Sandro Lanfranco, Department of Biology, University of
Malta, Msida, Malta.
Introduction: environment of the
Maltese Islands
The Maltese Archipelago consists of a group of small,
low-lying islands located in the central Mediterranean, approximately 96km
South of Sicily and 290km east of Cape Bon. The islands outcrop from the
shallow shelf seas of the Malta-Hyblean Platform which forms the foreland
margin of the African Plate The archipelago extends for 45km in a NW-SE
direction and covers a total land area of 316km2. The largest
islands are Malta (length 27km; area 245.7km2) and Gozo (14.5km;
67.1km2). The other islands of the archipelago are much smaller
and comprise Comino (area 2.8km2), St Paul's Islands (10.134
ha.), Cominotto (9.864 ha.), Filfla (2.024 ha.) and General's Rock (0.687
ha.).
The geological structure of the islands represents a
simple layer-cake succession of shallow water marine carbonates deposited
during the late Oligocene and the Miocene. No rocks of Pliocene age have
as yet been conclusively identified, indicating that the archipelago was
exposed during this period. The stratigraphic sequence of the islands is
in places overlain by unconformably bedded Quaternary deposits of lacustrine,
alluvial or terrestrial origin laid down following emergence of the archipelago
above sea level.
Local climatic patterns are biseasonal, characterised
by a wet season and a dry season. The wet season normally extends from
October to March and approximately 86% of total annual precipitation is
recorded during this period. The dry season extends from April to September
and is unfavourable for plant growth due to high temperatures and low precipitation.
Annual precipitation is low, of the order of 585.3mm
annually. October is the wettest month (mean 110.0mm; s.d.83.0mm) and July
the driest (mean 1.3mm; s.d. 3.7mm). Much variation occurs from year to
year, with some years being excessively wet and others excessively dry.
During the wet season, much of the rainfall occurs as brief, heavy showers
with intervening dry periods that may last for several weeks. The number
of rain days is highest in January (mean 14.0d; s.d. 4.4d) and lowest in
July (mean 0.4d; s.d. 0.57d) Individual showers may contribute a much as
10% of the total annual rainfall in a single day. Thunderstorms occur most
frequently in October (mean 5.8 storms; s.d. 2.7 storms).
Temperatures are generally stable from year to year (mean
18.7°C; s.d. 0.4°C). August is the hottest month (mean 26.4°C;
s.d. 1.0°C) and February the coldest (mean 12.4°C; s.d. 1.0°C).
Continuous cloud cover is very rare and extended periods of sunshine in
winter tend to raise soil and air temperatures. Relative humidity is high,
ranging from 51%-85% with little seasonal variation.
Ephemeral regime freshwater rockpools
in the Maltese Islands
Temporary freshwater pools occur in hollows (kamenitzas)
on karstified limestone substrata which solutional erosion of rock by percolation
of acidified groundwater. Freshwater derived from precipitation and runoff
accumulates in such solution hollows forming the pool environment. Pools
supplied exclusively by these freshwater inputs are characterised by a
typically autumnal cycle with the alternation of an aquatic phase with
a desiccated phase. These cyclic changes are correlated with the biseasonal
climate of the Maltese Islands.
When the pools are filled with water a diverse macro
and microplanktonic resident biota is present and as they dry a flora of
terrestrial macrophytes may colonise the substratum.
Pool biota
The ephemeral nature of the pool habitat restricts colonisation
and several groups that are generally regarded as being typical of freshwaters
(as by WILLIAMS, 1975) have not been recorded from local pools. These include
gastropods, bivalves, oligochaetes and malacostracans (LANFRANCO, in prep.)
The absence of these groups is a consequence of their
inability to survive the inherent instability of the habitat which places
the constraint of a prolonged period of desiccation on the biota.
The resident biota therefore comprises highly-fecund
organisms with short life-cycles and rapid attainment of reproductive capability.
These features have been listed by WILLIAMS (1985) as biotic adaptations
of organisms colonising lentic waters in arid and semi-arid regions.
The fauna of the study pools is depauperate relative
to the pool of potential regional colonists. Lowered species richness is
a consequence of two principal factors:
The resident biota of pools in the Maltese Islands is dominated by microcrustaceans (Branchiopoda, podocopid Ostracoda, cyclopoid Copepoda) and chlorophytic algae (Chlorophyceae, Zygnemataceae, Characeae). Semi-transient biota that occupy the pool environment as larval stages include insects (Chironomidae, Culiciidae, Ephemeroptera, Odonata) and tadpoles. Several adult insects visit the pools for brief periods.
