IntroductionIntroduction 

Picophytoplankton, phytoplankton 3µm and smaller in size, are the dominant photosynthesizers in the ocean. Thus picophytoplankton can have significant impacts on global carbon cycling. They typically dominate in the open ocean, however little is known about conditions which might cause picophytoplankton to dominate in a coastal shelf environment. Theoretically, picophytoplankton’s small dimensions can have both benefits and drawbacks based on varying environmental factors.

Light- Picophytoplankton do better in low light due to a higher surface-to-volume ratio, but in high light picophytoplankton will be inhibited (Fogg 1991). Nutrients- Picophytoplankton will take up nutrients more quickly at lower concentrations but cannot store these nutrients at high nutrient concentrations. Therefore, picophytoplankton can outcompete larger phytoplankton at lower nutrient concentrations (Moore et al. 2002).Predation- Picophytoplankton are susceptible to heavy grazing by small-sized grazers (Raven

1998).

An investigation into the response of picophytoplankton in relation to light, nutrient, and grazing treatments can aid in a better ecological understanding of these essential organisms.

HypothesesHypotheses

LightHo: There will be no difference between light treatments.H1: In the low light, the picophytoplankton will have the greatest growth response as they have been shown to dominate under low light conditions without nutrient additions.

NutrientsHo: There will be no difference between nutrient treatments.H1: Picophytoplankton will be outcompeted by larger phytoplankton in the nutrient- addition treatments.

Grazing RateHo: There will be no difference between the grazing rate onshore and offshore.H1: The onshore grazing rate will be greater due to a larger zooplankton population.

ConclusionConclusion

Under conditions of prevalent urea, picophytoplankton (particularly Synechococcus) dominated the overall phytoplankton assemblage both onshore and offshore.

The results from this study aid in explaining the trends of larger phytoplankton onshore and picophytoplankton offshore as nitrates are found more readily onshore while organic nitrogen sources are found offshore.

Acknowledgements

I would like to thank Dr. Nolan for all her help- running down to Wallops, helping in the lab, and all the long hours pouring over data and peering into microscopes! And to Danny who helped us determine how upset phytoplankton get when they are shipped up to York College!

I would like to thank NASA and the Marine Science Consortium for the use of their ship and time in collecting my samples.

I would like to thank Sally Hoh for letting me in and out of labs and helping me find the oddest things for my research.

I would like to thank Graham, my family and my friends for their continual support and proofreading skills.

MethodsMethods

Samples in 50mL conical tubes kept in growth chamber and a

fluorescence measurement was taken once every other day for twoweeks using a TD-700 Fluorometer.

ResultsResultsCollected samples on the Virginia Shelf from two stations (onshore and offshore) (Figure 1.)

Put samples into respective treatment groups-

3 replicates for each treatment group both onshore

and offshore

Samples preserved in Formalin and cell counts were

performed via fluorescence microscopy to determine

phytoplankton composition ReferencesReferences

Fogg, G.E. 1991. Tansley Review #30. The Phytoplanktonic Ways of Life. New Phytologist. 118(2):191-232.  Moore, L.R., Post, A.F., Rocap, G., Chisholm, S.W. 2002. Utilization of different nitrogen sources by the marine cyanobacteria Prochlorococcus and Synechococcus. Limnol. Oreanography. 47(4):989-996.  Raven, J.A. 1998. The twelfth Tansley Lecture. Small is beautiful: the picophytoplankton. Function Ecology. 12:503-513. 

Light

-Control (110µE m-2 s-1)

-High (420µE m-2 s-1)

-Low (40µE m-2 s-1)

(No nutrients added)

Nutrients

-NO3

(3.75mg/50mL)

-PO4 (.25mg/50mL)

-Urea (1.33mg/50mL)

Predation(Dilution

Experiment)

-25%

-50%

-75%

(See figure 2)

Figure 1. Map of the Virginia Shelf. Samples were taken from beginning and end of transect (in red).

Figure 2. Dilution series. Vials filled with given percentage of sample and remainder then filled with filtered (0.2um) sample water.

Figure 5. Onshore and offshore cell counts. Two-way Anova with matching Bonferroni post test revealed a statistically significant count of Synechococcus over diatoms, dinoflagellates, and picophytoplankton in general (picophytoplakton is a broad term encompassing all other picophytoplankton excluding Synechococcus). This graph shows mean ± SEM, the data was analyzed via a 2-way ANOVA and a Bonferroni post test. A separate analysis utilizing an unpaired t-test, compared onshore versus offshore Synechococcus yielded no significant difference.

An Examination of Onshore and Offshore Picophytoplankton Communities from the Virginia Shelf

Amanda Williams and Dr. Jessica NolanAmanda Williams and Dr. Jessica NolanDepartment of Biological Sciences, York College of PennsylvaniaDepartment of Biological Sciences, York College of Pennsylvania

Light

Light treatments both onshore and offshore showed an increase in fluorescence in low light samples which may reflect their adaptation to low light.

Nutrients

Significantly more growth occurred in the urea treatment then all other treatments.

Cell counts in the urea nutrient treatment revealed a significantly higher number of Synechococcus present.

Predation

There is no significant difference in grazing rate onshore and offshore, perhaps this is a reflection of a larger population of small-sized grazers offshore.

Figure 3. Relative fluorescence of light and nutrients samples over the two week experiment. A. Onshore light treatment. B. Offshore light treatment. C. Onshore nutrient treatments. D. Offshore nutrient treatments. The graph shows mean ± SEM, the data was analyzed via a 2-way ANOVA and a Bonferroni post test. An * indicates a significant difference among treatments.

A B

C D

Future QuestionsFuture QuestionsWould Synechococcus respond to urea the same way throughout the year or are they limited to summer samples?

Are the grazers of the phytoplankton assemblages focused on a specific size of phytoplankton?

Will Synechococcus respond to other available forms of organic nitrogen (i.e. ammonia) with such rapid growth?

Figure 4. Growth and grazing measured onshore and offshore. Data analyzed via an unpaired t-test.