Analysis of the horizontal distribution of harmful red tides along the Pacific coast of Hokkaido, Japan, in autumn 2021
Analysis of the horizontal distribution of harmful red tides along the Pacific coast of Hokkaido, Japan, in autumn 2021 - Proposal of a hypothesis on the mechanism of occurrence of large-scale harmful red tides -
・Succeed the analysis of the horizontal
community structure of harmful red tide algae on the Pacific coast of Hokkaido
during the fall season of 2021.
・Estimated the mechanism of red tide
occurrence in the east Hokkaido sea area based on a literature survey of past
red tide occurrence.
・Pointed out the importance of early
detection by satellite algorithms and preparation of control measures to reduce
damage from red tide.
A research group led by associate professor
YAMAGUCHI Atsushi and assistant professor MATSUNO Kohei of the Faculty of
Fisheries Sciences, Hokkaido University, and assistant professor IIDA Takahiro of
the Training Ship Ushio-Maru attached to the Faculty of Fisheries Sciences, has
successfully analyzed the community structure of harmful red tide algae in the
Pacific coast of Hokkaido in the autumn season of 2021. Surface water samples
were taken at 32 points along the Pacific coast of Hokkaido last October by the
Ushio-Maru, and phytoplankton communities were observed. The communities were
divided into four groups, and the cell count density of the community dominated
by the red tide-causing algae Karenia selliformis was higher. A
significant positive relationship was observed between chlorophyll a, an
indicator of phytoplankton abundance, and cell number density of K. selliformis.
Analysis of the relationship between environmental factors and cell number
density of K. selliformis showed a significant positive relationship
with nutrient phosphate concentration.
Red tides have been reported along the
Pacific coast of Hokkaido in the fall of 1972, 1983, 1985, and 1986. The common
factor in the occurrence of red tides in each year was the depletion of
nutrients in the surface layer under conditions where the water temperature was
1-3°C higher than usual and a thermocline*1 had developed.
Phytoplankton needs both nutrients and light for photosynthesis to proliferate,
but under these conditions, it is difficult for phytoplankton such as diatoms,
which do not have the ability to move, to proliferate because of the lack of
nutrients in the light-rich surface layer.
On the other hand, because K. selliformis
has the ability to move by flagellum, it is able to dominate by moving to the
lower layers at night to replenish nutrients and then proliferate by
photosynthesis in the surface layer during the daytime. The subsequent collapse
of the density dynamic layer was thought to have resulted in the supply of
nutrients from the lower layers, causing a large-scale harmful red tide. This
study points out the importance of early detection by satellite and control
measures while K. selliformis is still at low density in order to reduce
the damage caused by red tides.
The results of this research were published
in the May 25, 2022, issue of Bulletin of the Japanese Society of Fisheries
(a) Sea surface water temperature at the
time of the survey. Low-temperature Oyashio water existed in the eastern area
(b) Sea surface chlorophyll a at the time
of the survey. It was higher in the low-temperature Oyashio waters of the east
Hokkaido and coincided with the horizontal distribution of K. selliformis.
The “large-scale harmful red tide” that
occurred in the Pacific coastal area of Hokkaido in the fall of 2021 caused the
death of salmon in fixed nets and the mass mortality of short-spined sea
urchins. The damage amounted to 27,900 salmon and 2,800 tons of sea urchins,
and the total amount of fishery damage in the entire Hokkaido region was
reported to be 8.19 billion yen as of February 28, 2022.
The dinoflagellate Karenia selliformis
is believed to be the causative algae of this large-scale harmful red tide. So
far, red tides caused by Karenia species in Japan have been reported
mainly in the Seto Inland Sea and Kyushu coastal areas of western Japan, where
damage was caused by Karenia mikimotoi. The occurrence of this K.
mikimotoi in Hokkaido has been reported in Hakodate Bay in southern
Hokkaido and is believed to have been transported by the Tsushima Warm Current
water moving northward in the Japan Sea. On the other hand, K. selliformis,
the algae causing the large scale harmful red tide of 2021, is a species
described from the South Island of New Zealand in 2004. Red tide formation has
been reported in the Gulf of Mexico, New Zealand, Australia, Tunisia, and
Kuwait, suggesting that K. mikimotoi and K. selliformis probably
have a pan-global distribution.
For the 2021 large harmful red tide, genetic
analysis and cell size of K. selliformis, as well as estimation
oceanographic characteristics of the red tide formation area and of the origin
area using particle tracking models, are being conducted. For the large-scale
harmful red tide K. selliformis, a red tide caused by a genetically
identical strain was reported to have occurred on the eastern coast of the
Kamchatka Peninsula, Russia, in the fall of 2020. In the coastal areas of east
Hokkaido, outbreaks of red tides caused by dinoflagellates have been reported
in 1972, 1983, 1985, and 1986. The mechanism of these red tide outbreaks in the
coastal areas of east Hokkaido is considered to be a “rainfall-type red tide”.
