Resource availability affects temporal variation of phytoplankton size spectral slopes in the Kuroshio east of Taiwan
Fan-Sian Lin, Pei-Chi Ho & Chih-hao Hsieh
Institute of Oceanography, National Taiwan University
A study in the Kuroshio east of Taiwan led by master student Fan-Sian Lan, Dr. Pei-Chi Ho, Profs. Chih-hao Hsieh and Sen Jan at National Taiwan University, shows that the temporal variation of phytoplankton size spectral slops were affected by resource availability. This study is published (August, 2019) in Limnology and Oceanography.
Previous studies indicated that size structure of phytoplankton community changed in response to variation of environmental factors. In this study, the team employed size spectral slope as an index of phytoplankton size structure. The phytoplankton size spectrum is constructed as follows: sorting phytoplankton body size (4-200 μm) from small to large cells, classifying into size classes, and measuring the frequency in each size class. Size spectral slopes were fitted with linear regression (Figure 1). Responding to environmental factors, phytoplankton size spectral slope may become large (shallower) or small (steeper). A shallower size spectral slope indicates that the frequency of bigger cells is relatively equal to that of small ones, and vise versa.
A core objective in aquatic ecology is to explain how phytoplankton size structure varies along gradients of water temperature and resource availability. Previous studies showed that phytoplankton size spectral slopes were shallower with decreasing water temperature (temperature-size relationship, Figure 2A) or/and increasing resource availability (resource-size relationship, Figure 2B). Which relationship dominates in seas? The debate has continued for decades. Most studies focused plankton community in temperate zone and experimental setup. A better way to distinguish the effects of resource availability and water temperature is to study phytoplankton size structure in a system with large variation of one factor but low variation of the other factors.
To fill this knowledge gap, the research team investigated phytoplankton size spectral slope in the Kuroshio Current, an oligotrophic environment. The results indicate no significant relationship between NBSS slopes with temperature, providing little support for the temperature-size rule. In contrast, a positive relationship between NBSS slopes and total biomass for most of the year lends general support to the resource-size relationship, except during the winter and early spring. To explain this exception, the team hypothesizes that resource pulses occurring during the cold seasons are used more efficiently by small cells and promote faster growth of small relative to large phytoplankton because these pulses take place after a long period of resource depletion in oligotrophic seas; thus, the size spectral slopes become much steeper than would be expected from a positive resource-size relationship. This deviation can be considered as non-steady state in terms of phytoplankton size structure relative to resources.
The findings are summarized in Figure 3. In early spring, as the resource availability pulse in winter and early spring after long starvation, phytoplankton size spectral slopes were steeper. Later on, as resource becomes sufficient, large phytoplankton can outcompete the small phytoplankton; the phytoplankton size spectral slopes were shallower. Between summer to autumn, the result exhibited resource-size relationship- phytoplankton size spectral slopes were steeper with decreasing resource availability. This work reveals that resource effects override the temperature effects in affecting phytoplankton size structure in the oligotrophic area.
Fan-Sian Lin, Pei-Chi Ho, Akash R. Sastri, Chung-Chi Chen, Gwo-Ching Gong, Sen Jan, Chih-hao Hsieh. Resource availability affects temporal variation of phytoplankton size structure in the Kuroshio east of Taiwan. Limnology and Oceanography, Online Published: 02 August 2019. https://doi.org/10.1002/lno.11294
Figure 1. Phytoplankton size spectral slopes vary with changing environmental factors.
Figure 2. (A) Temperature-size relationship means that phytoplankton size spectral slopes decrease with increasing water temperature. In this study, the results did not support the temperature-size relationship. (B) Resource-size relationship means that phytoplankton size spectral slopes increase with increasing resource availability. The results generally support the resource-size relationship.
Figure 3. Seasonal variation of resource availability (μg-C/L) (black line), water temperature (blue line) and size spectral slopes (green circle).