The effect of environmental factors on the chemical constitution of a naturally-occuring phytoplankton population prior to and during the spring diatom increase in Saanich inlet.
Date
1977
Authors
Roberge, Marlene Helene Marie
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Abstract
Certain environmental factors controlling the initiation and development of the spring diatom maximum in Saanich Inlet, B.C. were investigated in detail between March 22 and May 30 , 1971 . Changes in the selected physical and chemical characteristics of the water column to 50 m were measured at two-day intervals and were correlated with changes in phytoplankton biomass and biochemical composition.
During this period two peaks in phytoplankton biomass occurred. A minor increase in algal standing stock began to develop in mid-April under the influence of a calm, sunny period. This was interrupted in early May by the recurrence of changeable weather and unstable water conditions. A brief period of nutrient regeneration followed. Through early and mid-May, phytoplankton levels remained low, while weather continued unstable. The observations indicate that sub-optimal weather, water column instability and heavy grazing pressure combined to suppress an early diatom maximum.
In late May weather stabilized and a strong density stratification pattern was set up in the upper 30 m of the water column. Initiation of the second and larger peak in diatom biomass coincided with the penetration of the 8.5°C. isotherm into deeper water. Light conditions in the same period increased from 400 ly/day to 600 ly/day.
In situ conditions during this period were near optimal for the growth of the bloom-forming diatom Thalassiosira rotula Meunier, which at the peak of its growth in late May reached a doubling time of 10 hours, very close to the minimum doubling time observed by Schöne (1972) in culture under similar conditions of light and temperature.
The progress of this diatom increase was marked by a number of changes in the relative amounts of chemical constituents in the filtered material. These changes were consistent with the progress of a phytoplankton population from active log-phase growth through steady-state and into senescence, and could be related to increased self-shading and to nutrient deficiency. In the progress of the bloom carotenoid to chlorophyll-a ratios rose, as did protein to carbohydrate ratios. At the same time carbohydrate to chlorophyll-a ratios, protein to chlorophyll-a ratios and protein to carbohydrate ratios all dropped. Observations of these phenomena in culture experiments by other workers have been associated with nitrogen limitation.
A strong inverse relationship exists between the daily incident radiation budget and the ambient nitrate level in surface waters for the first part of the study, March 22 to April 22. This effect was not observed after daily light budgets surpassed 500 ly/day in the latter part of May, instead a strong inverse relationship was formed between water column temperature and the nitrate levels found at specific depths. These two relationships show similar slopes but are complementary in nature. The shift from one effect to the other occurs at light levels between 500 and 600 ly/day, water temperatures between 9 and 16°C. and -3 at nitrate-nitrogen levels of 12 to 16 µg-at N0 3 Nm⁻³
The warming of the water column that brought about the second, temperature-related, diatom increase was the result of more prolonged weather stability towards the end of May and the resultant increase in water stability.
From this it can therefore be seen that light and temperature were the two factors which most markedly affected the induction of the spring diatom increases in Saanich Inlet, while stability of weather and the water column played a secondary role in this process.