Breaking out of the comfort zone: El Niño-Southern Oscillation as a driver of trophic flows in a benthic consumer of the Humboldt Current ecosystem

Abstract

The trophic flow of a species is considered a characteristic trait reflecting its trophic position and function in the ecosystem and its interaction with the environment. However, climate patterns are changing and we ignore how patterns of trophic flow are being affected. In the Humboldt Current ecosystem, arguably one of the most productive marine systems, El Niño-Southern Oscillation is the main source of interannual and longer-term variability. To assess the effect of this variability on trophic flow we built a 16-year series of mass-specific somatic production rate (P/B) of the Peruvian scallop (Argopecten purpuratus), a species belonging to a former tropical fauna that thrived in this cold ecosystem. A strong increase of the P/B ratio of this species was observed during nutrient-poor, warmer water conditions typical of El Niño, owing to the massive recruitment of fast-growing juvenile scallops. Trophic ecology theory predicts that when primary production is nutrient limited, the trophic flow of organisms occupying low trophic levels should be constrained (bottom-up control). For former tropical fauna thriving in cold, productive upwelling coastal zones, a short time of low food conditions but warm waters during El Niño could be sufficient to waken their ancestral biological features and display massive proliferations.

Keywords: Argopecten purpuratus; El Niño-Southern Oscillation; P/B ratio; climate change ecology; population blooms; trophic ecology.

Figure 1.

Location of the study area. Bahía Independencia, Peru. Dotted line shows the distribution of the scallop population in La Pampa.

Figure 2.

Interannual changes in population parameters of Argopecten purpuratus in Bahía Independencia, Peru. (a) Production to Biomass (P/B) ratio; (b) annual changes in growth performance and somatic production. Years highlighted with a black star represent severe El Niño anomalies (Multivariate ENSO Index ≥ 2), whereas years highlighted with a white diamond denote years with strong La Niña anomalies (MEI ≤ −1).

Figure 3.

(a) Biplot of two principal components (PC1 and PC3) after the PCA analysis of environmental variability and the superimposed vectors (grey lines) of environmental factors (Sal: salinity in surface waters averaged for the Pisco region; isDO: in situ-dissolved oxygen; SST: sea surface temperature reported by IMARPE in a fixed station near the study area; MEI: multivariate ENSO index; isSBT: in situ-sea bottom temperature ENCI: El Niño-Coastal Index; isSST: in situ-sea surface temperature). Black stars denote years with severe El Niño anomalies (MEI ≥ 2); white diamonds denote years with strong La Niña anomalies (MEI ≤ −1), and grey squares represent neutral periods. (b) Correlation between the P/B ratio of Argopecten purpuratus and the El Niño-Coastal Index.

Figure 4.

Monthly changes in population parameters of Argopecten purpuratus in Bahía Independencia between March 1992 and November 2007; (a) mean abundance (black line) and individual mass (grey line) calculated for a standard individual (65 mm shell length) from the monthly length-mass relationship; (b) boxplot showing the distribution of body sizes (minimum, 25th percentile, mean, 75th percentile, and maximum shell length). Grey panels show periods of severe El Niño (EN) anomalies (Multivariate ENSO Index ≥ 2) and strong La Niña (LN) anomalies (Multivariate ENSO Index ≤ −1).

Figure 5.

Boxplots showing differences in abundance, individual mass, and shell length of Argopecten purpuratus between neutral, El Niño, and La Niña periods. Central markers represent the mean, box limits: ± s.e., whiskers: ± s.d. Different capital letters in boxplots indicate significant differences (p < 0.05) between periods after paired comparisons. (Online version in colour.)

Figure 6.

Compilation of studies reporting on interannual changes in P/B ratio of molluscs facing different environmental and mixed stressors. Black box represents the mean P/B ratio and error bars the range observed during the years of study (number of years in parentheses). ZB: Zostera bed; SA: sandflat area. References: a: this study; b: [40]; c: [41]; d: [42]; e: [29]; f: [43]; g–m: [5].