Climate Change and Phenology in Narragansett Bay Phytoplankton. Introduction

Transcription

1 Sarah Blackstock OCG561: Biological Oceanography Susanne Menden-Deuer Research Project 3 December 2013 Climate Change and Phenology in Narragansett Bay Phytoplankton Introduction As time-series on plankton abundance lengthen, it has become increasingly clear that climate change can influence certain characteristics of these marine organisms. One parameter that can be particularly sensitive to a warming climate is plankton phenology, the timing of periodic (e.g. seasonal) life cycle events. The seasonal arrival and departure of plankton, fish, and other marine species has long been studied by the oceanographic community. For phytoplankton, seasonality is generally controlled by physical characteristics of the marine environment, such as nutrient and light availability (Sverdrup 1953). The growing awareness of climate change has sparked interest in how warming waters might alter the seasonal cycle of the marine environment (for example, thermal stratification and daily irradiance), and consequently, the seasonal cycle of biological events. Such observations have already been made worldwide (Ji 2010). Seasonal variability is usually strongest at mid-latitude to polar regions. Organisms in these areas are adapted to exploit the most favorable conditions of the year (Ji 2010). For example, in temperate climates like Narragansett Bay, a winter-spring plankton bloom occurs when thermal stratification and daily irradiance peak, and the fall bloom occurs when increasing vertical mixing replenishes the mixed layer with nutrients. Mid-latitude and polar regions also experience a strong effect of climate change. Waters off the coast of New England have experienced a rise in average temperature of at least 1.2 C since 1970 (Nixon et al. 2003). As a temperate area with measurable long-term temperature increase

2 and almost half a century of plankton abundance data, the Narragansett Bay is an ideal study site for the effects of warming temperature on the timing of phytoplankton events. This project assesses whether increasing sea surface temperature in Narragansett Bay has caused a shift in phytoplankton blooms. I predict that as temperature has increased, the peak phytoplankton blooms (both spring and fall), measured by dates of maximum chlorophyll concentration in the bay, has shifted earlier in the year. Methods Data Source The chlorophyll and SST data used in this experiment were taken from the Graduate School of Oceanography's ongoing weekly plankton time series, as well as the historic time-series for the same survey (The Plankton of Narragansett Bay). Combined, these data sets encompass four decades ( ), with weekly samples off the dock at GSO. There have been some procedural changes since the survey's beginning, but since the procedures were consistent within each year, this characteristic should not be confounding. Missing values in this data set include the years , and the second half of Statistical Analysis For each year (from ) the Chl data was split into two sections, the first 183 days, and the last 182 days. This allowed for the detection and analysis of the two distinct blooms, spring and fall. For each section, the day in the year of maximum chl concentrations was determined. This provided a 40-year time-series describing the day of maximum chl in each year for each bloom. For the SST data, annual averages were calculated. Though using annual averages decreased the resolution of the SST data, it provided a time-series with one point per year, which could then be correlated with the day of max chl datasets.

3 Linear regressions and pearson correlations were performed to describe the interactions of time, temperature, and plankton bloom timing. The day of chl max data and temperature were each regressed against time to detect temporal trends. Next, the day of chl max timeseries was correlated with temperature. Cross-correlation analysis was used to determine if there was a lag between temperature change and shifts in plankton bloom timing. Results The SST in Narragansett Bay has increased about 1.14 C since 1973 (R 2 = 0.196, P = 0.005; Fig 1), indicating a strong upward trend in SST in the last 40 years. Figure 2 shows the weekly chl concentrations in Narragansett Bay over seven recent years (2001 to 2003). This plot provides little information about the phenological shifts of the plankton in this environment. However it does illustrate the bimodal seasonal signal of chl concentrations, which is typical of a mid-latitude nutrient-limited estuary. The spring bloom timing also showed a positive, though not significant, trend with time (R 2 = , P = 0.21). Thus, the spring bloom may be moving later in the year. The fall bloom showed a trend in the opposite direction as the spring bloom (earlier in the year), but this trend was also insignificant (R 2 = , P = 0.51). When the spring and fall bloom timing were correlated with temperature, neither relationship was significant (spring: ρ = 0.133, P = 0.425; fall: ρ = , P = 0.24), but the trends were in the same direction as the temporal analysis: the spring bloom moved later in the year with increasing temperature and the fall bloom moved earlier (Table 1, Figures 3 and 4). Though the spring bloom timing showed no evidence of a lagged reaction to SST, the fall bloom timing correlated significantly with SST with a seven-year lag (ρ = , P = 0.015, Table 1).

