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Biodiversity, functional structure and conservation of the deeper water benthic assemblages of the Maltese Islands (TRANSDEEP)

The sublittoral marine benthic assemblages of the Maltese Islands have only recently started being investigated. All these studies were made using SCUBA. Therefore, practically all work has focused on the infralittoral, mainly at depths not exceeding 40m, which is the limit for safe diving using normal SCUBA techniques. The scanty data on the lower infralittoral and circalittoral biotic assemblages which is available is mainly descriptive and is based on studies carried out more than a century ago and on the more or less casual observations of sports divers and fishermen. This present project aims to investigate the deeper water benthic assemblages of the Maltese Islands and involves the following main tasks:

(i) Mapping of the bottom types present in selected areas of deeper water. This will focus on areas of the seabed that have not been previously investigated scientifically.
(ii) Drawing up of an inventory of the biota and the biotic assemblages present. 
(iii) Assessment of the resource potential of the bottoms investigated. This will focus on the ‘service’ role of the ecosystem rather than direct economic value as a fishing ground. This aspect has been neglected so far even if such bottoms as seagrass meadows and maerl beds provide important feeding grounds, breeding grounds, nursery areas and refugia for a number of commercially important species.
(iv) Assessment of any anthropogenic impacts on these bottoms. Maerl bottoms, for example, are very fragile and easily disrupted by the use of inappropriate fishing gear. By understanding something of the nature of the ecosystems present, recommendations for their preservation may be made.





Maerl beds are one of the habitats studied in the project "Biodiversity, functional structure and conservation of the deeper water benthic assemblages of the Maltese Islands". This work was undertaken as part of BIOMAERL, an international research project funded by the European Commission of the European Union under the Marine Science and Technology (MAST III) component of the Fourth Framework Programme (1994-1998). The BIOMAERL group linked research groups from the Universities of London (UK), Bretagne Occidentale (France), Alicante and Santiago (Spain), and the University of Malta.
The term ‘maerl’ refers to biogenic sediments characterized by accumulations of live and dead unattached coralline algae. The maerl-forming algae can take the form of rhodoliths (see plate), which are nodules or unattached branched growths. The complex morphology of rhodoliths provides a very heterogeneous habitat. Maerl grounds form a unique ecosystem with a high benthic biodiversity supporting many rare and unusual species. Surveys carried out as part of this project discovered an extensive maerl bed off the northeastern coast of Malta and Gozo (see map), covering an area of about 20km2 of the seabed, at depths of between 40m and 80m. This maerl ground is well known to local fishermen as being highly productive for demersal and pelagic species that are exploited by off-bottom fishing.  This high productivity may be related to the structure and dynamics of the maerl ecosystem.  Hence it is in the interest of the fishing industry that maerl ecosystems be maintained in a state of health.  That substantial degradation of maerl beds in some European countries has already taken place due to inappropriate anthropogenic impacts is then a matter of grave concern.



Map showing the location of the maerl bed (shaded) off the northeastern coast of Malta and Gozo.


Rhodoliths of the coralline alga Lithothamnion minervae from the maerl ground off Is-Sikka l-Bajda (northeastern coast of Malta). The largest rhodolith has a diameter of ca.40mm.



Summary of macro/megabiota species diversity in major taxa on maerl grounds studied during the BIOMAERL programme.  Note: groups which are likely to be underestimated due to incomplete knowledge are marked with an asterisk (*)


