In order to assess the phytoplankton size distribution in the Puget Sound, flourometric determination of chlorophyll a concentrations for four different size classes was used on water samples taken at nine different sites within the Puget Sound on 4-9 April 94. The cell size classes of 0-10 mm, 10-70 mm, 70-150 mm, and > 150 mm had population percentages of 21 %, 36 %, 30 %, and 13 %, respectively. The average phytoplankton distributions of the south Puget Sound and the north Puget Sound were roughly the same. Compared to the open ocean, the average phytoplankton distribution in the Puget Sound consisted of larger phytoplankton.

Introduction

It is well known that phytoplankton size distributions are related to phytoplankton biomass and the structure of food chains in the ocean (Kiorboe 1990). For this reason, phytoplankton size distributions, and factors affecting the size distributions, such as variation in upwelling and nutrient levels, need to be studied and understood. It is seen that large phytoplankton dominates in areas of high and variable nutrient levels and high phytoplankton biomass, and smaller phytoplankton dominates in areas of lower and stable nutrient levels (Varela 1987; Morris 1980). Therefore, in open ocean conditions, smaller phytoplankton are usually prevalent, while in neritic waters, larger species make up a greater fraction of the phytoplankton population (Malone et al. 1973).

The purpose of this project was to examine the phytoplankton size distribution of Puget Sound by measuring the chlorophyll a concentrations of the different sized phytoplankton. In the Puget Sound there it is expected that netplankton dominate the phytoplankton population due to the high nutrient levels, vertical mixing, and large phytoplankton biomass.

Material and methods

Plankton samples were taken at sites 16-20 and 43 in the Strait of Juan de Fuca and northern Puget Sound from the R/V Thomas G. Thompson on 8 and 9 April 94 and in the southern Puget Sound at sites TN (Tacoma Narrows), C1 (Colvos, cast 1), C2 (Colvos, cast 2), and EP (East Passage) from the R/V Clifford Barnes on 4-6 April 94 (Table 1 and Fig. 1). Water samples were collected with Niskin bottles and an attached CTD. The CTD measured light transmittance, fluorescence, temperature, and conductivity.

The samples were first filtered through 3 mesh sizing filters to separate the phytoplankton into 4 size classes: 0-10 mm, 10-70 mm, 70-150 mm, and > 150 mm. These sized samples were then filtered through a Whatman GF/F glass microfiber filter to collect the phytoplankton. To extract the pigments, the filters were then put into vials containing 10 ml 90% acetone and kept frozen and in the dark for 24 to 48 hours. A Turner 112 fluorometer was used to determine the chlorophyll a concentration of the sample as described by Lorenzen (1966).

Results

The data collected and analyzed revealed that the average phytoplankton size distribution in the Puget sound consisted of mainly large sized phytoplankton. The average phytoplankton distributions in the south Puget Sound and in the north Puget Sound were similar, although great variations (i.e. up to 32 % difference) in phytoplankton size distributions were observed in the different sampling sites (Table 2). The average size distribution at the given depth of 8.5 m in the Puget Sound was 13 % for phytoplankton greater than 150 mm, 36 % for phytoplankton between 150 mm and 70 mm, 30 % for phytoplankton between 70 mm and 10 mm, and 19 % for phytoplankton smaller than 10 mm (Figs. 2 and 3).

Discussion

Ultraplankton ( < 3 mm ) and nanoplankton ( < 20 mm ) compromise the majority of the chlorophyll a in the open ocean with contents of up to 98 % of the chlorophyll standing stock (Glover et al. 1985; Takahashi & Bienfang 1983). Conversely, in neritic waters, netplankton ( > 20 mm ) constitutes a majority of the chlorophyll a with up to 54 % in open bays (Cole et al. 1986), up to at least 80% in estuarine areas (Furnas 1983), and up to at least 80 % for phytoplankton > 10 mm in the fjord-like estuary of Puget Sound (Fig. 3). The data show that at the time of sampling, phytoplankton size distribution in the Puget Sound has a far greater abundance of larger sized phytoplankton than the open ocean and has a similar size distribution compared to other estuaries or bays.

Upwelling is an important factor in phytoplankton size distributions, for it brings up nutrients and suspends larger phytoplankton into the euphotic zone and promotes phytoplankton population growth. A study off the coast of Mexico shows that upwelling areas of the open ocean fluctuate in their phytoplankton size distributions, having the smaller ultraplankton and nanoplankton dominate over the netplankton at all times except during very strong upwelling (Gonzales-Morales & Gaxiola-Castro 1991). Other studies show that the netplankton dominates over the nanoplankton and ultraplankton when the phytoplankton biomass is high and the area has high seasonal nutrient levels, as where the smaller phytoplankton become dominant as the total biomass decreases and nutrients remain at steady-state conditions (Varela 1987; Morris 1980). Therefore, as the ultraplankton and nanoplankton remain relatively constant in abundance, the netplankton varies greatly, increasing population at up to four doublings per day when waters are high in nutrient levels and have changed recently (Malone et al. 1973). In the spring, the Puget Sound has high nutrient levels in the upper water column due to the upwelling caused by winter storms. Combining this high nutrient level with plenty of light results in a massive phytoplankton population growth, especially in the larger sized phytoplankton.

