The Adriatic seafloor is composed of
more coarse grained material while sediments in the
Limski are smaller in size – sometimes mud – and ex-
posed to higher sedimentation rates and therefore less
favourable for larvae settlement. Only the channels
mouth yielded sediments similar to those of the northern
Adriatic Sea. While the Adriatic Sea generally offers a
higher number of different habitats than the Limski (e.g.,
caves, different substrates), the new data show much
accordance between the inside of the channel and sam-
pling places outside.
The Limski must be characterised as a dominantly
marine habitat, despite the sediment barrier and the head-
land separating the channel from the open water of the
Adriatic Sea. Both northern and southern side show a
good agreement concerning hydrographical data [7]. The
channel itself can be divided into three sections: i) the
western entrance, mostly influenced by the Adriatic Sea,
ii) the central part of the channel with minimal water
movement, and iii) the eastern end, characterised by
small grained particles forming a muddy bottom, and
easy water mixing due to low width and depth. Minor
ecological gradients occur from west to east, from north
to south (especially PAR – due to the orientation of the
channel) and from the water surface to the bottom. Both
distribution and abundance of most of the 42 Porifera
species were linked to those gradients [6]. The interact-
tion of several parameters may influence species diver-
4.6. Sponge Fauna
Brümmer et al. [12] provided the most recent overview
on the sponge fauna in the Limski. Several authors also
investigated the benthos in the northern Adriatic Sea [37,
38,56]. Challenges that a sponge normally faces are re-
lated to ecological conditions such as light exposure,
currents and sedimentation. Due to the limited depth of
25-30 m, photophilic species were most common
(Aplysina aerophoba, Chondrilla nucula and different
‘”Keratosa” sponges). Chondrosia reniformis, but also
Tethya citrina, Tethya aurantium, Dysidea avara, Axi-
nella polypoides and Petrosia ficiformis were also very
abundant species in the Limski .
Sponge diversity in the Limski was highest in the cen-
tral part of the channel (around L36), compared to its
western entrance (high water movement affected by the
Adriatic Sea) and eastern end (high sedimentation,
muddy bottom) (Figure 4a). Here, only Cladocera case-
pitosa benches (often settled by Terpios gelatinosa,
abundant from the center to the eastern end) provided
adequate substrate [12].
Copyright © 2011 SciRes. OJMS
Both coastlines (N and S) show decreasing abundance
(Figure 5) but increasing diversity (Figure 4(b)) of most
sponges with depth (to 15 m). Green algae are their
strongest competitors in well-lit areas and their abun-
dance is slightly higher at the northern than the southern
coast [56]. Thus, the most frequent photophilic species
have a higher biodiversity and abundance in the shallow
waters of the southern side (6-10 m) than on the northern
side to avoid the competing situation (Figures 4 and 5).
The maximum biodiversity on the northern coast oc-
curred between 10 and 15 m. Below, values were always
higher than those at comparable depths of the southern
side. Due to wave-cut notches, supplying shaded areas
directly under the surface (2 m), diversity was also high-
est (minimal competition with algae) when compared
with the biodiversity in similar depths at the southern
side (Figure 4(b)). This habitat is not only settled by
sciaphilic sponges but also by most common photophilic
species, albeit in lower numbers. A similar situation is
reported from caves on the islands Banjole, Sv. Katarina
and Sv. Ivan in the Adriatic Sea [56,63].
The western channel opening is exposed to the water
movements of the Adriatic Sea (for current velocity see
Figure 3). Thus, mainly current-resistant encrusting
sponges, e.g., Crambe crambe, Chondrilla nucula or
boring ones like Cliona celata (Figures 5(a), 6(a)), were
found in surface waters, whereas tall and branching ones,
e.g., Axinella polypoides, settled deeper sites (Figure
5(b)). This species is able to face high sedimentation
rates but low current velocities (Figure 3) due to their
Light (PAR, Figure 3) acts as a limiting factor related
to depth, exposure and substrate inclination. Aplysina
aerophoba and Chondrilla nucula, the species with the
highest abundance in both Limski and northern Adriatic
Sea, are known to contain photosynthetic active cyano-
bacteria. Others like Petrosia ficiformis and Chondrosia
reniformis (Figure 5(b)) may also live without this asso-
ciation in shaded places, e.g., caves. The abundance of
most photophilic species like A. aerophoba and C. nu-
cula decreased with depth in the Limski (Figure 5(a)),
while that of sciaphilic ones such as A. polypoides and D.
avara increased (Figure 5(b)). Although PAR was al-
most absent at 15 m depth (Figure 3), a few A.
aerophoba specimen were found. A. polypoides and A.
cavernicola are sciaphilic species living at depths of 25
m or more. They could be found from the central part to
the eastern end of the Limski, even in shallow water;
primarily in caves or under notches (A. cavernicola).
