Antofagasta Bay Research Articles

Spatial and temporal changes of subtidal benthic communities in Antofagasta Bay (SE Pacific) stressed by permanent human disturbances

Human activities have impacted the coastal area of Antofagasta Bay for decades, particularly miners’ activities (loading of mineral concentrates of Cu, Zn, Pb, and Mg), port operations, tourism, desalination plants, artisanal fishing, and sewage discharge. Subtidal coastal benthic communities were assessed in 7 sectors with different magnitudes of anthropogenic disturbances between the years 2008 to 2011 at Antofagasta Bay. Transennella pannosa, Incatella cingulata, Crassilabrum crassilabrum, and Leukoma thaca dominated the communities in all years of study. Based on the type of feeding behavior, the highest biomass was evidenced for the suspensivore group in all years of study, followed by the carnivore species. The highest average richness and average Shannon diversity were observed in sectors with medium and high levels of disturbances. In contrast, the highest average species dominance was registered in a sector with low disturbances. Community structure differed significantly between the seven sectors over the four years. The analysis of classification and ordination showed the conformation of three groups of sectors based on their dissimilarities. The differences were best explained by the sectors with low and high disturbances and by the presence of bivalves, ascidians, gastropods, and algae. It was demonstrated that the reduction or absence of fishing pressure could explain species’ high richness and diversity. Likewise, the sectors under low disturbance pressure showed the highest biomass and dominance of species. The information obtained in the current work could be used to design management strategies for conservation, monitoring, and restoration programs.

Spatial Distribution of Cyclograpsus cinereus Dana 1851 on the Rocky Shores of Antofagasta (23°27′ S, Chile)

The decapod fauna in the intertidal zone of the rocky shores of Chile is highly diverse, especially along the northern and central mainland coasts, where the influence of the cold Humboldt Current results in high productivity. One of the most abundant species in these ecosystems is the decapod Cyclograpsus cinereus Dana, 1851. The aim of the present study, carried out in the spring and summer seasons between 2018 and 2020, was to determine the spatial distribution patterns of the decapod C. cinereus in different sites along the rocky shores of Antofagasta bay, northern Chile, in order to establish probabilistic models that explain its distribution at each site. Individuals were counted in random quadrants in the intertidal zone. The data thus obtained were processed by application of the variance/mean ratio to determine whether the distribution of individuals was random, aggregated or uniform, associated with Poisson, negative binomial or positive binomial distributions, respectively. The data revealed aggregated (negative binomial) distribution in 15 sampling events, and uniform (positive binomial) distribution in 4 events. The sampling sites were located on rocky shores in four sectors of an urban zone, and two in a protected zone; no significant differences were found between the densities of the sites in the two zones. The results of the interpretative probabilistic models indicated aggregated distribution patterns, agreeing with previously reported interpretative probabilistic models for the distribution of decapods on the rocky shores of central and southern Chile.

Potential mixotrophic relations of copepods and dinoflagellates and their associations with spectral properties of the water bodies in Antofagasta Bay (23°S, Chile), investigated with the aid of remote sensing

Abstract Antofagasta Bay is characterized by having a high primary productivity due to the presence of the cold Humboldt Stream. However, due to the current global climate change, also the previously generated biodiversity patterns are changing. The aim of the present study was analysing Antofagasta Bay in order to determine the existence of correlations between optical, i.e., spectral, properties recorded through remote sensing on the one hand, and the presence of dinoflagellates and zooplankton, on the other. The studies were done during two periods, i.e., in February 2019 and February 2020. The results revealed the existence of three main groups of sites: a first group comprising sites sampled in February 2019, with high Band 3 and Band 4 reflectance, high chlorophyl values, and high abundances of Dinophysis and copepods; a second group combining four sites visited in February 2019, with high Gymnodinium and Ceratium, high reflectance in Band 1, Band 2, Band 5, Band 6 and Band 7, and low zooplankton abundance; and finally a third group encompassing all sites sampled in 2020, with high Prorocentrum abundance, and low zooplankton abundance, as well as low reflectance. A similar situation with regard to zooplankton and optical properties was earlier reported for Chilean Patagonian lakes.

