Ecosystem responses in the southern Caribbean Sea to global climate change

Abstract

Over the last few decades, rising greenhouse gas emissions have promoted poleward expansion of the large-scale atmospheric Hadley circulation that dominates the Tropics, thereby affecting behavior of the Intertropical Convergence Zone (ITCZ) and North Atlantic Oscillation (NAO). Expression of these changes in tropical marine ecosystems is poorly understood because of sparse observational datasets. We link contemporary ecological changes in the southern Caribbean Sea to global climate change indices. Monthly observations from the CARIACO Ocean Time-Series between 1996 and 2010 document significant decadal scale trends, including a net sea surface temperature (SST) rise of ∼1.0 ± 0.14°C (±SE), intensified stratification, reduced delivery of upwelled nutrients to surface waters, and diminished phytoplankton bloom intensities evident as overall declines in chlorophyll a concentrations (ΔChla = −2.8 ± 0.5%·y −1 ) and net primary production (ΔNPP = −1.5 ± 0.3%·y −1 ). Additionally, phytoplankton taxon dominance shifted from diatoms, dinoflagellates, and coccolithophorids to smaller taxa after 2004, whereas mesozooplankton biomass increased and commercial landings of planktivorous sardines collapsed. Collectively, our results reveal an ecological state change in this planktonic system. The weakening trend in Trade Winds (−1.9 ± 0.3%·y −1 ) and dependent local variables are largely explained by trends in two climatic indices, namely the northward migration of the Azores High pressure center (descending branch of Hadley cell) by 1.12 ± 0.42°N latitude and the northeasterly progression of the ITCZ Atlantic centroid (ascending branch of Hadley cell), the March position of which shifted by about 800 km between 1996 and 2009. ecosystem state change | oceanography | plankton productivity P hytoplankton support over 95% of marine food webs and are responsible for about half of the Earth's conversion of CO 2 to biomass through net primary production (NPP) (1). Long-term declines in phytoplankton biomass and production in over 70% of the global ocean have been inferred recently from satellite imagery and century-long shipboard records of water clarity (2, 3). These reports of large-scale changes are at odds with trends directly observed at specific locations within the same ocean domains.

Phytoplankton adapt to changing ocean environments

“…The phytoplankton community shifted to smaller cells not identified in this time series, and many species that were tracked dropped in abundance 50-300-fold. This change appears to be a result of a change in grazing rates (35) and is not linked to sudden changes in temperature or the availability of nutrients. Using presence data rather than abundance means our niche models were not affected by the change in species abundances.…”

“…Monthly sampling at Station CARIACO recorded temperature, nitrate concentration, and the abundance of 67 dominant phytoplankton species (30,35). Irradiance in the mixed layer was estimated from monthly SeaWiFS PAR and k 490 data.…”

“…Inter-Tropical Convergence Zone (35). We divided the time series into an early, cooler period and a late, warmer period and examined the stability of the realized niches of phytoplankton species between these two periods ( Fig.…”

Monitoring Ocean Change in the 21st Century

“…The temporal scale of these shifts was on the order of several decades, a cycle that would have gone unnoticed without a consistent and comprehensive monitoring program (Russell et al, 1971;Southward, 1995). Similarly, using 15 years of repeat measurements in the southeastern Caribbean Sea (CARIACO time--series station), Taylor et al (2012) showed how the upwelling--favorable trade winds have diminished due to the northward migration of the ITCZ, warming the ocean and decreasing primary productivity with effects cascading through the local ecosystem. In the coastal Pacific Ocean (time--series stations IMARPE, Peru and MBARI, California), the El Niño--Southern Oscillation (ENSO) has been shown to affect primary productivity in surface waters (Chavez et al, 2011), while in the oligotrophic Pacific (Hawai'i Ocean Time--series --HOT), a shift in the phytoplankton community was reported in the early 1980s that was presumably linked to changes in the North Pacific climate system (Karl et al, 2001;Corno et al, 2007).…”

Climate change and marine ecosystems

“…The opposite northward migration is predicted during a warmer world (2). In PNAS, Taylor et al (4) describe the response of a marine ecosystem in the marginal southern Caribbean Sea to present-day northward migrations of the ITCZ (Fig. 1).…”