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Effects of temperature and red tides on sea urchin abundance and species richness over 45 years in southern Japan

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By Nicole Suren, SRC intern

Between 1963 and 2014, scientists in Japan have conducted 45 years of near continuous monitoring of the abundance (number of individuals), species richness (number of species), and developmental abnormalities of the sea urchins around Hatakejima Island. Hatakejima Island has been a marine protected area since 1968, meaning that humans are forbidden from harvesting sea urchins in the area. Removing fishing pressure makes this area the ideal study site to examine the effect of abiotic factors such as sea surface temperature and red tide events on sea urchin population dynamics, which is important since echinoderms (the family containing sea urchins) are often keystone or dominant species in an ecosystem.

Figure 1. Location of Hatakejima Island within Tanabe Bay, Japan. (Source: Ohgaki et al., 2018)

Urchin populations near Hatakejima Island were monitored using three complementary methods. The first was a quadrat study, where the urchins in a permanent underwater quadrat were counted once every year. The second was a coastal survey, where a more general sea urchin count was conducted all along the coast of Hatakejima Island six times total. The third was a developmental assay, where eggs and sperm were collected, fertilized in vitro, and the resulting embryos were monitored for early developmental abnormalities. Overall, the scientists found that the sea surface temperature increased over thirty years, and that developmental abnormalities coincided with the occurrences of red tides.

Figure 2. Population trends of the three most common species of urchin from the study over time. (Source: Ohgaki et al., 2018)

Red tide events, temperature, and ocean currents were found to be closely related to the abundance of the three most common species of urchin: H. crassispina, E. moralis, and Echinometra spp. Exact effects varied depending on species, but red tide events were found to decrease abundance (likely due to the developmental disruption of urchin larvae), while warmer temperatures and proximity to the Kuroshiro current had positive effects on abundance and species richness.

Although this population of sea urchins is not subject to fishing pressure, it is far from unaffected by humans. An increased incidence of red tide events in the area may be attributable to an increase in aquaculture nearby. Furthermore, chemicals like tributyltin (TBT) and other organotin compounds used in fish nets and ships are being introduced to the water, which may also have negative developmental effects that decrease population size. In addition to the other human effects, anthropogenic climate changes also affect urchin abundance and species richness in this area because of their dependence on a particular temperature range. Studies like this one are essential to determining the full extent of human impacts on ecosystems, and should continue to be employed so we can decide how best to mitigate those impacts (Ohgaki et al., 2018).

Work Cited

Ohgaki, S. I., Kato, T., Kobayashi, N., Tanase, H., Kumagai, N. H., Ishida, S., … Yusa, Y. (2018). Effects of temperature and red tides on sea urchin abundance and species richness over 45 years in southern Japan. Ecological Indicators, (January), 0–1. https://doi.org/10.1016/j.ecolind.2018.03.040

Harmful algal blooms

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by James Komisarjevsky, RJD intern

Harmful Algal Blooms (HABs) are something that affects almost every coastal region in the world. Many people know of the these HABs as red tides (Figure 1). These HABs not only affect coastal regions but have also been found to affect open ocean, brackish ecosystems, and freshwater ecosystems. Most of these HABs have been found to be caused by blooms of microscopic algae or by blooms of phytoplankton.

Figure 1. This figure depicts a red tide caused by an algal bloom. (Source: Erin Watson, University of Tasmania)

There has recently been an increase in the number of HABs due to an increase in a waters nutrient supply. An increase of nutrients such as nitrogen, phosphorous, silicate, and micronutrients can lead to a HAB. There is a concern today for the relationship between HABs and the increase of nutrient supply  (eutrophication). Humans are a big component of the increase of nutrients because of sewage, groundwater and atmospheric inputs, and agriculture and aquaculture runoffs. When compared to preindustrial and preagriculture times, it has been estimated that phosphorous levels have increased three times in the world’s oceans. An example of this can be seen in China where there have been major changes in the quality of its coastal waters. Due to Chinas increasing populations and its rise in agriculture, there has been a fourfold increase of nitrate concentrations and a 30% increase of phosphate concentration in the past 40 years at the mouth of the Changjian River. The increase of nutrients due to humans has led to a significantly higher algal biomass and also a change in the phytoplankton community.A HAB is considered to be a concept rather than a scientific definition. There are certain criteria, which if met, an event is considered to be a HAB. The criteria contains causing injury to human health, causing socioeconomic interest, or changes certain components of an ecosystem.  These HABs can cause illness or the deaths of marine organisms by the transfer of toxins through the food web (bioaccumulation). They can also cause an injury to human health due to the consumption of seafood which contain these toxins.

Climate change and overfishing have also caused concern for the increase of HABs. Prediction of the effects of climate change can only be speculated. This is because there is little long-term data of algal blooms for a specific region. Generally, at least 30 years of data is needed to establish trends on HABs. It is thought that some harmful species of algal blooms may become more prevalent while others may be negatively affected. Climate change will pose challenges for scientist because it is likely to cause changes in species composition, abundance, and the timing of HABS.

In recent years there has been an increase in the study ecogenomics and HAB species genes in order to further study HABs. Ecogenomics have caused interest because it gives scientist a way to study HABs and its surrounding ecosystem. This will allow for further information to be discovered about the consequences of HABs ecologically and their affect on other species evolutionarily. Also studying the genetics of HAB species will help scientist locate which genes are responsible for certain toxin production. This will help to classify the different species, which contain or do not contain the genes responsible for toxin production.

There is an increasing demand for the study of HABs and the species which cause them. Due to the anthropogenic affects such as eutrophication and overfishing, an increase in the number of HABs can have many affects on humans. Further and more studies of HABs will hopefully provide insight into helping us lower the numbers of HABs in the future.

REFERENCES

Anderson, Donald M., Allan D. Cembella, and Gustaaf M. Hallegraeff. “Progress in Understanding Harmful Algal Blooms: Paradigm Shifts and New Technologies for Research, Monitoring, and Management.” Annual Review of Marine Science 4 (2012): 143-76. Print.