However, the large-scale harmful red tide outbreak in 2021 is not confined to
one particular area of ocean, and the damage area covers a wide geographical
area from off Nemuro to Cape Erimo, so it is necessary to reconsider the cause
of its formation and the mechanism of its occurrence.
In this study, we clarified the cell number
density of K. selliformis and the horizontal distribution of
phytoplankton communities that appeared in samples collected from the west
coast of Cape Erimo to offshore Akkeshi over a wide area from October 6-12,
2021, and summarized previously reported red tides in the east Hokkaido coastal
area, and discussed conditions under which large-scale harmful red tides occur.
From October 6-12, 2021, 1 L of surface
seawater was sampled by the Training Ship Ushio-Maru (179 tons), which is
attached to the Faculty of Fisheries Sciences of Hokkaido University, at a
total of 32 points between the west coast of Cape Erimo and offshore Akkeshi.
Water samples were fixed by adding glutaraldehyde to reach a final
concentration of 1%, and the number of phytoplankton cells was counted.
Chlorophyll a, a pigment contained in phytoplankton, was also measured. To
clarify the conditions for high density of the red tide-causing algae K. selliformis
in the field, a generalized linear model was used to analyze environmental
factors: water temperature, salinity, nitrate, nitrite, ammonium, phosphate,
and silicate concentrations as independent variables, and K. selliformis
cell number density as the objective variable. After creating a similarity
matrix based on cell number density data for each species or genus of
unicellular organisms, a dendrogram was created using the mean linkage method,
and phytoplankton communities were separated by arbitrary similarity levels.
The GCOM-C "Shikisai," a climate change observation satellite
provided by JAXA, was used to evaluate the horizontal distribution of sea
surface temperature and chlorophyll a during the study period.
High densities of K. selliformis
were distributed at fixed sites except off Hiroo, along the Kushiro coast, and
near the shore of Akkeshi. In situ sea surface chlorophyll a concentrations (X:
µg L-1) ranged between 3.6-39.8 µg L-1 and the cell
number density of K. selliformis (Y: cells mL-1), Y = 27.07 X
-110.82, a significant positive relationship with a contribution of 85%. Using
the slope of this regression equation, the intracellular chlorophyll content of
K. selliformis was estimated to be 37 pg cell-1. A
generalized linear model analysis with cell number density of K. selliformis
as the objective variable and environmental factors as explanatory variables
revealed a positive relationship with phosphate among the various nutrients.
Throughout the study area, sea surface
phytoplankton densities ranged from 38-9033 cells mL-1. Cluster analysis
based on cell count densities for each species divided the phytoplankton
communities into four communities, A-D. Of the four phytoplankton communities,
18 of the 32 stationary sites were in community A, which had the highest number
of stationary sites, and community A was dominated by the dinoflagellate K.
selliformis, which accounted for 92% of cell count density, with an average
cell count density of 999 cells mL-1, outperforming the other
communities (77-152 cells mL-1). Sea surface temperatures in the
study area ranged from 13.9-18.1°C and salinities from 27.6-33.7. The sea
surface temperature and chlorophyll a concentration based on satellite data
during the survey period also indicated that high chlorophyll a concentrations
were found in the low-temperature water mass east of Cape Erimo and beyond.
Red tides have been reported to have
occurred along the east coast of Hokkaido in the fall seasons of 1972, 1983,
1985, and 1986. Although the period of the red tides varied from year to year,
they were reported to have occurred from September 3 to October 1, in the
Tokachi coast as the sea area, with dinoflagellates as the causative algae, and
with reduced catches of salmon in set nets as the damage. It is noteworthy that
whenever red tides occur in these eastern Hokkaido waters, there is always a
description that the water temperature is higher than usual.
The genus Karenia has the ability to
move by means of two flagella. Karenia mikimotoi can move vertically
around the water depth of 20 m per day at a speed of 2.2 m h-1,
while K. brevis can move at a speed of 1 m h-1. Karenia
selliformis has also been observed to have extremely high locomotion under
the microscope. The cell size of K. selliformis is about twice as large
as that of K. mikimotoi and K. brevis, suggesting that the
diurnal vertical migration capacity of K. selliformis is high.