All microcrustaceans recorded from local pools belong to Group I of WIGGINS et al. (1980) and are widely distributed throughout the Mediterranean littoral. All locally-occurring branchiopods that have been identified to species are also present in Italy and Algeria (LANFRANCO et al. 1991) although some of these have a wider distribution.
Species of local or regional ecological
significance
Species that occur exclusively in temporary waters are
limited in their occurrence and distribution in the Maltese Islands by
a paucity of colonisation sites which is a consequence of the scarcity
of karstified terrain. Several temporary-pool species are therefore of
considerable ecological significance in a local context. The designation
RDB refers to the status of the species as listed in the Red Data Book
for the Maltese Islands (SCHEMBRI & SULTANA, 1989)
Crustacea: Branchiopoda: Anostraca (2 species)
Branchipus schaefferi Fischer [RDB: restricted
distribution]
This anostracan is euryhaline (LANFRANCO et al.,
1991), and has been recorded from inland pools as well as mesohaline coast-fringing
habitats. Occurrence is however sporadic. A previously unrecorded epizoic
association between B.schaefferi and Lyngbya sp. (Cyanophyta)
was observed at Ghadira s-Safra in 1990.
Branchipus visnyai Kertesz [RDB: very rare with
a restricted distribution]
This species has only been recorded from three pools
situated in different localities: Wied ir-Raheb in Gozo (1984), Ghadira
s-Safra (1989) and il-Qaliet (1995). B.visnyai was syntopic with
B.schaefferi at Ghadira s-Safra and il-Qaliet and sympatric with
the same species at Wied ir-Raheb. For the purposes of the present work,
B.visnyai has been determined according to the scheme of COTTARELLI
(1969), in line with previous records from the Maltese Islands.
Crustacea: Branchiopoda: Conchostraca (1 species)
Cyzicus tetracerus Krynicki [RDB: restricted distribution]
Although recorded in low abundances from several pools
throughout the Maltese Islands, this species is nevertheless absent from
many other habitats that could be considered potential sites for colonisation.
Crustacea: Branchiopoda: Notostraca (1 species)
Triops cancriformis (Bosc) [RDB: rare with a restricted
distribution]
The only records of this species in the Maltese Islands
are from the main island, Malta. The last records of this species are of
one live individual and a single dead specimen from Tal-Wej in December
1994. Prior to that, the only verified sightings in recent years were two
individuals from Ghadira s-Safra in December 1989 and a single individual
from Mosta in December 1990.
Crustacea: Branchiopoda: Cladocera (at least five genera)
The Cladocera are not featured in red data lists of the
Maltese Islands since local records of this group are scant. This is mainly
a consequence of the insufficient attention paid to this order. Following
a recent survey, a number of species from different families were collected
and are being identified by Prof. F.G. Margaritora in Rome.
Chydoridae (2 species): Occur in high abundances in the benthos of temporary pools.
Daphniidae (1 species; possibly Ceriodaphnia): Frequently occurring open-water form.
Moinidae (1 species): only recorded from a single pool in Gozo. The carapace of all specimens collected was covered with Colacium sp. (Euglenophyta).
Macrothriciidae (1 species): recorded in very low abundance from a single pool in Gozo.
Bryophyta: Hepaticae (1 species)
Riella helicophylla (Bory & Mont.)
A very rare and endangered liverwort recorded on a single
occasion from Ghadira s-Safra in 1990 (LANFRANCO E. & LANFRANCO S.,
in prep.). This species is listed in Appendix 1 of the Berne Convention
as a "Species to be strictly protected" but is not included in local Red
lists since its discovery post-dates the compilation of the only published
edition of the Red Data Book. R.helicophylla has recently been included
in a list of local flora covered by protective legislation (Government
of Malta Legal Notice 49, 1993).
Angiospermae: Alismataceae
Damasonium bourgei Cosson [RDB: Vulnerable with
a restricted distribution in the
Mediterranean and in the Maltese Islands]
Confined to temporary rainwater pools. This species is
of very restricted Mediterranean distribution.
Angiospermae: Elatinaceae
Elatine gussonei (Sommier) Brullo, Lanfranco,
Pavone & Ronsisvalle
[RDB: Rare with a restricted distribution in the Mediterranean
and in the Maltese Islands]
A Pelago-Maltese endemic confined to the Maltese Islands
and Lampedusa.
Angiospermae: Poaceae (1 species)
Crypsis aculeata (L.) Aiton [RDB: endangered with
a restricted distribution]
Recorded from Ghadira s-Safra.
