Biokarst on Limestone Coasts, Morphogenesis and Sediment Production

Abstract. Biokarst-forms on limestone coasts are developed and arranged according to the bionomic zonation. The development of biokarst is the result of bioerosion, a synergistic effect of biological corrosion by endoliths and biological abrasion by grazers.
The cumulative effect of biogenic carbonate destruction leads to coastal destruction with a resulting highly profiled morphology on the limestone surfaces along the coastal profile. Under the influence of environmental factors a zonation of organisms develops which brings in turn a zonation of erosion rates (0.1-1.1 mm a^-1) resulting in biokarst-forms such as rock holes, rock pools and notches.
Products of bioerosion on limestone coasts are dissolved carbonate (by biological corrosion, 10–30% of the decomposed limestone) and particulate carbonate (by biological abrasion, 70–90% of the decomposed limestone) both of which contribute directly or indirectly to nearshore sedimentation. Size and shape of the bioerosional grains are determined by the boring pattern of the endoliths. The fine-grained sediments (maximum within the fraction 20–63 μm) contribute 3–25 % to the nearshore sediments.
Drastic changes in the biological zonation (like the mass invasion of the sea urchin Paracentrotus lividus in the Northern Adriatic since 1972 which eliminated nearly the entire macrophyte zone) due to unknown factors or pollution can have a profound effect on the bioerosion rates, altering them by as much as a factor of ten.     

Assessing potential abiotic and biotic complications of crayfish-induced gravel transport in experimental streams

Biogeomorphology adds the element “biological dynamics” (of populations or communities) to chemical and physical geomorphic factors and thus complicates the framework of geomorphic processes. Such biological complications of the animal-induced transport of solids in streams should be particularly important in crayfish, as crayfish affect this transport through their overall activity and intraspecific aggression levels, which could be modified by shelter availability or the establishment of dominance hierarchies among individuals not knowing each other. Using experimental streams, we tested these hypotheses by measuring how shelter availability or residential crayfish group invasion by unknown individuals affected the impact of the crayfish Orconectes limosus on the (i) transport of gravel at baseflow (during 12 experimental days); (ii) sediment surface characteristics (after 12 days); and (iii) critical shear stress causing incipient gravel motion during simulated floods (after 12 days). The two potentially important factors shelter availability or residential group invasion negligibly affected the crayfish impact on gravel sediments, suggesting that habitat unfamiliarity (a third potentially important factor affecting crayfish activity) should increase the crayfish-induced sediment transport. Because habitat unfamiliarity is associated with sporadic long-distance migrations of a few crayfish individuals, this third factor should play a minor role in real streams, where crayfish biomass should be a key factor in relations with crayfish effects on sediments. Therefore, we combined the results of this study with those of previous crayfish experiments to assess how crayfish biomass could serve in modelling the gravel transport. Crayfish biomass explained 47% of the variability in the baseflow gravel transport and, in combination with the coefficient of variation of the bed elevation and algal cover, 72% of the variability in the critical gravel shear stress. These results encourage more research on the topic, as an increasing number of eliminations of abiotic and biotic factors that could complicate the animal-induced sediment transport in streams would facilitate the use of biological variables (e.g., bioturbator biomass) in future modelling of the transport of solids.     

Two bioerosion ichnofacies produced by early and late burial associated with sea-level change

In bioerosion, as in trace fossils as a whole, deeply emplaced structures have greater survival value than shallow structures. That is to say, tiering (the relative depth to which rasping, etching and boring organisms penetrate their substrate) is of paramount importance for the preservation potential of individual trace fossils. An Entobia ichnofacies is established for trace fossil assemblages dominated by deep tier borings and arising from long-term bioerosion, such as occurs on sediment-free submarine cliffs or hardgrounds. A Gnathichnus ichnofacies comprises assemblages containing all tiers, including superficial sculptures produced by radulation that have very little preservation potential. Such assemblages occur in short-term bioerosion situations as on shell surfaces and hardgrounds buried early by sedimentation. Correspondence to: R. G. Bromley     

Substrate Effects on the Bioeroding Demosponge Cliona orientalis.1. Bioerosion Rates

Abstract. Bioeroding sponges are highly specialised to live in and to erode various natural and man-made calcareous substrates. They encounter very different substrate features. Previous field observations suggest that damage caused by sponge bioerosion may vary with substrate density and architecture. This study aims to experimentally investigate influences of structurally different calcareous substrates on bioerosion activities of Cliona orientalis Thiele, 1900 , an important eroder of inshore Great Barrier Reef calcium carbonate. Blocks were made of the corals Goniopora tenuidens , massive Porites sp., Astreopora listeri, Favites halicora, Favia pallida, Goniastrea retiformis and Cyphastrea serailia , and of the clam Tridacna squamosa . They were grafted with C. orientalis tissue and re-examined after 9 months. Block weight loss, increase of pore volume and differences in breaking stability were measured as indicators of sponge erosion.Erosion caused by C. orientalis differed between substrates and was significantly more pronounced in denser materials with lower pore volume and in coral blocks with more structural barriers. Coral substrates with imperforate thecae and thicker dissepiment walls were more strongly eroded than those with perforate thecae and thinner dissepiments. At similar growth rates, more material has to be removed in denser material with more barriers compared to more porous substrates. Existing pores will be occupied, resulting in lower erosion rates. Erosion capabilities of the sponge could best be detected by the blocks' loss in dry weight, but the sponges also significantly reduced block breaking stability. Change in pore volume was not found to be a reliable parameter to investigate sponge erosion.     

