Analysis of the inter-annual variability and southward expansion of red tides in Zhejiang coastal waters from 1981 to 2018
Abstract: A time series dataset spanning 39 years (1981 to 2018) on red tide events in Zhejiang coastal waters was used to study the characteristics of inter-annual spatial and temporal variations. A distinct inter-annual pattern characterized by low frequency, explosive growth and fluctuating decline stages was found over the studied time scale. Most red tide events occurred in parallel to the bathymetric contour, and 95.4% were located to the west of the 50-m isobath. Additionally, the high-incidence area of red tides is expanding southward. In this paper, local sea surface temperature (SST), mariculture area and secondary industry growth rate are introduced and identified as the main factors influencing the nutrient and hydrometeorological conditions. A multivariate nonlinear regression equation based on these factors was constructed, and the goodness of fit coefficient was 0.907. The causes of the annual variation and high-frequency area in the southward expansion were quantitatively analyzed based on the proposed regression model. Finally, the results indicated that 68.7% of the annual occurrence variation of red tide was due to the SST and mariculture area, which are the main impact factors; however, secondary industry growth could compensate for the nutrient deficiency caused by the sharp mariculture area reduction and decreased SST. The background nutrient level, which is elevated by coastal economic development, especially secondary industry, is the main determinant of the southward expansion. Although the trend of the southward expansion of high-frequency areas has not changed, the red tide frequency in coastal cities has decreased by half and remained at a stable level after 2010 due to substantial economic restructuring and environmental protection.
Table 1. List of the main impact factors that are closely related to the long-term change of red tide
Data category Data record elements Data cycle Domain or station Specifications or technical standards Notes on information Red tide events Time, location, maximum area, dominant species 1981–2015 Zhejiang coastal waters Technical specifications for red tide monitoring(HY/T069—2005) Dataset from 1981 to 2006 is reorganized by the Zhejiang Ocean Monitoring and Forecasting Center, and dataset from 2007 to 2015 is provided by the Zhejiang Ocean Monitoring and Forecasting Center. 2016–2018 Monitoring data of 2016–2018 is provided by East China Sea Monitoring Center. Marine hydrometeorology SST 1981–2018 Shengshan and Nanji stations Specification for coastal observation (GBT14914–2006) Daily operational observation data from 1981 to 2018 provided by the East China Sea Forecast Center. SSTA Sea surface temperature anomaly SSTA10a 10–year average SSTA ENSO index Niño3.4 area Monthly mean SSTA in Niño3.4 area National Oceanic and Atmospheric Administration (https://www.cpc.noaa.gov) Marine water environment Proportion of class IV to classes less than IV 2000–2018 Zhejiang coastal waters Sea water quality standard (GB3097–1997) Zhejiang Marine Environmental Bulletin issued by the Department of Ecology and Environment of Zhejiang Province (http://sthjt.zj.gov.cn.) Indicators of socio-economic development Gross domestic product (GDP) 1981–2018 Zhejiang Province Added-value index of primary, secondary and tertiary industries Statistical Yearbook of China issued by the State Statistical Bureau (http://data.stats.gov.cn.) Ningbo City Table 3–2 GDP development index over the years 2019 Ningbo Statistical Yearbook issued by the Ningbo Municipal Statistics Bureau (http://vod.ningbo.gov.cn/) Wenzhou City Table 1–10 GDP development index over the years 2019 Wenzhou Statistical Yearbook issued by the Wenzhou Municipal statistics bureau (http://wztjj.wenzhou.gov.cn.) Maricultural area 1981–2016 Zhejiang Province Total maricultural area for agriculture Statistical Yearbook of China issued by the State Statistical Bureau (http://data.stats.gov.cn.)
Table 2. Nonparametric test statistics of the normal distributions of red tide occurrence and factors
Factor Shapiro-Wilk Skewness Kurtosis Statistics Freedom Significance Statistics Statistics YAORT 0.835 38 0 1.34 1.215 XSST 0.957 38 0.147* 0.365 –0.344 XNino3.4 0.985 38 0.881* 0.257 –0.339 Xmar 0.923 36 0.015* 0.3 –1.416 Xwq 0.96 19 0.572* 0.577 –0.264 Xpri 0.886 38 0.001 0.674 –0.838 Xsec 0.814 38 0 0.924 –0.633 Xter 0.763 38 0 1.44 1.051 Note: * When the significance is >0.05, the factor exhibits normal or partially normal distributions.
Table 3. Rank correlation coefficient of red tide occurrence and factors
Variable code (YAORT) 7 SST Nourishment Water Quality XSST XNiño3.4 Xmar Xpri Xsec Xter Xwq 1 8 2 3 4 5 6 YAORT Correlation coefficient 1 .326* 0.107 .743** .394* .696** .461* 0.22 Sig (two-sided) . 0.046 0.524 0 0.016 0 0 0.366 N 38 38 38 36 37 37 37 19 Note: * When the confidence coefficient (two-sided test) is 0.05, the correlation is significant; ** when the confidence coefficient (two-sided test) is 0.01, the correlation is significant.
Table 4. ANOVA of the multivariate nonlinear regression equation
ANOVA Source Sum of squares Degrees of freedom Mean square Regression 26 504.633 9 2 944.959 Residual 1 410.367 26 54.245 Total before correction 27 915 35 Total after correction 15 127.543 34 Note: R2=1–(residual sum of squares)/(corrected sum of squares) = 0.907
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