This might be also caused by the higher turbidity in this
area serving as radiation protection.
Abundance and diversity were minimal at the muddy
bottom, typical for the eastern channel end. Antho invol-
vens had its largest occurrence in this rear part. The high
turbidity (radiation shield) may favour the distribution of
sciaphilic sponges (Figure 6(b)). Most species could be
found on rocks (Hemimycale columella, Sarcotragus
foetidus, Aplysina aerophoba and Crambe crambe). A
few Geodia cydonium specimen emerged from the bot-
tom. They had a furry skeleton of spicula avoiding that
particles block the pores. Chondrilla nucula was most
abundant in the eastern part compared with other species,
most likely because of its capability to produce mucus
which encloses and removes particles from the sponge
surface ([19], Figure 6(a)). Especially Terpios gelati-
nosa and Dysidea avara used the expanded C. caespi-
tosa-benches as substrate. These observations and the
higher diversity on the shaded southern side and below
rocky protrusions (N) support the hypothesis that the
sponge biodiversity is inversely dependent on the avail-
ability of light and substrate [6].
Very often, two or more sponges form a stable asso-
ciation to avoid competition (epibiosis; e.g., Aplysina
aerophoba and Chondrilla nucula). Both species share
the same ecological niche. They can be found in shallow
and light-rich waters, on hard or detritic bottom (Figure
5(a), [12,47]). Both face the problem of high sedimenta-
tion in different ways (A. aerophoba: tubular form,
smooth surface; C. nucula: mucous trapping, rythmic
contractions, [19,64]). Overgrown sponges may even
share their aquiferous systems in some sort of coopera-
tion [65].
The Limski kanal and the Adriatic Sea seem to be very
similar in physicochemical features. Many species could
be found all over the channel and the Adriatic Sea (e.g.,
A. aerophoba, C. nucula, Chondrosia reniformis, Cliona
celata, Geodia cydonium and Ircinia spp.). This applied
to Oscarella lobularis too, but this species prefers greater
depths and notches. It is likely that the open water of the
Adriatic Sea – despite the homogenous water character-
istics – provide more small-scaled variations due to
morphology of the coasts (e.g., adequate substrates) and
the small islands compared to the Limski.
5. Conclusions
Environmental conditions, availability and type of sub-
strate, and the interactions between the organisms define
the structure of a benthic community. Presence, abun-
dance and coverage of a species are strictly related to its
adaptability to ecological conditions. Based on detected
differences in sponge diversity and abundance between
the sampling sites, the Limski kanal could be divided in
three parts from the western opening to the eastern end
and between the northern and the southern shores. The
highest biodiversity was found in the central part at the
Copyright © 2011 SciRes. OJMS
southern coast at 6 - 10 m depth. At the light exposed
northern coast, biodiversity was highest at 2 m depth
(under shading rocky protrusions) and at 15 m depth.
PAR, available substrate algal competition and sedimen-
tation, water current, pH, oxygen, temperature and salin-
ity were the deciding ecological parameters responsible
for the diverse sponge patterns. This and the higher di-
versity on the shaded southern side and below rocky pro-
trusions (N) support the hypothesis of the sponge biodi-
versity being inversely dependent on the availability of
light and substrate. A similar rich and diverse sponge
fauna like in the Limski kanal was not described in the
6. Acknowledgements
This work was part of the BIOTECmarin project sup-
ported by the Federal Ministry of Education and Re-
search (BMBF, 03F0414D) and the Universität Stuttgart.
We thank Dr. R. Batel for the excellent logistic support
at the Center for Marine Research (Ruđer Bošković In-
stitute) in Rovinj, Croatia, including provision of the
research vessel “Burin” with captain D. Devescovi. Fur-
thermore we thank D. Mlinek from the Croatian Hydro-
meteorological Survey for providing the meteorological
data of Rovinj.
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