Showing papers in "Oceanography and Marine Biology in 2003"

Oxygen minimum zone Benthos: Adaptation and community response to hypoxia

Abstract: Mid-water oxygen minima (� 0.5 ml l � 1 dissolved O2) intercept the continental margins along much of the eastern Pacific Ocean, off west Africa and in the Arabian Sea and Bay of Bengal, creating extensive stretches of sea floor exposed to permanent, severe oxygen depletion. These seafloor oxygen minimum zones (OMZs) typically occur at bathyal depths between 200 m and 1000 m, and are major sites of carbon burial along the continental margins. Despite extreme oxygen depletion, protozoan and metazoan assemblages thrive in these envi- ronments. Metazoan adaptations include small, thin bodies, enhanced respiratory surface area, blood pigments such as haemoglobin, biogenic structure formation for stability in soupy sedi- ments, an increased number of pyruvate oxidoreductases, and the presence of sulphide-oxidising symbionts. The organic-rich sediments of these regions often support mats of large sulphide- oxidising bacteria (Thioploca, Beggiatoa, Thiomargarita ), and high-density, low-diversity meta- zoan assemblages. Densities of protistan and metazoan meiofauna are typically elevated in OMZs, probably due to high tolerance of hypoxia, an abundant food supply, and release from predation. Macrofauna and megafauna often exhibit dense aggregations at OMZ edges, but depressed densities and low diversity in the OMZ core, where oxygen concentration is lowest. Taxa most tolerant of severe oxygen depletion ( � 0.2 ml l � 1 ) in seafloor OMZs include calcare- ous foraminiferans, nematodes, and annelids. Agglutinated protozoans, harpacticoid copepods, and calcified invertebrates are typically less tolerant. High dominance and relatively low species richness are exhibited by foraminiferans, metazoan meiofauna, and macrofauna within OMZs. At dissolved oxygen concentrations below 0.15 ml l � 1 , bioturbation is reduced, the mixed layer is shallow, and chemosynthesis-based nutrition (via heterotrophy and symbiosis) becomes important. OMZs represent a major oceanographic boundary for many species. As they expand and con- tract over geological time, OMZs may influence genetic diversity and play a key role in the evo- lution of species at bathyal depths. These ecosystems may preview the types of adaptations, species, and processes that will prevail with increasing hypoxia over ecological and evolution- ary time. However, many questions remain unanswered concerning controls on faunal standing stocks in OMZs, and the physiological, enzymatic, metabolic, reproductive and molecular adap- tations that permit benthic animals to live in OMZs. As global warming and eutrophication reduce oxygenation of the world ocean, there is a pressing need to understand the functional consequences of oxygen depletion in marine ecosystems.

Ecology of whale falls at the deep-sea floor

Abstract: The falls of large whales (30-160 t adult body weight) yield massive pulses of labile organic matter to the deep-sea floor While scientists have long speculated on the ecological roles of such concentrated food inputs, observations have accumulated since the 1850s to suggest that deep-sea whale falls support a widespread, characteristic fauna Interest in whale- fall ecology heightened with the discovery in 1989 of a chemoautotrophic assemblage on a whale skeleton in the northeast Pacific; related communities were soon reported from whale falls in other bathyal and abyssal Pacific and Atlantic sites, and from 30 mya (million years ago) in the northeast Pacific fossil record Recent time-series studies of natural and implanted deep- sea whale falls off California, USA indicate that bathyal carcasses pass through at least three successional stages:

The effects of sedimentation on rocky coast assemblages

Abstract: Sedimentation is a widespread and increasing process on most rocky coasts. The literature on its effects is reviewed and support is found for the general conclusion that sedimen- tation is an important ecological factor for hard bottom organisms. Sediments deeply affect the composition, structure and dynamics of rocky coast assemblages, and increased sediment load as a consequence of anthropogenic activities can be a threat to their diversity and functioning. Sediments that accumulate on rocky substrata are important agents of stress and disturbance. They can cause burial, scour and profound modifications to the characteristics of the bottom surface, and interact with other important physical and biological processes. The effects of sedi- mentation are complex, because they involve both direct outcomes on settlement, recruitment, growth or survival of individual species and indirect outcomes through mediation of competitive and/or predator-prey interactions. Not all species and assemblages are equally affected by sedi- mentation and responses vary over space and time, depending on the characteristics of the depo- sitional environment, life histories of species and the stage of development of individuals and assemblages, and in relation to variable physical factors, including hydrodynamics, light intens- ity and bottom topography. Recent studies have much improved our ability to detect and under- stand the effects of sedimentation on rocky coast assemblages. However, little is still known about the underlying mechanisms. Overall, our present ability to make generalisations and pre- dictions is limited by a paucity of quantitative and experimental research, and by the scant atten- tion devoted to measuring the regime of perturbation by sediments and responses of organisms at relevant spatial and temporal scales. Predicting the magnitude of the effects that different sed- imentation regimes have on rocky coast organisms and the critical levels above which detrimen- tal effects become manifest remains a key issue for the ecology of rocky coasts and a challenge for future studies.

