the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.Temperature toleranceThermal adaptations of the life-history stages, mainly for growth, reproduction and survival requirements,are key factors explaining the geographic distribution of benthic algae. In general, two majoraspects characterise temperature demands of macroalgae distributed along the HCS: (1) there is aclose correlation between the upper survival temperature of gametophytes and the northern distributionlimits of the species and (2) algae occurring between 18°S and 40°S show higher temperaturetolerance (18–28°C) than algae south of 40°S (17–23°C), coinciding with the latitudinal influenceof the HCS and the boundaries of the two traditionally recognised biogeographic regions (Chilean-Peruvian and Magellan provinces) (Santelices 1980, Wiencke & tom Dieck 1990, Peters & Breeman1993, Santelices & Marquet 1998, Martínez 1999). For example, brown algae with Antarctic/subantarctic distribution commonly found between 18°S and 40°S (e.g., Adenocystis utricularisand Scytothamnus fasciculatus) have maximal survival temperatures close to 18°C and 24°C(Wiencke & tom Dieck 1990). These lethal values are above the mean monthly temperaturesmeasured around 18°S (14–19.5°C) and 37°S (11.7–15.5°C) (Gorshkov 1985). However, when theupper limit for gametogenesis is regarded, these species do not reproduce at temperatures >13–15°C(Peters & Breeman 1993). Such thermal requirements suggest that they may survive moderate ENepisodes, but probably a recovery through sexual reproduction may become limited. In the case ofspecies restricted to the HCS (e.g., the red alga Chondracanthus chamissoi), the growth of bothgametophytes and sporophytes increases at temperatures of 25°C, which is 5–6°C higher than watertemperature in northern Chile (Bulboa & Macchiavello 2001). This is a known phenomenon inmacroalgae and may reflect a reasonable safety limit to survive unpredictable increases of temperaturefor a long time (e.g., months during EN) or may represent a potential for shifting thedistribution boundaries northward. It must be emphasised that most of the macroalgae from northern-centralChile studied so far show a capability of growth at very low temperatures (>10°C),indicating clearly an adaptation to the cooling effect of the HCS and reflecting their subantarcticaffinities (Wiencke & tom Dieck 1990, Peters & Breeman 1993, Santelices & Marquet 1998).Physiological and morphofunctional adaptationsIn general, there are only a few ecophysiological studies addressing adaptations of the life historyof macroalgae to varying light and nutrient conditions and they have normally been restricted tothe genera of commercial importance (e.g., Lessonia, Gracilaria, Gelidium, Mazzaella and Porphyra)(Oliger & Santelices 1981, Hoffmann & Santelices 1982, Hoffmann et al. 1984, Correaet al. 1985, Hannach & Santelices 1985, Avila et al. 1986, Bulboa & Macchiavello 2001, Vélizet al. 2006). Although physiological performances (measured as growth or photosynthesis) arecomparable to those of species from other biogeographical regions, much of the existing informationis site-specific and has been gathered from individual species, indicating adaptations to narrowranges of environmental variability. However, in genera such as Gracilaria and Porphyra, whichare exposed to highly changing environmental conditions in enclosed bays, estuaries or upper littoralzones, broader ranges of environmental tolerance may be expected (Gómez et al. 2004, 2005a).Due to its physical configuration, the coast along the HCS is characterised by high energy, andhence physical perturbations such as wave action or sand erosion/accretion fluctuations are importantand often govern the population dynamics of various infralittoral algae such as Lessonia,Mazzaella and Gymnogongrus. Thus, algae have developed a suite of morphofunctional adaptationssuch as alternation of crustose and erect morphs, large size and seasonal regulation of abundance(Santelices et al. 1980, Jara & Moreno 1984, Santelices & Ojeda 1984, Santelices & Norambuena1987, Gómez & Westermeier 1991, Westermeier et al. 1994, Vega & Meneses 2001). In many cases,these adaptive strategies operate in the early development of the life cycle and in both isomorphic276
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEand heteromorphic phase expressions. Processes such as coalescence of spores (Santelices et al.1999, 2003), regrowth from crusts or vegetative propagation (Hannach & Santelices 1985, Gómez& Westermeier 1991, Macchiavello et al. 2003), selective mortality of early developmental stages(Martínez & Santelices 1998), differential phase ratios (Vega & Meneses 2001) and synchronisationof spore release (Edding et al. 1993, Tala et al. 2004) have been described in algae from northerncentralChile, which are related to potential selection under changing environmental conditions.While the seasonal and latitudinal gradients in environmental conditions along the HCS arerecognisable, the magnitude of their impact on the physiological and reproductive biogeographyof benthic algae remains diffuse. Therefore, comparative studies focused on assemblages fromdifferent sites along the HCS are needed in order to define, for example, the environmentalthresholds involved in reproductive fitness, physiological adaptation and recovery capacity.A special case: enhanced solar radiationOzone depletion (with the concomitant increase of UV-B radiation) over the Antarctic region, whichin spring can reach areas as far north as 36°S, has opened the debate about its consequences onthe marine biota of cold and temperate regions (Madronich et al. 1995, Sobolev 2000). Shortwavelengths (UV-B) affect photosynthesis in different ways and have detrimental effects on DNAand other key cell components (Bischof et al. 2006). Recent studies indicate a potential impact ofcurrent solar UV radiation on photosynthesis of intertidal macroalgae from the southern limit ofHCS (39°S; Gómez et al. 2004, Huovinen et al. 2006). In northern Chile (30°S), zoospores, gametophytesand embryonic sporophytes of subtidal Lessonia trabeculata and the intertidal L. nigrescensshow elevated sensitivity to UV exposure, leading to high spore mortalities and decreases ingermination under current UV doses (Véliz et al. 2006). In general, UV sensibility of early stagescorrelates with the depth-distribution patterns of the parental sporophytes, suggesting that this factormay play a relevant role in depth zonation of benthic algae in this region (Gómez et al. 2005b).Recent surveys indicate that intertidal species display various photoprotective mechanisms, inparticular the ‘dynamic photoinhibition’, regarded as a down-regulation of the photosyntheticapparatus to quench the impact of excess energy (Gómez et al. 2004). Some intertidal algae alsohave noticeably high contents of UV-absorbing substances (e.g., mycosporine-like amino acids)(Huovinen et al. 2004). Whether algae from different latitudes exhibit a differential susceptibilityto UV radiation in the context of the HCS remains unclear and future work should give new insightsinto the ecophysiological strategies of macroalgal assemblages in scenarios of climate change.A brief history of exploitation of natural resources in the HCSFirst coastal settlersEvidence of coastline occupation off Peru and Chile may date from as early as 11,000 yr ago(Llagostera 1979, Muñoz 1982, Báez et al. 1994, Keefer et al. 1998, Sandweiss et al. 1998, Méndez2002, Méndez & Jackson 2004, Santoro et al. 2005). One of the first studies of the earliest humanhabitation along the coast of northern-central Chile (Arica to Coquimbo) was carried out by JuniusBird in 1941 during excavations of shell middens. This author (Bird 1943, 1946) showed that theearliest evidence of human settlements on this coast went back to ~6000 yr before present (BP)and people used highly specialised artifacts with an efficient maritime adaptation. According toSantoro et al. (2005) evidence of the earliest inhabitants (~11,000 yr BP) is difficult to find sincemost prehistoric sites are now on drowned landscapes. However, there is clear evidence that coastalpopulations in northern-central Chile used marine natural resources during the Holocene (Llagostera1979). It is thought that early fishermen dived in subtidal waters to collect molluscs and they also277
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