Relations between optical properties, dinoflagellates and zooplanktonic crustaceans in Antofagasta Bay (23°S, Chile)

Abstract Antofagasta Bay is characterized by a high primary productivity due to the presence of the cold Humboldt Stream that is associated with a high diversity in flora and fauna in the benthic and pelagial environments of the Chilean west coast. Nevertheless, due to the global climate changes, the existing biodiversity patterns change as well. The aim of the present study was to analyse Antofagasta Bay for determining the existence of patterns between optical properties of the water, and the phytoplankton and zooplankton. The results show that one site had high chlorophyll concentrations, high reflectance, a high abundance of the dinoflagellate genus Dinophysis, and a high abundance of copepodites, whereas five sites had low chlorophyll concentrations, a low reflectance value, low copepodite abundances, and high abundances of dinoflagellates of the genera Ceratium, Gymnodinium and Prorocentrum. These results are similar to earlier observations for the coastal waters along northern Chile.

Decline of a non-native ecosystem engineer and its replacement with a native on rocky shores: effects on the diversity and structure of benthic communities

Ecosystem engineer species create enhanced habitat resources, and therefore host exceptionally diverse communities. We studied the case of two ecosystem engineers: a non-native ascidian, Pyura praeputialis, and the native mussel, Perumytilus purpuratus, in the rocky shores of Antofagasta Bay, northern Chile. P. praeputialis was once dominant in mid- and low-intertidal zones, but human harvesting has severely reduced the extent of the P. praeputialis band, allowing for competition from P. purpuratus, which subsequently expanded and replaced P. praeputialis. We compared the macrobenthic community structure between these two ecosystem engineers at five sites throughout the bay. Our results suggest not only an important share of species composition between the ecosystem engineers (68.1%) but also dissimilarity due to differential changes in taxa abundance. Taxonomic richness and diversity were always high in both ecosystem engineers when analyzing communities in areas without these engineers. Diversity recorded in P. praeputialis was slightly higher than those in P. purpuratus; however, the re-establishment of the native mussel could prevent a substantial drop in diversity as the invasive bioengineer disappears. From a biogeographical standpoint, all recorded taxa associated with both ecosystem engineers were native, thus allowing us to conclude that P. praeputialis did not further facilitate the presence of invasive species; rather, this bioengineer could be considered a reservoir of native fauna. Our study highlights the importance of the coexistence of multiple ecosystems engineers and their interactions in the maintenance of biodiversity in rocky shores, especially in cases where human disturbance reduces the more dominant but invasive ecosystem engineer.

The Decreasing Availability of Settlement Surfaces Affects the Transition From Larvae to Early Recruitment of the Scallop Argopecten purpuratus Through El Niño and La Niña Episodes

Despite their importance as a larval source for the aquaculture industry in several countries, natural stocks of pectinids have been heavily impacted by a variety of natural and anthropogenic stressors. Here we studied the key transition from larvae to early recruitment using the largest natural population of the scallop Argopecten purpuratus (Lamarck, 1819) in Chile. We assessed whether thermal anomalies associated to El Nino-La Nina (EN-LN) cycle had a signal in the temperature regime at La Rinconada Marine Reserve and the consequences of this variability for the settlement of competent larvae of A. purpuratus to artificial and natural substrates in this area. For this, we gathered historical data (February 1950-December 2017) regarding satellite-derived thermal anomalies in the El Nino 1+2 region and in situ measurements of sea surface and bottom temperature in the study area. Moreover, we sampled the larval abundance of A. purpuratus in the water column and the settled spat on artificial collectors and on red algae Rhodymenia spp. –their natural settlement surface– between April 1996 and January 2000. Thermal anomalies associated to El Nino-La Nina cycle were strongly related to interannual changes in winter and summer temperature in Antofagasta Bay, with a clearer signal in winter than summer temperature. The increased reproductive activity during September to April seemingly foster the observed pulses in the larval abundance of A. purpuratus, as expected in a semi-enclosed system. In contrast to previous studies, the larval abundance did not show clear responses to the local El Nino-La Nina -related shifts in temperature regimes. Larval settlement of A. purpuratus in artificial collectors increased through the study period while that on Rhodymenia spp. showed a contrasting, decreasing trend. At the same time, the abundance of Rhodymenia spp dropped continuously. All these temporal trends and our statistical analysis implied that, whether EN-LN-driven thermal shifts had an effect on the larval settlement of A. purpuratus, it is masked by the strong and continuous reduction in the availability of Rhodymenia spp. as a settlement surface. This reduction may have consequences for the stability of natural stocks of A. purpuratus in Chile.

