Environmental and seasonal dynamics altering the primary productivity in Bingo-Nada (Bingo Sound) of the Seto Inland Sea, Japan

Abstract

Monthly phytoplankton monitoring with primary productivity estimation using pulse amplitude modulation (PAM) fluorometry was conducted from May 2014 to March 2018 in Bingo-Nada, center of the Seto Inland Sea, Japan, where oligotrophication has been a concern. The average chlorophyll a concentration, 7.24 µg L −1 , was even higher than the values of previous studies; therefore, it should not be assumed that the primary productivity has decreased. However, some seasonal trends seemed to drastically change; one of them was notable diatom blooms from December to January. PAM fluorometry revealed that high Fv/Fm of the diatoms due to rich nutrients and high α values (initial slope of ETR versus light) enabled them to actively photosynthesize even under low-light conditions in winter. This winter bloom further caused severe nutrient depletion in spring, which resulted in notable drops in overall photosynthesis and the ETR-based primary production rate (PPR ETR) in March or April. From May, Fv/Fm and rETR max , which began to increase from their minimum values in April, peaked in July or August when the highest PPR ETR was recorded. However, over 70% of the total phytoplankton population consisted of dinoflagellates, raphidophytes and silicoflagellates, not diatoms. The PPR ETR then drastically decreased from August to September, regardless of the fact that there were rich nutrients in the entire water column. Rather, decreased light level, as well as increased turbidity caused by non-phytoplankton particles, especially those abundant in this season, might hinder photosynthesis in autumn, further resulting in nutrient-rich conditions leading to the winter bloom.

High light stress under phosphorus limitation in summer may accelerate diatom shift from Skeletonema to Chaetoceros in an oligotrophic coastal area of Japan

“…Indeed, even under prolonged exposure to HL, C. lorenzianus actively induced NPQ, which resulted in the maintenance of higher qP levels than in S. costatum. Based on recent reports that have described a decreasing trend in phosphorus in the Seto Inland Sea (Hayashi et al, 2000;Ohara et al, 2020), we employed continuous cocultures of S. costatum and C. lorenzianus under P-limited condition to simulate the typical summer environment where severe P deficiency and high light occur. The results showed a population shift from S. costatum to C. lorenzianus under both ML and HL conditions, which could support several of the observed phenomena of Chaetoceros succession from Skeletonema in the field, presumably due to ocean oligotrophication (Tada et al, 2014).…”

“…One of these factors might be a change in the photic conditions in the sea. Field observations by Ohara et al (2020) showed that, while Chaetoceros spp. were typically more abundant than Skeletonema spp.…”

The effects of sea-bottom plowing on phytoplankton assemblages: a case study of northern Hiroshima Bay, the Seto Inland Sea of Japan

“…To evaluate the effect of sea-bottom plowing on primary productivity, photosynthetic carbon fixation in the water columns was estimated using a pulse amplitude modulation (PAM) fluorometry based on protocols reported by Maung-Saw-Htoo-Thaw et al ( 2017) and Ohara et al (2020). Conceptually, electron transport rates (ETR) from the photosystem II in a plant (in our case, phytoplankton) under various light intensities were determined for the water samples obtained from 1, 3, 5, 10, and bottom-1 m depths by a cuvette-type PAM fluorometer (Water-PAM, Heinz Walz, Effeltrich, Germany).…”