4 Discussion and Conclusions Despite decades of research, the causes for the timing of the spring and fall blooms in Narragansett Bay are not fully understood, a shortcoming that burdens our ability to speculate on the forcers of long-term phenological shifts (Nixon, 2009). In this project, we see observable, though statistically insignificant delays of the winter-spring bloom, and early occurrence of the fall bloom. The direct relationship of these shifts to temperature change proved weak. Oviatt et al. found a weakening or even loss of the winter-spring bloom in especially warm years, hypothesizing that the warm waters increased grazing pressure such that this bloom could not develop (2002). I hypothesized that, as warming sets in earlier, the spring bloom would occur earlier, implicating physiological (e.g. temperature preference) mechanisms. However, the shift of the spring bloom later in the year rather than earlier, along with the work of Oviatt and others, suggest that perhaps grazing pressure is a top-down forcer of plankton phenology in Narragansett Bay, while physiological preferences are less important. The shifts in the fall bloom to earlier dates are somewhat more mysterious. Given that this bloom is usually triggered by nutrient mixing, it is possible that changes in wind (such as those discussed in Nixon, 2008) or other variables may contribute to this trend. Alternatively, the composition of phytoplankton population of Narragansett Bay is known to be different in the spring, summer, and fall (Durbin, 1975). While the spring bloom seems to depend on light availability and grazing, the plankton in the fall bloom may depend on temperature-mediated physiological responses in their life strategies, an explanation that supports my original hypothesis. The physiological dependence might cause them to bloom earlier, if their optimal temperature occurs earlier. The seven-year

5 time lag observed in this project should be interpreted with caution, as there are few mechanisms that could explain such a lag. This project, though limited in scope, suggests important underlying trends in the phenology of Narragansett Bay phytoplankton. However, even when combined with previous studies on plankton phenology, it leaves many gaps in our understanding of how climate change might affect the community dynamics of Narragansett Bay. Future work could include an examination of phenological shifts in species or species groups, rather than simply chlorophyll. For example, understanding how diatom shifts as compared to dinoflagellates. These taxa have different adaptations and phenology, so might be expected to change independently of each other. Specifically, understanding which taxa would fare better in warmer temperatures might allow for predictions of how the rest of the biological community will change. An assessment of shifts in zooplankton phenology may also be a useful follow-up to this project. Such information might support or invalidate the top-down hypothesis for the delayed spring bloom. Furthermore, it could address the question of trophic mismatch in the bay. As might be expected, shifts in plankton phenology can reverberate through higher trophic levels, creating trophic asynchrony. Thackery et al. demonstrated "a broad scale of differential phenological change among trophic levels" (2010). Most studies of this type indicate that secondary consumers shift phenologically more slowly than primary producers. This has serious implications for the entire marine community, and is thus important to understand. An assessment of whether the zooplankton in Narragansett Bay have "kept pace" with the phenological shifts in phytoplankton could provide important information about changes we can expect from further increases in SST.

6 References Durbin, E. G., R. W. Krawiec, and T. J. Smayda. "Seasonal studies on the relative importance of different size fractions of phytoplankton in Narragansett Bay (USA)." Marine Biology 32.3 (1975): Greve, Wulf, et al. "Predicting the seasonality of North Sea zooplankton." Senckenbergiana maritima 31.2 (2001): Ji, Rubao, et al. "Marine plankton phenology and life history in a changing climate: current research and future directions." Journal of plankton research32.10 (2010): Nixon, S., Granger, S., Buckley, B., The warming of Narragansett Bay. Universityof Rhode Island. 41_N, The Magazine of the RI Sea Grant and Land Grant. Nixon, Scott W., et al. "The impact of changing climate on phenology, productivity, and benthic pelagic coupling in Narragansett Bay." Estuarine, Coastal and Shelf Science 82.1 (2009): Programs 2, vol2no1/baywarming.html. Oviatt, C., A. Keller, and L. Reed. "Annual primary production in Narragansett Bay with no bay-wide winter spring phytoplankton bloom." Estuarine, Coastal and Shelf Science 54.6 (2002): The Plankton of Narragansett Bay. URI Graduate School of Oceanography. accessed: 15 November, Sverdrup, H. U. "On conditions for the vernal blooming of phytoplankton."journal du Conseil 18.3 (1953):

7 Tables and Figures Table 1. Correlation coefficients, R 2 values, and associated p-values among temperature, time, day of chl maximum in both summer and fall. Statistically significant relationships are shaded. Temperature Day of Spring Chl Max Day of Fall Chl Max Time R P Temperature ρ P Temperature, ρ lag = 7 years P Figure 1: Temperature in Narragansett Bay from 1973 to present.

8 Figure 1. Weekly Chl concentrations in Narragansett Bay from Weekly Chl Concentrations in Narragansett bay from 2001 to 2003 Count (million cells/l) Days into Year Figure 2. Temperature and spring bloom timing in Narragansett Bay as a function of time. The straight black line is the fit of a linear regression of day of chl max and time.

9 Figure 3. Temperature and fall bloom timing in Narragansett Bay as a function of time. The straight black line is the fit of a linear regression of day of chl max and time.