the humboldt current system of northern and central chile - figema

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MARTIN THIEL ET AL.whiplash effect of fronds and stipes pushes herbivores away from the plants, reducing grazingpressure. On the other hand, spawning of egg cases of elasmobranchs on L. trabeculata ties thestipes together, thereby reducing the whiplash effect and thus permitting grazers to approach kelpplants. Additionally, this ‘tie effect’ modifies plant shape toward the tree-like morph, and plantsare more easily dislodged by water movement (Vásquez 1992).Longevity of kelps from northern Chile in the field is not well known since they do not showany evident age-related structure. Nevertheless, individual Lessonia plants can survive in the fieldfor as long as 5 yr (J.A. Vásquez personal observations), and Macrocystis integrifolia has beenreported as a perennial species in northern Chile (Buschmann et al. 2004b). Several factors generatesignificant biomass loss in the field: grazing pressure, wave impact, and spore release, which takesplace mainly during summer (Santelices & Ojeda 1984, Edding et al. 1994).Lessonia and Macrocystis populations in northern-central Chile grow throughout the year butexhibit growth peaks during spring–summer (Buschmann et al. 2004b, Tala et al. 2004). Growthpatterns are modified by wave impact, quantity and quality of light, water temperature and nutrientconcentration (Buschmann et al. 2004b, Vega 2005). Local factors such as intraspecific interactions(Santelices & Ojeda 1984), herbivory (Vásquez & Buschmann 1997, Vásquez et al. 2006) andcoastal upwelling events (González et al. 1998, Vásquez et al. 1998) can modify seasonal patternsof abundance and distribution (see also Graham et al. 2007). Large-scale phenomena such as ENSOproduce interannual variability in abundance and could eventually generate local extinctions, asobserved after the EN events of 1982–1983 and 1997–1998 (Soto 1985, Tomicic 1985, Vega 2005,Vásquez et al. 2006). Major impacts of EN were observed in kelp beds from lower latitudes(18–21°S). For example, a kelp bed occupying an area of ~40 ha at 18°S during the 1970s (IFOP1977) disappeared as a consequence of EN 1982–1983 (Soto 1985) and has not recovered since.Similarly, during EN 1997–1998, the density of adult sporophytes on subtidal hard bottoms at 21°Sdecreased rapidly and linearly with increasing positive thermal anomalies (Figure 14). Six monthslater the site remained completely devoid of adult sporophytes, and no recolonisation occurred inthe subtidal zone during the study period. In areas south of 23°S positive thermal anomaliesregistered during EN 1997–1998 had only limited effects on kelp beds (Figure 14). As a result, thespatiotemporal abundance patterns of M. integrifolia sporophytes in northern-central Chile is highlyvariable (Figure 14).Kelp conservation and human activitiesMany of the diverse kelp-associated species have significant socioeconomic importance for humanpopulations along the coast in north-central Chile and have been subject to harvesting by localhuman communities since pre-Columbian times (Jerardino et al. 1992, Vásquez et al. 1996). Spatialand temporal dynamics of kelp beds are significantly affected by anthropogenic impacts producedby both intense harvesting and severe pollution with organic as well as mining waste (Faugeronet al. 2005; Vásquez & Vega 2005). Lessonia nigrescens, L. trabeculata and Macrocystis integrifoliaare commercially exploited between 18°S and 32°S. These species account for >95% of landingsof macroalgae and basically are used for alginic acid extraction. Until 2002, collected biomass indry weight (dry wt) amounted to ~200,000 t, almost exclusively based on stranded kelps, resultingfrom natural mortality of plants with holdfasts that are weakened by grazing and then detached bystrong bottom currents and waves. Since 2003, however, due to international needs for raw (dry)materials and also due to increasing demands for fresh algae (to sustain aquaculture of herbivorousinvertebrates in northern Chile), harvesting of natural kelp increased to ~300,000 t dry wt per year.This has led to the recent implementation of new administrative rules in order to mitigate the impacton natural kelp populations. Regulations aim at the establishment of a sustainable kelp fishery,applying the following strategies: (1) harvest management (Vásquez 1995, 1999, 2006), (2) stock242
THE HUMBOLDT CURRENT SYSTEM OF NORTHERN AND CENTRAL CHILESubtidal kelp 54Intertidal kelp3251San Marcos (21°S)0Abundance of adult sporophytes (ind m −2 )43210543210543210543210543210543210Abundance of adultsporophytes (ind m −2 )Camarones (18°S)1996 1997 1998 1999 20001−1−3−55Constitución (23°S)31−1−3−55Playa Blanca (28°S)31−1−3−55Los Choros (29°S)31−1−3−55San Lorenzo (30°S)31−1−3−55Los Vilos (32°S)31−1−3−51996 1997 1998 1999 2000Thermal anomalies (°C)531−1−3−5Thermal anomalies (°C)Figure 14 Temporal variation (between 1996 and 2000) of abundances of adult sporophytes of M. integrifolia(●) and thermal anomalies estimated in situ (line) over a latitudinal gradient in northern Chile. Note: At SanMarcos, an intertidal kelp population appeared after the El Niño event (▫), while the subtidal kelp bed didnot recover. At Camarones (top) no sporophytes were observed during the study period. (Modified from Vega2005)243
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the humboldt current system of northern and central chile - figema

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