Biogeography and phenology of satellite-measured phytoplankton seasonality in the California current |
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| Institution: | 1. School of Life Sciences, Xiamen University, Xiamen 361102, China;2. State Key Laboratory of Marine Environmental Science, Xiamen University, Xiamen 361102, China;3. Key Laboratory of the Ministry of Education for Coastal and Wetland Ecosystems, Xiamen University, Xiamen 361102, China;4. Key Laboratory of Marine Environment and Ecology, Ministry of Education, Qingdao 266100, China;5. Laboratory for Marine Ecology and Environmental Science, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China |
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| Abstract: | Thirteen years (1998–2010) of satellite-measured chlorophyll a are used to establish spatial patterns in climatological phytoplankton biomass seasonality across the California Current System (CCS) and its interannual variability. Multivariate clustering based on the shape of the local climatological seasonal cycle divides the study area into four groups: two with spring-summer maxima representing the northern and southern coastal upwelling zones, one with a summer minimum offshore in mid-latitudes and a fourth with very weak seasonality in between. Multivariate clustering on the seasonal cycles from all 13 years produces the same four seasonal cycle types and provides a view of the interannual variability in seasonal biogeography. Over the study period these seasonal cycles generally appear in similar locations as the climatological clusters. However, considerable interannual variability in the geography of the seasonal cycles is evident across the CCS, the most spatially extensive of which are associated with the 1997–1999 El Niño-Southern Oscillation (ENSO) signal and the 2005 delayed spring transition off the Oregon and northern and central California coasts. We quantify linear trends over the study period in the seasonal timing of the two seasonal cycles that represent the biologically productive coastal upwelling zones using four different metrics of phenology. In the northern upwelling region, the date of the spring maximum is delaying (1.34 days yr?1) and the central tendency of the summer elevated chlorophyll period is advancing (0.63 days yr?1). In the southern coastal upwelling region, both the initiation and cessation of the spring maximum are delaying (1.78 days yr?1 and 2.44 days yr?1, respectively) and the peak is increasing in duration over the study period. Connections between observed interannual shifts in phytoplankton seasonality and physical forcing, expressed as either basin-scale climate signals or local forcing, show phytoplankton seasonality in the CCS to be influenced by changes in the seasonality of the wind mixing power offshore, coastal upwelling in the near-shore regions and basin-scale signals such as ENSO across the study area. |
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| Keywords: | Eastern boundary current Phytoplankton Remote sensing Food webs structure and dynamics Chlorophyll Clustering analysis |
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