The specific gravity of seawater becomes
lighter under high water temperature and low salinity conditions. This means
that when the sea surface is warmer than usual in the low-salinity Oyashio
region, the thermocline will develop strongly. When the water temperature
dynamic layer develops, nutrients in the shallow areas below the layer are
depleted, making it difficult for phytoplankton (diatoms, etc.), which do not
have the ability to move, to proliferate. On the other hand, dinoflagellates of
the genus Karenia, which have high mobility, so enabling diurnal
vertical migration which distributed in the surface layer during the daytime
for photosynthesis, and dive to the depths below the thermocline at night to
replenish nutrients. In 1972, 1983, 1985, and 1986, red tides are explained as
a “rainfall-type red tide” that when the water temperature was higher than
usual and the thermocline developed, only dinoflagellates with high mobility
were able to increase for a long time causing
the species composition was simple, and increased river flow after rainfall
provides nutrients to the coastal zone, which allows a single species to
proliferate and form a red tide.
Large-scale harmful red tides of K.
selliformis in 2021 were observed extensively in the open ocean, and it is
difficult to interpret them as transient “rainfall-type red tides” along the
coast. Considering these factors, the mechanism of K. selliformis red tide
in the Pacific coast of Hokkaido in the autumn of 2021 is as follows: “Seawater
temperature rises → Thermocline strengthens → Diatoms (competing organisms)
decrease → Karenia selliformis with the ability to migrate increase by
supplying nutrients through diurnal vertical migration →Surface community
dominated by K. selliformis→passage of low
pressure → weakening of stratification / vertical mixing / increase in nutrients
in the luminous layer → red tide by K. selliformis” is a possible
Expectations for the future
Satellite monitoring of ocean surface
chlorophyll a has shown that high concentrations of chlorophyll a had already
begun in late August in the case of the large toxic red tide of 2021. The genus
Karenia, due to its auxiliary pigment, has been reported to be
detectable based on satellite data for K. mikimotoi on the Seto Inland
Sea and the south coast of Ireland, and K. brevis on the west coast of
the Florida Peninsula. Based on the algorithm using these satellite data,
detection of Karenia spp. in the east Hokkaido area should be possible.
Control measures for red tide include the promotion of diatom (competing
organisms) germination by seabed cultivation, algal bed creation
(microbiological control), and the application of active clay (coagulation
removal). What is effective as a control measure for K. selliformis
awaits further research and evaluation. However, it is important to detect
water masses dominated by K. selliformis using an auxiliary dye
algorithm on satellite data and to apply some control measures prepared in
advance before the density becomes as high as a red tide.
Title: Horizontal distribution of harmful
red-tide Karenia selliformis and phytoplankton community along the
Pacific coast of Hokkaido in autumn 2021
(1 Faculty of Fisheries
Sciences, Hokkaido University, 2 Graduate School of Fisheries
Sciences, Hokkaido University, 3 Arctic Research Center, Hokkaido
University, 4 Training Ship Ushio-Maru, Faculty of Fisheries,
Journal name: Bulletin of the Japanese
Society of Fisheries Oceanography (Refereed Japanese journal published by the
Japanese Society of Fisheries Oceanography)
DOI: Scheduled to be granted one year after
the print media is published.
Date of publication: Wednesday, May 25,
2022 (Online publication will be available one year later)
Fig. 1 Relationship between chlorophyll a
concentration (X: µg L-1) and number density of Karenia selliformis
cells (Y: cells mL-1).
This equation indicates that the amount of
chlorophyll a in the cells of Karenia selliformis is 37 pg cell-1.
Using this formula, it is possible to estimate the cell number density of K.
selliformis from the chlorophyll a concentration that can be detected from
*1 Themocline: The specific gravity
(density) of seawater is mainly determined by water temperature and salinity,
and the specific gravity of seawater with high water temperature and low
salinity is light, while that of seawater with low water temperature and high
salinity is high. When the ocean surface becomes warmer due to solar radiation,
a discontinuous layer of density due to the difference in water temperature
develops between the warmer water and the cooler water of the deep sea. This pyconocline
caused by the difference in water temperature is called the “thermocline”. The
fact that when you heat a bath, the surface is warm while the bottom remains
cold is an example of the density difference in water caused by the difference
in water temperature.
The area shallow of the thermocline is
called the mixing layer, where seawater mixing occurs, but the density
difference between the shallower and the deeper the thermocline prevents the
mixing of seawater beyond the thermocline. Due to this difference in the mixing
of seawater between the shallower and the deeper the thermocline, the
concentration of nutrients such as nitrogen and phosphorus, which are essential
elements for phytoplankton to carry out photosynthesis, differs greatly between
the depths below the thermocline and the depths above the thermocline.
Nutrient concentrations in the shallow part
of the thermocline are low because phytoplankton consumes nutrients through
photosynthesis, while nutrient concentrations in the deep part are high because
phytoplankton consumes less nutrients and bacteria decompose organic matter.