Ecological context of temporary
pools
Ephemeral-regime freshwater pools represent impermanent
aquatic islets in a terrestrial habitat and the pool biota is essentially
restricted to highly localised patches of suitable habitat situated throughout
the Maltese Islands. These patches are characterised by varying physico-chemical
conditions but are all underlined by the common factor of ephemerality
which sets the behavioural and physiological limits required of potential
colonisers. A subset of these colonisers (most typically the non-cladoceran
branchiopods) are almost exclusively restricted to such ephemeral waters,
although exceptions have been recorded (VILLACLARA et al. , 1990).
Any major disturbance of this highly-localised, and therefore
highly vulnerable, point habitat results in a loss of diversity by compressing
the virtual physico-temporal hypervolume within which these species can
operate in an active state.
Threats to the pool environment
The most significant disturbances of the pool environment
are directly or indirectly derived from anthropogenic activity. Isolated
climatic events (e.g. flash-flooding following heavy storms) may also exert
considerable disruption on temporary pools, although the effects of such
phenomena are generally far less persistent than those deriving from human
interference. The following are the principal sources of disturbance for
pools in the Maltese Islands:
Encroachment of human constructions
Growth of the human population in the Maltese Islands
has been particularly rapid in the last forty years resulting in an increase
in the area occupied by buildings. Several karstified outcrops that harboured
temporary rockpools have been converted into residential areas with consequent
obliteration of these habitats.
The sharply-enhanced volume of tourism since 1978 has
necessitated upgrading of infrastructural facilities resulting in construction
of several new hotels and upgrading of existing ones. Several of these
have been built over coastal karstland. A recently-approved extension to
a major coastal hotel includes development of the area in which the il-Qaliet
pool is situated.
Reclamation of land
Several attempts at converting areas of natural landscape
into agricultural or forested land have been made. Such ventures invariably
involve the covering of land perceived as ‘barren’ (actually karstland
with typical garigue vegetation) with soil resulting in the eradication
of pool habitats (SCHEMBRI & LANFRANCO, 1993).
Quarrying
The rapid growth of the construction industry has led
to an increase in the number and size of quarries. A total of 96 licensed
quarries were operative in 1992, 26 of which exploit karstland (MALTA STRUCTURE
PLAN, 1991). There is also a sizeable number of inoperative and illegal
quarries.
Runoff
Flash-flooding during and following heavy rains has occasionally
been observed to flush out smaller rockpools almost completely with the
resultant loss of all sediment and biota (LANFRANCO, 1990). Infilling of
kamenitzas with water-borne soil has also been noted, although the effects
are generally transient, as the soil is redistributed by successive floods.
A more persistent threat is the accumulation of leachate
from agricultural and industrial land in the pool sediment. Biocides derived
from agriculture may inhibit the development of pool vegetation with a
consequent reduction in faunal diversity. Leachate from waste dumps may
also interfere with the establishment and maintenance of pool communities.
Illegal dumping
Dumping of organic waste in pool hollows is a frequent
occurrence. Consequent rapid growth of bacterial populations leads to a
decline in oxygen content of poolwater and results in impoverishment of
the biota.
Protective legislation
Although there is no specific legislation for the conservation
of temporary freshwater rockpools in the Maltese Islands, these habitats
and some of their resident species may be used in conjunction with other
features in order to determine the Ecological Protection Rating of a locality
(MALTA STRUCTURE PLAN 1992a, b). A small number of temporary pools (e.g.
Ghadira s-Safra) have been indirectly protected through designation of
the area within which they occur as a nature reserve. This measure is however
mainly cosmetic since only one site is regularly wardened (STEVENS et
al., in press).
Only two of the species which occur in temporary pools
are formally protected by legislation. The Painted Frog (Discoglossus
pictus pictus Otth) and Riella helicophylla (Bory & Mont.)
have been protected since 1993 (Government of Malta Legal Notice 49, 1993).
Present legislation does not specifically cover any of the resident fauna
of temporary pools.
References
COTTARELLI, V., (1969). Osservazioni tassonomiche su
Branchipus visnyai Kertesz e diagnosi preliminare di Branchipus pasai n.sp.
(Crustacea, Euphyllopoda, Anostraca). Fragm. ent. 6: 63-68.
LANFRANCO, E. & LANFRANCO, S. (in preparation). Riella
helicophylla (Bryophyta: Hepaticae); a new addition to the Maltese
flora.
LANFRANCO, S., (1990). The ecology of freshwater rockpools
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LANFRANCO, S., (in preparation). Ecological succession
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