the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.mortalities, extinction-recolonisation processes, and variations in population connectivity are presumablyof critical significance, but they are just beginning to be explored (e.g., see Martínez et al.2003; see also Population connectivity on p. 252ff. and Biogeography, p. 255ff.); and (5) interannualvariations related to EN and LN may be strongly related to both small-scale processes such as theMadden-Julian Oscillation (Madden & Julian 1971) and large-scale processes such as the PacificDecadal Oscillation (PDO; e.g., Trenberth & Hurrel 1994, Zhang et al. 1997) or the AntarcticOscillation (e.g., Gong & Wang 1999), with possible biological implications for benthic communitydynamics that are virtually unknown.Up to now, the ecological knowledge of EN impacts on the marine communities from northernChile continues to be mainly descriptive, focused on a reduced number of species and places.Nonetheless, prior studies have shed some light on the wide biological scope and geographicalextent of such impacts and the need for comparative, multiscale and long-term approaches to obtainmeaningful results.Physiological adaptations of marine invertebratesPhysiological variation is the result of genetic, developmental and/or environmental influences(Spicer & Gaston 1999). Thus, physiological diversity and adaptations are linked to environmentalcharacteristics and variability. The understanding of how living organisms function (i.e., theirphysiology) is aided by comparing the way different animals deal with environmental constraints(Schmidt-Nielsen 1997).Major environmental factors affecting the animal’s physiology are temperature, oxygen andenergy (food) availability. The HCS in northern-central Chile is an interesting scenario for runningphysiological studies due to changing environmental characteristics, such as (1) the occurrence ofa latitudinal temperature gradient, (2) extended zones with permanent and/or seasonal upwelling(cold seawater temperature and low oxygen content), (3) some closed bays with relatively hightemperatures (e.g., Antofagasta Bay) compared with the surrounding areas and (4) the occurrenceof thermal anomalies like ENSO. The HCS is characterised also by the occurrence of oxygenminimumwaters, where physiological adaptations of organisms should be expected, even of speciesfrom shallower (10–50 m) waters, which may occasionally be confronted with low oxygen concentrations(when there is upwelling of oxygen-deficient waters). Surprisingly few studies areavailable on physiological adaptations to hypoxic conditions of benthic organisms from the HCS.One of these studies was the characterisation of the pyruvate oxidoreductase enzymes involved inthe biochemical adaptation to low oxygen conditions in nine benthic polychaetes from the HCS(González & Quiñones 2000). Pyruvate oxidoreductase enzymes permit the metabolism to produceadenosine triphosphate (ATP) at high rates under environmental or physiological hypoxic conditions(Livingstone 1983). Interestingly, these enzymes were found to be more numerous and with differentpyruvate consumption rates in the most abundant and worldwide distributed polychaete species(Paraprionospio pinnata) (González & Quiñones 2000). Another study of biochemical adaptationsto hypoxic conditions in the HCS was done on two key species (in terms of trophodynamics andabundance) of this system, the euphausiid Euphausia mucronata and the copepod Calanus chilensis(González & Quiñones 2002). The enzyme lactate dehydrogenase (LDH, a key enzyme of theanaerobic pathway) from Euphausia mucronata was two orders of magnitude higher than that ofCalanus chilensis. Higher activities of the LDH indicate higher anaerobic capacities, and this mayenable Euphausia mucronata to conduct daily vertical migration through the oxygen-minimumlayer (see also Zooplankton consumers, p. 214ff). In contrast, low LDH activities restrict Calanuschilensis to oxygenated waters (González & Quiñones 2002). The importance of the interactionbetween oxygen and temperature has been explored in recent studies of the brooding behaviour ofdecapod crabs (Baeza & Fernández 2002, Fernández et al. 2006b).262