Preliminary estimates of intertidal limestone erosion,one tree island,southern great barrier reef,australia

Estimations of surface lowering of 0.2-3.8 mm a^?1 were gained from measurements of pedestal heights under sedentary organisms. The role of erosion by chitons, estimated at 0.2-2.9 mm a^?1 for home scars and 0.2-0.7 mm a^?1 for grazing, and other bioerosive organisms, is briefly discussed.     

Neoichnology and implications for stratigraphy of reworked Upper Oligocene oysters,Antigua, West Indies

The Late Oligocene oyster Hyotissa antiguensis (Brown) is locally common in the Antigua Formation of Hughes Point, eastern Antigua, Lesser Antilles; it was not commonly bored at that time. Its valves and shells are robust, and reworked into the shallow water near-shore environment in Antigua; it could potentially be incorporated into younger rocks. Its neoichnology includes clues that would facilitate identification of these oysters as reworked fossils. The suite of modern borings found in these specimens includes common Caulostrepsis taeniola Clarke, Gastrochaenolites isp. cf. G. turbinatus Kelly and Bromley and Entobia isp., and rare Oichnus simplex Bromley and Rogerella? isp. The latter three taxa are limited to oyster shell substrates. Of the common ichnotaxa, Caulostrepsis and Gastrochaenolites are particularly prominent in limestone clasts and limestone cemented to oyster shells, which would be an indicator of reworking if found in a post-Oligocene lithified deposit. Caulostrepsis and Gastrochaenolites are relatively less common in oyster shells and valves, and in many specimens are seen to terminate against the shell. Entobia is the only common boring limited to the shell substrate. The fidelity of preservation of modern borings is also superior in limestone clasts. This suite of borings is comparable with those found in the Neogene of the Antillean region.     

Crayfish and fish as bioturbators of streambed sediments: Assessing joint effects of species with different mechanistic abilities

Many studies illustrate that bioturbating animal species individually affect aquatic sediments through diverse mechanistic abilities, whereas assessments of joint effects of such species on sediments are relatively rare. Such joint effects have implications for real systems, in which different bioturbators coexist, but are difficult to predict for two reasons. First, they can be additive (being the sum of the individual effects of each species) or they can be positive or negative interactive (being greater or smaller than the sum of the individual effects). Second, if interactive, they can depend on biotic interactions that affect the bioturbating activities of the species and/or they can depend on physical interactions among bioturbator-induced sediment modifications. Using experimental streams, we assessed such joint effects on gravel–sand sediments for flow and sediment conditions preferred by barbel (Barbus barbus) but also used by gudgeon (Gobio gobio) and, in a second experiment, for flow and sediment conditions preferred by both male crayfish (Orconectes limosus) and gudgeon. These species have different mechanistic abilities to affect gravel and/or sand in stream beds. In each experiment, we measured (i) the transport of gravel and sand at baseflow (during 12 experimental days); (ii) four sediment surface characteristics (after 12 d); and (iii) the critical shear stress (τ_c) causing incipient gravel and sand motion during experimental floods (after 12 d). Gudgeon contributed differently to the joint effects in the two experiments, which related to its individual weight, prevailing baseflow shear stress, sediment particle weight, and sediment mixture (availability of surface sand). Overall, the species pairs had predominantly negative interactive joint effects on the sediment variables assessed by us. Both a literature survey and observations during the experiments provided no evidence for direct biotic interactions between barbel and gudgeon or crayfish and gudgeon, so one would reasonably associate their negative interactive effects on the sediments with physical interactions among bioturbator-induced sediment surface modifications. Individually, each species reduced the percentage of sand in the surface layer and the surface algal cover to relatively low values so that the species pairs could not accomplish much greater joint effects on these variables, explaining their negative interactive effects on them. As algal cover particularly affected the τ_c for gravel and sand, the negative interactive effects of the animals on this surface variable chained toward the τ_c for the sediments, on which the species pairs also had negative interactive effects. Such chained negative interactive effects on sediment variables are seemingly a general pattern of joint bioturbator effects on aquatic sediments, i.e., the many so far described single-species effects should be smaller than their sum if the species coexists in nature.     