Phytoplankton, especially netplankton, use vacuoles to store nutrients, which are used during periods of non-steady state conditions. The vacuole increases in size and takes up a greater percentage of the volume of the phytoplankton cell as the cell volume increases (Smayda 1970). This allows the larger netplankton to make greater use of storage of nutrients. The sinking rates of netplankton are also higher than nanoplankton and ultraplankton (Smayda 1970), giving the netplankton a chance to get to deeper levels in the water column and to higher nutrient concentrations when upwelling or vertical mixing is absent. Therefore it is seen that when vertical mixing or upwelling is absent, as in the open ocean, the netplankton population is low in the surface layers and storing nutrients at depth, where as nanoplankton and ultraplankton, with their ability to stay suspended in the euphotic zone, and inability to store large amounts of nutrients, are compromising the bulk of all phytoplankton in the surface layers. As vertical mixing or upwelling occurs, as in the coastal, estuarine, or bay areas, the nanoplankton and ultraplankton increase in population slightly, while the netplankton are suspended back up to the surface layers and in to the photic zone where they are able to populate quickly and dominate the population of phytoplankton.

The implications of a large-sized dominated phytoplankton size distribution takes place in the resulting food webs. It was seen by Kiorboe (1990) that when smaller phytoplankton, such as photosynthetic flagellates and cyanobacteria, dominated the population, a longer, more complex food web was present. This longer food web had heterotrophic flagellates and protozooplankton as secondary consumers and at higher trophic levels came the larger zooplankton, such as the copepoda. In contrast, a shorter, more classical (phytoplankton-zooplankton-fish) food web was present when the larger netplankton dominated the phytoplankton population. The middle trophic levels were cut out, so that netplankton, such as the larger diatoms were directly consumed by the copepoda.

Conclusion

The Puget Sound distribution of phytoplankton size is comparable to most coastal areas where larger netplankton species are more prevalent than the smaller nanoplankton and ultraplankton. Blooms of netplankton occur when the total biomass is low and nutrients are abundant when vertical mixing is prevalent. In contrast, the open ocean is relatively void of vertical movement of water, nutrients, and netplankton, so that nanoplankton and ultraplankton dominate in the surface layers. With this in mind, a prediction is made that in the Puget Sound, the resulting food webs tend to be of the more direct, shorter type with fewer trophic levels, than the longer, less direct food webs of the open ocean.

Acknowledgments

I thank Lee Karp for her assistance and helpful reviews. I also thank Kathy Newell, Dick Sternberg, Emmanuel Bass, and Dr. Dean McManus for their valuble suggestions. Furthermore, I am indebted to the crews of the R/V Thomas G. Thompson and R/V Clifford Barnes for their help. Funding was provided by the University of Washington, School of Oceanography.

References

Cole, B., Cloern, J. and Alpine, A., 1986. Biomass and productivity of three phytoplankton size classes in San Francisco Bay. Estuaries, 9: 117-128.

Furnas , M.J., 1983. Nitrogen dynamics in lower Narragansett Bay, Rhode Island: Uptake by size-fractionated phytoplankton populations. Plankton Res., 5: 657-676.

Glover, H., Smith, A. and Shapiro, L., 1985. Diurnal variations in photosynthetic rates: Comparisons of ultraphytoplankton with a larger phytoplankton size fraction. Plankton Res., 7: 519-535.

Gonzales-Morales, A. and Gaxiola-Castro, G., 1991. Daily variation of physico-chemical characteristics, biomass and phytoplankton primary production in an upwelling coastal zone of Baja California. Science, 17: 21-37.

Kiorboe, T., Kaas, H., Kruse, B., Mohlenberg, F., Tiselius, P. and Aertebjerg, G., 1990. The Structure of the pelagic food web in relation to water column structure in the Skagerrak. Mar. Ecol. Prog. Ser., 59: 19-32.

Lorenzen, C., 1966. A method for the continuous measurement of in vivo chlorophyll concentration. Deep-Sea Res., 13: 223-227.

Malone, T., Garside, C., Anderson, R. and Roels, O., 1973. The possible occurence of photosynthetic microorganisms in deep sea sediments of the North Atlantic. J. Phycol., 9: 482-488.

Morris, I., 1980. The physiological ecology of phytoplankton. Boston, Mass., 625 pp.

Smayda, T., 1970. The suspension and sinking of phytoplankton in the sea. Ocean-ogr. mar. Biol. Ann. Rev., 8: 353-414.

Takahashi, M. and Bienfang, P., 1983. Size structure of phytoplankton biomass and photosynthesis in subtropical Hawaiian waters. Mar. Biol., 76: 203-211.

Varela, M., 1987. Phytoplankton size classes distribution in an upwelling area. Invest. Pesq. Barc., 51: 97-105.