Emergy evaluation of benthic ecosystems influenced by upwelling in northern Chile: Contributions of the ecosystems to the regional economy

Emergy evaluations of three benthic ecosystem networks found in Mejillones, Antofagasta and Tongoy Bays, located on the coast of northern Chile, were carried out with the intent of documenting the contributions of these coastal ecosystems to the economy. The productivity of these bays is strongly influenced by the Humboldt Current System, as well as by the loss of upwelled flows that occurs during El Nino events. The results of the emergy evaluations were expressed as emdollars (EM$), a combined emergy-money measure that can be used to examine the equity of the emergy exchanges between fishermen and the buyers of the harvested algae and shellfish. In addition, an estimate of the total ecosystem services provided by these coastal ecosystems was made. The emdollar (Em$y−1) and the hypothetical monetary value (US$y−1) of the nitrate nitrogen upwelled constituted the highest inflow of emergy to all three benthic ecosystems. The empower density expressed as Em$m−2y−1 was highest in Mejillones Bay; however, the natural capital (biomass) of the ecological components (EM$m−2) was highest in Antofagasta Bay, where La Rinconada Marine Reserve is located. The relationship between the coastal zone system and the regional economic system was assessed using the emergy benefit after exchange, EBE, which showed that there were net gains to the overall welfare of the sellers in two regions, 3,280,000 Em$ to those in Mejillones Bay, and 34,000,000 Em$ to those in Tongoy Bay, but a net loss of 2,000,000 Em$ to the sellers of algae and shellfish harvested from Antofagasta Bay. By supplying a clearer picture of the equity of trade relationships for individual organisms, fisheries and bays, emergy evaluation can help develop and implement management strategies for the conservation and preservation of coastal ecosystems to ensure that they are sustainable in the future.

Integrative taxonomy of Peniculus, Metapeniculus, and Trifur (Siphonostomatoida: Pennellidae), copepod parasites of marine fishes from Chile: species delimitation analyses using DNA barcoding and morphological evidence

Pennellidae is a family of copepod parasites of widely distributed marine fishes. The pennellid species are usually morphologically differentiated by cephalothorax, neck, trunk, and abdomen shape. These characters, however, show high polymorphism and therefore using only this type of data, delimitation at species level of this genus is difficult. In this study, we explored the genetic distances calculated from sequences of a DNA barcoding marker (COI mt) (678 base pairs). We also explored the genetic distances of 25 Peniculus specimens associated within nine marine fish species, four Metapeniculus specimens associated within one marine fish species, and four Trifur specimens associated within one marine fish species. All specimens were collected in Antofagasta Bay, Chile and were calculated from sequences of a DNA barcoding marker (COI mt) (678 base pairs). The genetic distance among the Peniculus specimens was 0.95% from the different host species, the Metapeniculus specimens distance was 0.44% and the Trifur specimens was 2.25%. Genetic difference between Peniculus and Metapeniculus was 17.86% and Peniculus differ from T. tortuosus by 18.16%. We analysed the barcoding gene fragment using Bayesian Inference (BI) for phylogenetic reconstruction using three outgroups. Based on the phylogenetic analysis an ultrametric tree was built and a general mixed Yule-coalescent (bGMYC) model was conducted for species delimitation. Morphometrics analyses were made with Bayesian statistics. Mean and credibility limit (95%) for each parameter was calculated. Results show that based on morphology the individuals collected can be assigned to P. cf. fistula von Nordmann, 1832, Metapeniculus antofagastensis Castro-Romero & Baeza-Kuroki, 1985, and Trifur cf. tortuosus Wilson, 1917. High morphological polymorphism was observed for the lineage of Peniculus associated to several host species of marine fishes. Similar results were obtained for Trifur cf. tortuosus parasites on Chilean marine fishes.