Antarctic marine benthic diversity

Abstract: Species lists have been compiled for all the major groups of Southern Ocean benthic marine invertebrates, eliminating synonymies where possible and providing a subjective estimate of completeness and reliability for each group Antarctic marine diversity (pelagic and benthic) is relatively high at the phylum and class level, with the gaps mostly comprising minor, meiofaunal or parasitic groups Most benthic diversity data come from the continental shelves, with relatively few samples from deeper water Even for the continental shelves, however, sampling is highly patchy with some areas hardly investigated at all Over 4100 benthic species have been reported from the Southern Ocean, with the most speciose groups being polychaetes, gastropods and amphipods Comparison with tropical and temperate regions suggest that decapods, bivalves and teleost fishes are poorly represented in the Southern Ocean benthic marine fauna, whereas pycnogonids, echinoderms and many suspension feeding groups are rich and diverse Some groups that are currently low in diversity were previously well represented in the Antarctic shallow water marine fauna, notably decapods and many fishes Other groups have undergone marked radiations in the Southern Ocean, including pycnogonids, amphipods, isopods and teleost fishes; in all cases, however, it is only some lineages that have diversified This indicates that evolutionary questions concerning the origin, diversification or extinction of the Southern Ocean marine fauna will have no single answer; the evolutionary history of each group appears to reflect a different response to the tectonic, climatic and oceanographic changes to which they have been subject through history The disposition of southern hemisphere continents makes it difficult to assess whether there is a latitudinal cline in shallow-water marine diversity to mirror that known from the northern hemisphere Within Antarctica, many species appear to have circumpolar distributions, and the long established biogeographical division into continental Antarctic, Antarctic Peninsula and sub-Antarctic regions have not been challenged by recent sampling For most groups the frequency distribution of species per genus ratios is typical, though none is well described by the predictions from current evolutionary or null models Where data are available, size spectra indicate that many Southern Ocean taxa are small, a few spectacular examples of gigantism notwithstanding, and species abundance plots are normal Knowledge of the Southern Ocean benthic marine fauna has reached a stage where we can now ask powerful evolutionary questions, and the development of new molecular techniques provides the mechanism for answering them

Detritus in the epilithic algal matrix and its use by coral reef fishes

Abstract: The epilithic algal matrix (EAM) is a ubiquitous component of coral reefs and is the primary grazing surface for many reef fishes. Detritus accounts for at least 10% to 78% of all the organic matter present in the EAM, variation being attributed to hydrodynamic forces such as wave energy and biological elements such as algal morphology. When compared with filamentous algae, the other major source of organic matter in the EAM, protein:energy ratios, C:N ratios and total hydrolysable amino acids all suggest that detritus is of higher nutritional value than the algae. Lipid biomarkers indicate that more than 70% of the detritus is derived from the filamentous algae but the addition of bacteria and microalgae add essential nutrients and improve the nutritional value of the detritus. The detritus is typically of an amorphic form with protein:energy ratios which indicate that it is capable of sustaining fish growth. Detritus within the EAM may be derived from dissolved organic matter, which reduces refractory material, enhancing the palatability and digestibility of detritus relative to filamentous algae. Detritus in the EAM may also come from settling material and fish faeces. Studies that quantified the amount of detritus ingested by fishes have identified at least 24 species from five families that predominantly ingest detritus. These species represent some of the most widespread and abundant EAM feeding fishes on coral reefs. It is estimated that detritivorous fishes account for at least 20% of individuals and 40% of the biomass of an EAM-feeding fish assemblage on the Great Barrier Reef. Comparisons of ingested material with the EAM indicate that many of these species selectively feed on detritus, particularly the small, organic rich particles <125 μm. Furthermore, analysis of lipids in body tissues of blennies and assimilation of nutrients from the alimentary canal of scarids and acanthurids provide strong evidence that detritus is assimilated by coral reef fishes. Consequently, a large percentage of EAM-feeding fishes on coral reefs can unequivocally be classified as detritivores. The ingestion and assimilation of detritus by these fishes represents a significant pathway for transferring energy from within the EAM to secondary consumers, making detritivorous fishes a critically important component of coral reef trophodynamics.

