UC San Diego

Phenology is the study of seasonal, biological events and how they are influenced by climate. Climate change has prompted an earlier arrival of spring in numerous ecosystems. It is uncertain whether such changes are occurring in coastal upwelling ecosystems, because these regions are subject to decadal climate oscillations and regional climate models predict later seasonal upwelling. To answer this question, chapter 1 investigated decadal changes in the phenology of 43 larval fish species in southern California. The first principal component of this dataset showed a progression towards the earlier appearance of larvae, although 18% of phenological events exhibited seasonal delays. These changes were best explained by a secular trend towards earlier warming of surface waters. Species with earlier phenology were characterized by an offshore, epipelagic distribution, while fishes with delayed phenology were more likely reside in coastal, demersal habitats. Chapter 2 focused on improving understanding of how oceanic factors affect fish spawning habitat. Spawning habitat models can be applied to examine variations in fish phenology. Using data from spring cruises conducted between 1998-2004, dynamic height was investigated as a variable affecting the spawning habitat of anchovy, sardine, and jack mackerel. The greatest probability of encountering anchovy, sardine, and jack mackerel eggs occurred at dynamic heights of 79-83 cm, 84-89 cm, and 89-99 cm, respectively. Dynamic height explained more variance than any other variable (e.g., temperature, salinity, chlorophyll, zooplankton volume, geographic currents, eddies) in models of sardine and anchovy spawning habitat. Chapter 3 examined variations in phytoplankton phenology across the North Pacific using a hindcast of the Community Earth System Model 1.0 (CESM1) forced with atmospheric observations. Comparisons with SeaWiFS chlorophyll indicated that CESM1 could simulate mean dates of phytoplankton bloom initiation with as much skill as it could predict mean bloom magnitude. The first principal component for each of five phenological metrics (bloom initiation, midpoint, termination, duration, and magnitude) was either correlated with the Multivariate ENSO Index or displayed a long-term trend. Compared to terrestrial ecosystems, long-term trends in phytoplankton phenology were noteworthy due to their rapid rate of change and greater prevalence of delayed phenology