Organic matter-clay interaction along a seawater column of the Eastern Pacific upwelling system (Antofagasta bay, Chile): Implications for source rock organic matter preservation

Natural Organic Matter (OM) preservation and sedimentary burial mechanisms are still poorly understood despite their importance in oil source rock characterization and carbon cycle issues. It is now recognized that mineral sorption contributes significantly to OM preservation by conferring steric protection and preventing enzymatic access. Most of the studies dealing with OM preservation have focused on settled OM, omitting aggregation in the water column. For the first time, we have tested the complexation of natural OM on 3 types of clays in seawater in-situ conditions. Samples of Na exchanged montmorillonite (Na MMt), synthetic montmorillonite (S Na MMt) and natural kaolinite (Kaol) were immersed in an upwelling zone (Antofagasta bay, Chile). Three depths were studied: 15m (photic zone), 40m (oxygen transition zone) and 80m (anoxic zone). The clayey samples were kept immersed for different durations and analyzed after removal, via GC/MS, Py-GC/MS, flash elementary pyrolysis, DOC analyzer and XRD. The main results are: (1) for the shortest immersion times, DOC analyzes performed on clays' proximal water indicated excessive DOC concentrations compared to seawater values. This attraction effect was however temporary, as values tended to rebalance with seawater concentrations for the longest immersion times. (2) Molecular analyses performed on seawater organic extracts indicated that fatty acids, alkanes, alkyl benzenes and methoxy-benzenes were the main dissolved lipid groups. (3) These lipids were also the main sorbed compounds identified on immersed clays, with a predominance of fatty acids. This is due to the higher availability and better affinity of fatty acids with the clayey samples. (4) Because of its higher specific area and CEC, Na MMt exhibited the best sorption rates compared to Kaol and SNa MMt. X ray analyses performed on Na MMt indicated d001spacing growths, but these growths are not attributed to OM intercalation. Sorbed OM is probably located on the external surfaces leaf edges of clays.

Hierarchical analysis of the population genetic structure in Concholepas concholepas, a marine mollusk with a long‐lived dispersive larva

AbstractFor most marine invertebrate species, dispersal is achieved mainly or exclusively by pelagic larvae. When the duration of the pelagic larval stage is long, genetic homogeneity over large geographic scales is expected. However, genetic structure has often been reported over small spatial scales, suggesting that more complex processes occur than a simple positive relationship between pelagic larval duration and gene flow. Concholepas concholepas has a larval stage that can last up to 3 months in the water column with a wide distributional range covering from 6°S to 56°S. We used a hierarchical sampling technique to test if the genetic homogeneity of this highly dispersive species is maintained throughout its total geographic range in spite of environmental heterogeneity. In the three studied regions (Antofagasta Bay, Valdivia and Patagonia), a spatial pattern of isolation by distance in conjunction with a spatial genetic structure was observed. Within each region, different spatial genetic patterns were detected. In Antofagasta Bay and Valdivia there was evidence of substantial gene flow among populations, whereas in Patagonia, populations showed genetic structure and a unique, genetically isolated location was identified. These results revealed the existence of spatial differences in the genetic patterns among regions with different coastal topographies in C. concholepas, and give us new insights into the inter‐relationships of larval dispersal potential, actual larval dispersal and physical processes. Regarding the sustainable management of C. concholepas, two important issues are derived from this study: (i) to highlight the need for a regional context in the management of C. concholepas, (ii) to determine the distinctiveness of the most austral population and to focus on the conservation efforts due to the relevance of this area.

Mass balanced trophic models and short-term dynamical simulations for benthic ecological systems of Mejillones and Antofagasta bays (SE Pacific): Comparative network structure and assessment of human impacts

Mass-balanced and dynamical multispecies trophic models were constructed for benthic systems of Mejillones and Antofagasta bays. We analysed data on biomass, P/B ratios, catches, food spectra, and the consumption and dynamics of commercial and non-commercial species or functional groups using the Ecopath with Ecosim framework. The biomass of the Rhodophyta group, which contributed 25.5% of the total system biomass, was the most abundant compartment in Mejillones bay, whilst the bivalve Transenella pannosa recorded the highest biomass (∼39%) in Antofagasta bay. Amongst the carnivores, the commercial gastropod Tais chocolata (∼2%) and the sea star Luidia magallanica (∼4%) were the compartment containing the highest biomass in Mejillones Bay and Antofagasta Bay, respectively. Based on the magnitudes of Pp/R, Relative Ascendency (A/C), Relative Overhead (Ov/C), Redundancy, Finn's cycling index and the system recovery time (SRT), the benthic system of Antofagasta Bay was more developed or mature and, in turn, less resistant against perturbations compared to Mejillones Bay. The outcomes of the mixed trophic impacts (MTI) indicate that the small epifauna carnivore (SEC) and the phytoplankton propagate the highest magnitudes of direct and indirect effects on the remaining compartments in the Mejillones and Antofagasta bays, respectively. According to the Ecosim short-term simulations (increasing 10, 30 and 50% the mortality by fishing and/or toxic substances) the snail Tegula spp. had the highest impact on the other compartments in Mejillones Bay, whilst the bivalve T. pannosa exhibited the highest impacts in Antofagasta Bay. Based on our results, we suggest that environmental studies should not only aim to promote and ensure the quality of physical components of nature, but should also consider the ecosystem properties and dynamics that emerge from complex networks.