Influence of marine allochthonous input on sandy beach communities

Abstract: This review provides an overview of the importance of beach accumulations of macrophytes and other organic beach-cast material on the ecology of sandy beach ecosystems. It describes the composition of these allochthonous subsidies, their abundance on beaches in relation to seasonal, lunar, tidal and spatial trends, their decomposition and utilisation by bacterial, meio- and macrofaunal communities. The paper then analyses the community structure and the species succession in both macrophyte wrack and carrion and reports the most important findings on individual wrack-inhabiting species (amphipods, isopods, dipterans). Other aspects, such as feeding and microclimatic preferences of certain species and their interactions in wracks, are also discussed. Links to vertebrate species and other secondary consumers that exploit beach-cast macrophytes and carrion as trophic resource are considered, and the importance of wrack in recycling nutrients to nearshore coastal ecosystems is stressed. The beneficial and detrimental effects of organic beach-cast material on both plants and animals of beach and nearshore communities and on the geomorphology of coastal beach-dune systems are pointed out. Another section is dedicated to human use of beach-cast macrophytes through harvesting of economically important species and of other stranded material through its exploitation for traditional reasons. The effects of harvesting on local faunal communities and on the stability of the dunes is discussed. A final section of the paper includes the positive and negative effects of man-made debris on sandy-beach ecosystems and briefly reviews the major findings.

The diet of harbour porpoise (phocoena phocoena) in the northeast atlantic

Abstract: The harbour porpoise (Phocoena phocoena) is probably the most abundant small cetacean in the northeast Atlantic and as such is an important top predator. It is also one of the most threatened species, particularly as a consequence of fishery by-catch. Porpoises feed mainly on small shoaling fishes from both demersal and pelagic habitats. Many prey items are probably taken on, or very close to, the sea bed. Even though a wide range of species has been recorded in the diet, porpoises in any one area tend to feed primarily on two to four main species (e.g. whiting ( Merlangius merlangus) and sandeels (Ammodytidae) in Scottish waters). Evidence for selective predation is equivocal. Many studies provide evidence of geographic, seasonal, interannual, ontogenetic or sexual differences in prey types or prey sizes, and such dif- ferences are often (speculatively) interpreted in terms of prey availability. A few studies demon- strate trends in diet selection that are consistent with changes in prey abundance. However, lack of availability of prey abundance data at an appropriate spatial and temporal scale is often a problem. Porpoise diets overlap extensively with diets of other piscivorous marine predators (notably seals). Many of the main prey species are also taken by commercial fisheries, although por- poises tend to take smaller fishes than those targeted by fisheries. Given their high abundance, porpoises clearly remove substantial quantities of fish. The literature on porpoise diets in the northeast Atlantic suggests that there has been a long- term shift from predation on clupeid fish (mainly herring Clupea harengus) to predation on sandeels and gadoid fish, possibly related to the decline in herring stocks since the mid-1960s. Evidence from studies on seals suggests that such a shift could have adverse health con- sequences. Food consumption brings porpoises into contact with two important threats - persistent organic contaminants and fishing nets, both of which have potentially serious impacts.

Exotic molluscs in the Mediterranean basin: Current status and perspectives

Abstract: An updated synthesis is presented for the records of introduced Mollusca in the Mediterranean basin. The rationale for taking molluscan records as significant is discussed. The Mediterranean Sea, with some 1800 native species of Mollusca, currently houses 139 exotic species, of which 85 form established populations, 52 are aliens recorded once or twice, and two are questionable. Ten species (the gastropods Cerithium scabridum, Rhinoclavis kochi, Strom- bus persicus and Bursatella leachi and the bivalves Pinctada radiata and Brachidontes pharao- nis in the eastern Mediterranean, the gastropod Rapana venosa and the bivalves Anadara inaequivalvis, Musculista senhousia, and Xenostrobus securis in the northern Adriatic and the western Mediterranean lagoons) are locally invasive. The bulk of the introduced species (118 species, of which 70 are established, 46 aliens, and two questionable) are species of Indo-Pacific origin found mainly in the eastern basin of the Mediterranean. Among these species, some which live in the Suez Canal are most likely to have spread by their own means through this waterway (these are the "lessepsian immigrants" in the most restricted sense). For other species, the intervention of transport by ship hulls or ballast water can be suspected. Only two of these Indo-Pacific immigrants are found, very locally, in the western Mediterranean. The process of immigration has become unprecedented in magni- tude since the 1970s and is not slowing down. The remaining introductions of marine species are connected with mariculture and/or ship- ping. These vectors account for the occurrence or dissemination of only 29 exotic species in the Mediterranean basin, but four of these are invasive. The data regarding the Mollusca do not support any substantial faunal change caused by an influx of subtropical faunal elements through the Straits of Gibraltar. Some local species bound- aries may have changed slightly in the past decades but not a single newcomer to the Mediter- ranean basin by this route could be detected in this survey. The open sea localities of the western Mediterranean remain virtually free of immigrant mollusc species. The areas most severely affected by the occurrence of exotic species (eastern Mediterranean, Adriatic and lagoons in the western Mediterranean) are those where the species richness of the native fauna is low. It is speculated that this low diversity is a crucial contribu- tion to the success of the newcomers. In the affected areas, the impact on the local fauna is con- siderable in terms of species composition of the assemblages but so far no native Mediterranean species can be reported as endangered as an effect of a biological invasion.