Study of the dynamic of phytoplankton growth and its relation to variability of solar radiation intensity and dissolved inorganic nutrient in Banten Bay

Abstract

The growth of phytoplankton in the waters mostly depends on sunlight intensity and dissolved inorganic nutrient present in euphotic zone. This research aimed to count phytoplankton growth including growth rate, doubling time, and generation time, and to analyze growth rate response of phytoplankton to solar radiation intensity and dissolved inorganic nutrient. Measurement of phytoplankton growth was conducted from April to September 2008 at five stations. Station 1 was situated at the mouth river, station 2 at the middle of the Bay toward the beach, station 3 at the middle of the bay, station 4 located at the middle toward outside the bay, and station 5 located outside the bay. Phytoplankton growth at each station was measured at various water depth. At station 1, measurement was done at three euphotic depth namely at 10%, 5%, and 1% from surface irradiance. At station 2, 3, and 4, growth measurement was conducted at 50%, 25%, 10%, and 5% from surface irradiance while at station 5, phytoplankton growth was measured at five euphotic depth namely 50%, 25%, 10%, 5%, and 1% from surface irradiance. Research result showed that phytoplankton growth occurred faster at intermediate light intensity and intermediate concentration of dissolved inorganic nutrient. However, the role of light intensity and dissolved inorganic nutrient in controlling phytoplankton growth was not synergic. Temporally, light intensity did not really control the growth of phytoplankton as compared to dissolved inorganic nutrient in term of DSi because sunlight intensity from April to September was relatively stable compared to DSi that tended to be various. Spatially, on the other hand, light intensity and dissolved inorganic nutrient in term of DIN, DIP, and DSi simultaneously controlled phytoplankton growth. This occurred due to light intensity was varied and tended to be homogenous from station 1 to station 4 and low at station 5. Different phenomena also occurred with DIN, DIP, and DSI which was varied and tended to be higher at station 1 and homogenous at station 1 to station 4. However, DSi tended to be more varied than DIN and DIP. Based on this finding, it was stated that DSi was more dominant in controlling phytoplankton growth in Banten Bay and categorized as fast growth rate ranging from 0.66-2.44 day-1, doubling time 0.34-2.04.day-1, and generation time ranging from 0.91-3.53 day-1.

Pertumbuhan fitoplankton di perairan sangat tergantung pada ketersediaan intensitas cahaya matahari dan nutrien inorganik terlarut yang berada di zona eufotik. Permasalahannya adalah ketersediaan intensitas cahaya matahari dan nutrien inorganik terlarut ini tidak selalu berada pada kondisi yang serasi untuk menunjang pertumbuhan fitoplankton. Hal ini tentu akan menyebabkan pertumbuhan fitoplankton di perairan selalu berada pada kondisi yang tidak optimal. Berdasarkan hal ini maka penelitian ini dilakukan dengan berbagai pendekatan dengan melakukan pengukuran pertumbuhan fitoplankton pada beberapa kedalaman eufotik. Penelitian ini bertujuan untuk menghitung pertumbuhan fitoplankton yang meliputi laju pertumbuhan, waktu doubling, dan waktu generasi fitoplankton serta menganalisis respon laju pertumbuhan fitoplankton pada perubahan intensitas cahaya matahari dan nutrien inorganik terlarut. Pengukuran pertumbuhan fitoplankton dilakukan dari bulan April sampai September pada lima stasiun. Stasiun satu berada pada muara sungai, stasiun dua berada pada perairan pantai, stasiun tiga berada pada bagian tengah teluk tapi lebih mengarah ke pantai, stasiun empat berada pada bagian tengah teluk tapi lebih mengarah ke luar teluk, dan stasiun lima berada pada luar teluk. Pengukuran pertumbuhan fitoplankton pada setiap stasiun dilakukan pada kedalaman yang berbeda-beda. Pada stasiun satu dilakukan pada tiga kedalaman eufotik yang secara berturut-turut terdiri dari kedalaman eufotik 10%, 5%, 1% dari cahaya permukaan. Pada stasiun dua, tiga, dan empat dilakukan pada empat kedalaman eufotik yang secara berturut-turut terdiri dari kedalaman eufotik 50%, 25%, 10%, dan 5% dari cahaya permukaan. Sedangkan pada stasiun lima dilakukan pada lima kedalaman eufotik yang secara berturut-turut terdiri dari kedalaman eufotik 50%, 25%, 10%, 5%, dan 1% dari cahaya permukaan.