Identifying keystone trophic groups in benthic ecosystems: Implications for fisheries management

Many species inhabiting the benthic marine ecosystems of the central and northern Chilean coast have been intensively harvested and this exploitation has increased considerably in recent years. Despite this harvest pressure, few studies have attempted to establish a more holistic, systems-based management plan. On the contrary, research continues to rely on population models in which the species of interest are isolated from their ecological context. This work offers several keystone indices in order to help multispecies fisheries management. The indices used are: (1) functional indices based on steady-state and dynamic trophic models; (2) structural indices based on bottom-up and top-down control mechanisms; and (3) qualitative keystone species indices using loop models (mixed control). The quantitative trophic models were constructed using Ecopath with Ecosim (EwE; v. 5.0) software, and the qualitative model was analysed using Loop Analysis. All models describe the interactions of the most representative species and functional groups inhabiting the benthic ecosystems of Tongoy Bay, La Rinconada Marine Reserve (Antofagasta Bay), and the kelp forest of Mejillones Peninsula (Antofagasta). Even though our results only represent the short-term dynamics of these systems, we have found keystoneness properties of several species and functional groups, including primary producers, herbivores, and top predators. Despite this wide variability of groups, we detected a different core set of species or functional groups, each of which contained prey–predator and plant–herbivore relationships. Because the traditional keystone concept of a single species is difficult to apply, we suggest shifting away from this view towards a more holistic alternative such as that of a keystone species complex. This kind of approach would facilitate the design and assessment of sustainable management strategies for ecological marine ecosystems. Despite the ecological relevance of our results, further experimental studies and modelling using other theoretical frameworks should be performed.

Trophic mass balanced models and dynamic simulations of benthic communities from La Rinconada Marine Reserve off northern Chile: network properties and multispecies harvest scenario assessments

Abstract Mass balanced trophic models and dynamic simulations of two benthic ecological systems from La Rinconada Marine Reserve (Antofagasta Bay, SE Pacific) were constructed. The scallop Argopecten purpuratus is the most important benthic resource in La Rinconada Marine Reserve, followed by the carnivorous snail Thais chocolata, and the filter‐feeder bivalves Tagelus dombeii and Transennella pannosa. Information on biomass, P/B ratios, catches, food spectrum, consumption, and dynamics of commercial and non‐commercial species was obtained and examined using Ecopath with an Ecosim software package. The bivalves A. purpuratus and T. dombeii represented the most abundant compartments in the studied subsystems. Of the carnivores, the snail T. chocolata was dominant, followed by the crabs Cancer spp. and the functional group of large epifauna. The two subsystems presented similar values of system throughput. The mean trophic level of their fisheries also reached similar magnitudes (2.0), showing that the harvests in each system concentrated on secondary producers. Likewise, both subsystems presented similar A/C ratios (29.9 and 30.3), suggesting that they were immature. The results obtained using mixed trophic impact (MTI) and Ecosim (increasing the fishing mortality Fi by four times) showed that only four species propagated the highest direct and indirect effects. Coincidentally, these species are the most economically important and the changes produced by the scallop A. purpuratus are noteworthy. With regard to the system recovery time (SRT) estimates, only three species or functional groups presented the highest magnitudes, from highest to lowest: the sea star Luidia magallanica, the scallop A. purpuratus, and the crabs Cancer spp. The topological keystone indexes of Jordán and Libralato had divergent results. According to Jordán's index, the keystone species were L. magallanica, Cancer spp., and detritus; whereas Libralato's index showed phytoplankton to be the keystone species. Based on the results obtained, it is concluded that trophic mass balanced models and simulated management scenarios have considerable value for planning interventions and manipulations or for planning more sustainable management strategies in La Rinconada Marine Reserve. Copyright © 2009 John Wiley & Sons, Ltd.