BIOMAERL team (1998) Maerl grounds: habitats of high biodiversity in European waters. In: Barthel, K.G.; Barth, H.; Bohle-Carbonell, M.; Fragakis, C.; Lipiatou, E.; Martin, P.; Ollier, G. & Weydert, M. [eds] Third European marine science and technology conference, Lisbon, 23-27 May 1998. Volume 1 Marine systems; pp.170-178. Brussels : European Commission. [The BIOMAERL team includes J.A.Borg, E.Lanfranco, M.Rizzo and P.J.Schembri from the University of Malta .]
Borg, J.A., Lanfranco, E., Mifsud, J.R., Rizzo, M. & Schembri, P.J. (1998) Does fishing have an impact on Maltese maerl grounds? Paper presented at ICES Symposium on Marine benthos dynamics: environmental and fisheries impacts. Heraklion, Crete 5-9 October 1998; [abstracts book p.18]
Schembri, P.J. (1998) Maerl ecosystems of the Maltese Islands . In: Dandria, D. [ed.] Biology abstracts MSc, PhD 1998 and contributions to marine biology. pp.35-37. Msida , Malta : Department of Biology, University of Malta ; iv+38pp.
Lanfranco, E.; Rizzo, M.; Hall-Spencer, J.; Borg, J.A. & Schembri, P.J. (1999) Maerl-forming coralline algae and associated phytobenthos from the Maltese Islands. The Central Mediterranean Naturalist 3(1): 1-6.
BIOMAERL Team (1999) BIOMAERL: Maerl biodiversity, functional structure and anthropogenic impacts. Final Report (in two volumes). [Co-ordinator: P.G.Moore, university Marine Biological Station, Millport, Scotland] EC Contract No. MAS3-CT95-0020. Vol 1 pp.1-541; Vol 2 pp.542-973 + Appendix.
Bordehore, C.; Borg, J.A.; Lanfranco, E.; Ramos-Esplá, A.; Rizzo, M. & Schembri, P.J. (2002) Trawling as a major threat to Mediterranean maerl beds. Proceedings of the First Mediterranean Symposium on Marine Vegetation ( Ajaccio , 3-4 October 2000); [Mednature 1] pp.105-109. Tunis , Tunisia : Regional Activity Centre for Specially Protected Areas.
BIOMAERL Team (2003) Conservation and management of northeast Atlantic and Mediterranean maerl beds. Aquatic Conservation: Marine and Freshwater Ecosystems 13 suppl.1: S65-S76.
[The BIOMAERL team includes J.A.Borg, E.Lanfranco, M.Rizzo and P.J.Schembri from the University of Malta.]
Sciberras, M; Rizzo, M; Cilia, R; Camilleri, K; Borg, JA; Lanfranco, E & Schembri, PJ (2008) Maerl assemblages as biodiversity hotspots in the Maltese Islands. Poster presented at Porcupine Marine Natural History Society Annual Conference, University of Wales, Bangor, 28-30 March 2008




Through its 1998 Participation Programme, UNESCO awarded the Marine Ecology Research Group a grant to carry out a detailed investigation of the benthic assemblages of the lower infralittoral to upper circalittoral transition zone, which are not readily accessible using normal SCUBA techniques and equipment.

The specific objectives of this project were to acquire a towable underwater television system and to adapt this gear for use in local conditions and to use it to map the bottom types present in selected areas of deeper water. 

The towable underwater television camera (manufactured by Elettronica Enne of Vado Ligure, Italy) was towed at a constant speed of c.1 knot along three transects laid at a bearing of 35° off the northeastern coast of the Maltese Islands (see map). The images transmitted by the TV camera were recorded on videotape on deck. In the laboratory, the video footage for each transect was viewed using normal speed and slow motion and stop-frame as appropriate, in order to identify and describe the benthic assemblages and estimate the abundance of the dominant mega and macrobenthos.

Plant assemblages were dominated by the phanerogam Posidonia oceanica up to a depth of 45m. Codium bursa, Vidalia volubilis, Cystoseira sp. and the non-indigenous species Caulerpa racemosa were also found in relatively shallow depths with a lower bathymetric limit of about 55m. Beyond a depth of 45m, maerl assemblages dominated the seabed, with Flabellia petiolata and Lophocladia lallemandii dominating the non-coralligenous macrophyte species in these deep water assemblages. Flabellia petiolata showed a particular distribution pattern along each transect; being very common at depths between 40m and 50m, decreasing sharply at a depth of about 60m, increasing again between depths of 70m and 80m, and diminishing beyond 80m. Rhodolith-forming algae occurred from depths of about 45-50m down to depths greater than 90m. The animal biota, which was dominated by echinoderms, was more or less evenly distributed along the whole area covered by the three transects, with the exception of gorgonians which were present down to a depth of 50m. The crinoid Antedon mediterranea appeared to be commoner along Transect 3. Beyond 90m, the seabed was characterized by drop-offs, making the seabed too deep for video surveys.


Transects 1, 2 and 3 off the northeastern coast of the Maltese Islands. The depths are given at intervals along the transects. 


Frame captured from one of the underwater video transects, showing the starfish Astropecten aranciacus feeding on the surface of a gravely sand bottom [Transect 2: 4km NE of St.Paul’s Bay; depth: 50-53m].


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