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILEClosed bays with relatively high temperatures and productivity in the HCS offer suitableconditions for the permanence and reproduction of the scallop Argopecten purpuratus, a speciesmore characteristic of warm waters. An increment of 2.5°C in bottom temperatures (normally15.5°C) during EN 1982–1983 in Tongoy Bay (30°S) augmented dramatically gonad mass andspawning, and as a consequence spat (juvenile) collection exceeded levels from previous years by300% (Illanes et al. 1985). However, total gonadal levels of lipids and proteins increased markedlyin A. purpuratus conditioned for reproduction at 16°C, but these increases were less pronouncedat 20°C (Martínez et al. 2000). Moreover, during gonad maturation muscle carbohydrate levelsdropped considerably, as well as the activity of a pyruvate oxidoreductase, the enzyme octopinedehydrogenase (Martínez et al. 2000). Muscle carbohydrate (i.e., glycogen) and glycolytic enzymeshave been shown to decrease greatly in other scallop species such as Chlamys islandica and Euvolaziczac (Brokordt et al. 2000a,b). This leads to a decrease in muscle metabolic capacity and thus inescape capacities, which is facilitated by muscle contractions. A reduction of escape capacitiesduring reproduction has been observed in Argopecten purpuratus as well as in Chlamys islandicaand Euvola ziczac (Brokordt et al. 2000a,b, 2006).In the intertidal and shallow subtidal zones of the HCS, temperature is the main variablechanging over various spatial and temporal scales, with unpredictable interannual patterns. Undernormal conditions, physical environmental conditions are relatively stable in the shallow subtidalbetween 18°S and 35°S (HCS), where salinity typically ranges between 34 and 35 and temperaturemay vary from 12°C to 22°C. However, due to terrestrial influence, the temperature conditions inthe intertidal are different along this latitudinal gradient. For example, the range of mean temperaturesregistered in high intertidal pools during the summer is ~13–33°C in Antofagasta (23°S),~13–30°C in Carrizal Bajo (28°S), and ~11–25°C in Las Cruces (33°S) (Pulgar et al. 2006). DuringEN, these differences in thermal conditions may be enhanced.To evaluate phenotypic plasticity or evolutionary responses of organisms to different habitattemperatures, comparative studies have typically focused on species distributed along latitudinalgradients (Vernberg 1962, Graves & Somero 1982, Stillman & Somero 2000, Pulgar et al. 2006).However, local thermal gradients (TGRs) can be formed by fine-scale variation in, for example,the marine intertidal vertical zones. The intertidal zone is characterised by important spatial andtemporal gradients of temperatures, which may be equivalent to those found over a large latitudinalgradient. Intertidal organisms have evolved physiological tolerance adaptations that are importantin determining the upper vertical distribution of the species. Studies of congeners or conspecificsallow adaptive variation to be clearly demarcated, independent of effects of phylogeny (Stillman& Somero 2000). Crabs of the genus Petrolisthes (Anomura: Porcellanidae) are widely distributednot only along the intertidal zone of the HCS, but also worldwide, covering huge latitudinalgradients. One of the few studies of physiological adaptations of marine invertebrates in the HSC(Las Cruces, 33°S) was done in five species of the genus Petrolisthes (P. granulosus, P. laevigatus,P. violaceus, P. tuberculatus and P. tuberculosus) (Stillman & Somero 2000). Each species is foundat different vertical levels, from the low (P. tuberculosus), mid-low (P. tuberculatus), middle(P. violaceus), mid-high (P. laevigatus) to the high (P. granulosus) intertidal. The limits of thermaltolerance (LT 50 ) were strongly correlated with the vertical position of the species in the intertidalzone (y = 36.02 − 1.88x, r 2 = 0.97) and with the maximal habitat temperature (Table 5) (Stillman &Somero 2000). Thus, species have adapted their upper thermal tolerance limits to coincide withmicrohabitat conditions. Interestingly, mid-high and high intertidal species (P. laevigatus andP. granulosus, respectively) live near their limits of thermal tolerance. While these LT 50 values offersome hints, it may be extremely interesting to explore at which temperatures these organisms entersuboptimal ranges, that is, where they may be able to survive but where growth and reproductionmay be compromised. Petrolisthes laevigatus from southern-central Chile dramatically reducesoxygen consumption between 18°C and 20°C (maximal average temperature range found in its263
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