Effects of water depth on survival and growth of Argopecten purpuratus (Lamarck, 1819) spat in northern Chile

Collection and suspended culture of Argopecten purpuratus spat in Japanese-type collectors was undertaken in the Rinconada Marine Reserve (Antofagasta, Chile) to determine growth variations between surface and bottom waters. Scallop spat was collected at 16-m depth and grown at 1- and 16-m depths. An initial settlement of ca. 13,000 post-larvae per bottom collector (2 cohorts) was observed on 3 February 2001. Two new cohorts settled in bottom collectors on 3 March, while no spat settlement occurred in surface collectors. The four cohorts exhibited substantial and different mortality rates over the study period, depending on the cohort and on-growing depth. The first and second cohorts exhibited mortality rates of 80.3 and 53.1% in bottom collectors, respectively, versus 29 and 43.6% in surface structures. The third and fourth cohorts exhibited mortality rates of 21.4 and 59.7% in bottom collectors, respectively. Mean shell height of scallop spat after 88 days of culture was 6.46 mm for the first cohort and 4.37 mm for the second cohort in bottom collectors, versus 10.33 and 7.31 mm in surface collectors, respectively. Mean post-larval growth rate in bottom collectors (86 μm day−1) was significantly lower than growth rate in surface collectors (146 μm day−1). Factors explaining the depth-related mortality and growth rates are discussed to improve scallop culture in the Reserve. It was concluded from the results that culture improvement in Antofagasta Bay would require collecting spat in bottom waters and raising it near the surface during initial cultivation stages.

First record of Cymatium keenae Beu, 1970 (Mollusca: Ranellidae) from Antofagasta Bay, northern Chile, in connection with El Niño events

Cymatium (Monoplex) keenae Beu, 1970, the neopolitan triton, is a Ranellidae predator distributed in tropical and subtropical waters. In this study we report this species for the first time in Antofagasta Bay, northern Chile. Adult specimens have been found since June 2003 at depths ranging from 5 to 15 m in La Rinconada Marine Reserve (23°28′18″S, 70°30′46″W) in Antofagasta Bay, near a scallop farming facility. The present findings clearly extend the geographic range of C. keenae, which previously had been reported from La Paz, Gulf of California (Mexico) to Galápagos Islands (Ecuador) in the SE Pacific. Previous evidence strongly suggests that the intrusion of this tropical immigrant can be related to warm El Niño episodes. We analyze the key role of the oceanographic characteristics of Antofagasta Bay as potential temporal refuge for this species.

Artificial collection and early growth of spat of the scallop <i>Argopecten purpuratus</i> (Lamarck, 1819), in La Rinconada Marine Reserve, Antofagasta, Chile

Artificial collection of early juveniles (“spat”) of the scallop Argopecten purpuratus in Japanese-type collectors was evaluated between January 2001 and July 2002 in the La Rinconada Marine Reserve, Antofagasta, Chile. This area of Antofagasta Bay has in the past been noted for the retention of scallop larvae by local gyres, in which their numbers can vary between 89 and 34175 larvae m -3 , producing larval sets of 400 to 15340 post-larvae (spat) per collector. The results showed no quantitative relationship between larval abundance in the water and the spat density collected per day in the collectors, although high settlement rates were associated with high numbers of umboned larvae in the water. Allowing collectors to remain in situ for extended periods of 88 and 159 days resulted in a severe loss of seed which had settled in the collectors during the first 28 to 40 days of immersion. These losses varied between 50.9 and 99.6% of the spat collected, and were more prejudicial for the smaller cohorts that had settled in the collectors at the end of the first immersion period. The growth rates measured among different cohorts for each immersion period varied between 81.3 and 235.2 mm/day for the first cohort (C 1 ) and between 64.0 and 167.4 mm/day for the second cohort (C 2 ). The highest growth rates occurred in collectors containing the lowest numbers of spat after the occurrence of spat losses during the long periods of immersion. Occurrence of intraspecific competition within the collectors is discussed as potentially responsible for the decreases in spat numbers and the variations observed in their growth rates.

Nearshore surface temperatures in Antofagasta Bay (Chile) and adjacent upwelling centers

Several years of sea surface temperature data (1997–2003) for the coast of Antofagasta Bay—a semi-enclosed bay in northern Chile—were analyzed in order to f...