Methods to quantify components of the excavating sponge Cliona orientalis Thiele, 1900

This study applied the loss after combustion (LAC) method and the acid decalcification (ADC) method to quantify different components of an excavating sponge. Samples of dried coral skeleton of Favia sp. invaded by the Indo‐Pacific excavating sponge Cliona orientalis Thiele, 1900 were used. The sponge tissue penetrated the 12‐mm‐thick samples to approximately 10 mm. The average proportional weight of organic matter, siliceous spicules, calcareous substrate and salts in the entire samples was found to be respectively 2.5%, 4.4%, 90.5% and 2.5% of dry weight applying the LAC method, and 2.9%, 5.9%, 89.0% and 2.3% of dry weight applying the ADC method. Respective volumetric proportions of the organic matter, spicules, substrate and salts were then calculated to be 6.4%, 5.5%, 85.2% and 3.0% of volume with the LAC method, and 7.4%, 7.2%, 82.7% and 2.7% of volume with the ADC method. The LAC method showed low variability of data and is simple and fast and therefore is recommended. The ADC method generated very similar results to the LAC method. However, due to the handling involved in the ADC method, more than half of the spicules may be lost and the method is therefore not recommended unless careful data corrections are considered. In addition, the buoyant weight method was used to quantify actual substrate weight in the fresh sponge‐substrate samples. This method was found to be at least 97% effective, revealing that buoyant weights can potentially be used to quantify bioerosion rates of excavating sponges. To our knowledge, this is the first study to systemically quantify organic and inorganic components of an excavating sponge and its calcareous substrate, providing improved standard methods for future studies.     

Environmental factors shaping boring sponge assemblages at Mexican Pacific coral reefs

Recent studies suggest a future increase in sponge bioerosion as an outcome of coral reef decline around the world. However, the factors that shape boring sponge assemblages in coral reefs are not currently well understood. This work presents the results of a 17‐month assessment of the presence and species richness of boring sponges in fragments collected from living corals, dead coral reef matrix and coral rubble from Punta de Mita and Isabel Island, two coral reefs from the central coast of the Mexican Pacific Ocean. Both localities have a high cover of dead corals generated by past El Niño Southern Oscillation events, but Punta de Mita was also highly exposed to anthropogenic impacts. Additionally, environmental factors (water transparency, water movement, temperature, sediment deposition, SST, and chlorophyll concentration) were assessed to test the hypothesis that environmental conditions which are potentially harmful for corals can enhance sponge bioerosion. Isabel Island and Punta de Mita showed a similar species richness (13 and 11 species, respectively) but boring sponge presence in both live and dead corals was higher at Isabel Island (57.6%) than at Punta de Mita (35.7%). The same result was obtained when each type of substrate was analysed separately: dead coral reef matrix (81.3% versus 55.5%), coral rubble (47.7% versus 20.0%) and living corals (43.7% versus 31.7%). A principal components analysis showed a higher environmental heterogeneity at Punta de Mita, as well as important environmental differences between Punta de Mita and Isabel Island, due to sediment deposition (2.0 versus 0.2 kg·m^?2·d^?1) and water movement (24.5% versus 20.5% plaster dissolution day^?1), that were also negatively correlated with boring sponge presence (r = ?0.7). By analysing the boring sponge assemblage, we found that environmental settings, together with habitat availability (i.e., dead coral substrate) differentiated assemblage structure at both localities. Major structural differences were largely due to species such as Cliona vermifera, Cliona tropicalis and Aka cryptica. In conclusion, factors such as habitat availability favored the presence of boring sponges but some environmental factors such as abrasion resulting from moving sediment acted restrictively, and exerted a major role in structuring boring sponge assemblages in the Mexican Pacific.     

Submerged notches and doline sediments as evidence for Holocene subsidence

The possibility of Holocene subsidence along the northern coast of the Corinth Gulf is often mentioned in the literature; however, systematic detailed evidence that submergence (e.g. of archaeological remains) does not simply depend from eustatic sea-level rise is most often missing. In this paper, a new detailed study of submerged tidal-notch profiles along the limestone coast has shown that periods of sea-level stability are intercalated with periods of rapid subsidence or gradual relative sea-level rise. It appears that most of the sites considered, seem to have been affected by a relatively recent co-seismic subsidence of about half a meter, whereas during the longer period, by stages of relative sea-level stability and/or gradual relative sea-level rise. This evidence of subsidence is confirmed by radiocarbon dating in doline sediments, suggesting that during certain periods, a relative sea-level rise was much faster than the raising suggested by glacio-eustatic or hydro-isostatic estimations. Juxtaposing a list of known earthquakes occurred in the area shows that several earthquakes (e.g. the 1981 one for the easternmost sites considered) are potential candidates for the recent co-seismic displacements and thus supporting the geomorphological interpretations.