Marine ecosystem engineering by the alien ascidian Pyura praeputialis on a mid-intertidal rocky shore

Engineer species transform ecosystems due to their own growth, constitute an integral part of altered environments, and provide new habitats for other species, thus affecting biodiversity and the ecosystem. On rocky shores inside Antofagasta Bay (Northern Chile), the alien ascidian Pyura praeputialis, an engineer species, creates broad belts and dense 3-dimensional matrices that modify the intertidal habitat structure. In all, 116 species of macro-invertebrates and algae inhabit this habitat, compared with the 66 species inhabiting adjacent intertidal rocky shores which lack P. praeputialis. Of the 145 species recorded at the seascape scale (encompassing both mid-intertidal habitat), 55% were found exclusively in intertidal P. praeputialis matrices. Along the coastal gradi- ent, patterns in β-diversity emerge due to the addition of a new set of species to the community inhab- iting the P. praeputialis matrices and, to a lesser extent, from spatial turnover. We found differences in the shape of the species frequency distribution between the communities inhabiting the engi- neered and non-engineered mid-intertidal habitats. However, within the same habitat type, there was no difference in the species frequency distribution between functional groups. Occurrence of macro-algae was not affected by habitat type, but occurence of macro-invertebrates increased sig- nificantly in P. praeputialis matrices. P. praeputialis increases species richness at local and seascape scales by providing a novel mid-intertidal habitat which is used by mobile and vagile macro-inverte- brates that otherwise would remain excluded from this intertidal level.

AN ALLOMETRIC TRIDIMENSIONAL MODEL OF SELF-THINNING FOR A GREGARIOUS TUNICATE

A tridimensional allometric model of self-thinning was tested in a tunicate population of Pyura praeputialis in the Antofagasta Bay, northern Chile. The theory tested follows the bidimensional allometric theory of Osawa and Allen for the self-thinning rule proposed for plants, except that in addition to mean individual mass we included three new concepts: the number of layers (an equivalent of the leaf area index, [LAI] used in plants), the effective unit area, and the density per unit effective area. We assumed constant tunicate mass per effective unit area and allometry for mean dimensions of the population. The tridimensional model can be reduced to a bidimensional one if density is expressed as the density per unit effective area, which corresponds to the number of individuals relative to the effective area occupied by individuals when they conform a monolayer. The model allows for random sampling and rules out the selection for samples or stands at maximum crowding. The self-thinning of P. praeputialis through a tridimensional model showed an exponent of −1.518 (95% ci,−1.635 to −1.401) for mean tunicate visceral mass, and an exponent of −1.489 (95% ci, −1.588 to −1.390) for the corrected bidimensional model. The two exponents were identical to those predicted from tridimensional allometric theory. The results of this new approach increase the variance explained in comparison to that of classical bidimensional models.

Coseismic coastal uplift and coralline algae record in Northern Chile: The 1995 Antofagasta earthquake case

Coralline algae that may be predominant in the upper part of the infralittoral zone along rocky shorelines proved to be a useful indicator of rapid coastal uplift. As these encrusting algae cannot survive desiccation, even for short periods at low tide, they can provide estimates of positive vertical motions like those which may accompany seismic events. The desiccation of a fringe of the algal encrustment, at the base of the intertidal area (=infralittoral fringe), combined with effects of solar radiation rapidly kills the organisms. This mortality results in a conspicuous alteration of the pigmentation from pink/beige/reddish to white. After the July 30, 1995 Antofagasta earthquake (M w 8.1), in northern Chile, such a white fringe appeared in some parts of the bay of Antofagasta and surroundings. The width of the dead algae fringe varied from 0 to more than 1.8 m. The widest observed widths are related to local parameters (exposition to wave splash, geometric disposition) that account for an amplification of the width of the dead algae fringe, and must be identified. Thus, a careful study of each locality led us to determine the extent of the coastal areas that had been uplifted, and to reconstruct, with a precision of the order of 2 cm, the amount of the vertical deformation along the Antofagasta Bay and southern Mejillones Peninsula. It was thus shown that the coastline bordering the town of Antofagasta suffered practically no coseismic uplift, while areas to the south and to the west of Antofagasta Bay proved to have been uplifted by as much as 25 and 40 cm, respectively. The maximum uplift (80 cm) was seen at the southwestern tip of the peninsula of Mejillones. These precise reconstructions are of great help for the calibration of geodetic studies performed independently and for the modelling of the coseismic deformation at a regional scale.