Qianguo Xing

World’s largest macroalgal bloom caused by expansion of seaweed aquaculture in China

In late June 2008, just weeks before the opening of the Beijing Olympics, a massive green-tide occurred covering about 600km(2) along the coast of Qingdao, host city for Olympic sailing regatta. Coastal eutrophication was quickly attributed with the blame by the international media and some scientists. However, we explored an alternative hypothesis that the cause of the green-tide was due to the rapid expansion of Porphyra yezoensis aquaculture along the coastline over 180km away from Qingdao, and oceanographic conditions which favoured rapid growth of the bloom and contributed to transport of the bloom north into the Yellow Sea and then onshore northwest to Qingdao. At its peak offshore, the bloom covered 1200km(2) and affected 40,000km(2). This is the largest green-tide ever reported, the most extensive translocation of a green-tide and the first case of expansive seaweed aquaculture leading to a green-tide. Given similar oceanographic conditions to those that occurred in 2008, these green-tides may re-occur unless mitigation measures such as those proposed here are taken.

Development of a Semi-Analytical Algorithm for the Retrieval of Suspended Particulate Matter from Remote Sensing over Clear to Very Turbid Waters

Remote sensing of suspended particulate matter, SPM, from space has long been used to assess its spatio-temporal variability in various coastal areas. The associated algorithms were generally site specific or developed over a relatively narrow range of concentration, which make them inappropriate for global applications (or at least over broad SPM range). In the frame of the GlobCoast project, a large in situ data set of SPM and remote sensing reflectance, Rrs(λ), has been built gathering together measurements from various coastal areas around Europe, French Guiana, North Canada, Vietnam, and China. This data set covers various contrasting coastal environments diversely affected by different biogeochemical and physical processes such as sediment resuspension, phytoplankton bloom events, and rivers discharges (Amazon, Mekong, Yellow river, MacKenzie, etc.). The SPM concentration spans about four orders of magnitude, from 0.15 to 2626 g·m−3. Different empirical and semi-analytical approaches developed to assess SPM from Rrs(λ) were tested over this in situ data set. As none of them provides satisfactory results over the whole SPM range, a generic semi-analytical approach has been developed. This algorithm is based on two standard semi-analytical equations calibrated for low-to-medium and highly turbid waters, respectively. A mixing law has also been developed for intermediate environments. Sources of uncertainties in SPM retrieval such as the bio-optical variability, atmospheric correction errors, and spectral bandwidth have been evaluated. The coefficients involved in these different algorithms have been calculated for ocean color (SeaWiFS, MODIS-A/T, MERIS/OLCI, VIIRS) and high spatial resolution (LandSat8-OLI, and Sentinel2-MSI) sensors. The performance of the proposed algorithm varies only slightly from one sensor to another demonstrating the great potential applicability of the proposed approach over global and contrasting coastal waters.

Interpreting the progressive eutrophication behind the world's largest macroalgal blooms with water quality and ocean color data

World’s largest macroalgal blooms (MAB) caused by Ulva prolifera outbreak every summer in the Yellow Sea since 2007, which bring potential damage to regional marine environment and economics. Assessing the evolution of historic eutrophication status is of importance for exploring the reasons for the blooms. In this study, we used an area-weighted nutrient pollution index (AWCPI-NP) and ocean color data to assess temporal changes in eutrophication status in the Yellow Sea basin. Results show that significant shifts occurred from the pre-MAB phase (2001–2006) to MAB phase (2007–2012): the average AWCPI-NP was higher than that in the pre-MAB phase (2001–2006) by about 45xa0%; chlorophyll a concentration, an indicator of eutrophication, increased by about 15xa0% in the region of MAB. Meanwhile, in the Jiangsu Shoal, the origin place of the floating macroalgae, the annual in situ nutrient concentrations increased rapidly from 2000 to 2011. The result obtained through our approach highlights that the MAB in the Yellow Sea might be primarily linked to the progressive eutrophication. In order to reduce this hazard, policy makers shouldxa0include this new knowledge for an effective countermeasure planning.

Long-term trend of Ulva prolifera blooms in the western Yellow Sea

Blooms of the green macroalga Ulva prolifera in the western Yellow Sea occurred every year since 2008, and they have been reported and studied extensively using a variety of means including remote sensing. However, to date, long-term bloom patterns have not been reported except for a few case studies showing examples in different years. Here, using MODIS observations and an objective method to perform statistical analysis, mean Ulva coverage in the western Yellow Sea has been derived and analyzed between 2007 and 2015 at both monthly and annual scales. On annual scale, mean Ulva coverage decreased after 2008, but increased rapidly after 2012 from 8km2 in 2012 to 116km2 in 2015 (the largest ever reported in history for this region). In the month of June the mean coverage increased from 18km2 in 2012 to 363km2 in 2015. Other than 2009 and 2010, the month of June showed maximum Ulva coverage in every year. These coverage estimates are significantly lower than previously reported values as they represent pure algae coverage after taking into account of partial pixel coverage. Several environmental factors were examined in an attempt to determine the reasons behind such long-term changes, yet the results are inconclusive, suggesting a strong necessity of further coordinated and multi-disciplinary researches.

Geochemical characteristics of phosphorus in surface sediments of two major Chinese mariculture areas: The Laizhou Bay and the coastal waters of the Zhangzi Island

Phosphorus (P) in surface sediments of the Laizhou Bay (LB) and the coastal waters around the Zhangzi Island (ZI) was analyzed. Six forms of P were separated - exchangeable or loosely sorbed P (Ads-P), aluminum-bound P (Al-P), iron-bound P (Fe-P), authigenic apatite plus CaCO3-bound P plus biogenic apatite (Ca-P), detrital apatite plus other inorganic P (De-P) and organic P (OP). The average contents of P in the LB were in the order: De-P>OP>Ca-P>Fe-P>Ads-P>Al-P; in the ZI, the corresponding order was De-P>OP>Fe-P>Ca-P>Ads-P>Al-P. Due to the high nutrient loadings from the surrounding rivers, TP contents in sediments of the LB were higher than in those of the ZI. The potential bio-available P (Ads-P and OP) accounted for 14.7% and 24.2% of TP in sediments of the LB and the ZI, respectively.

World's Largest Macroalgal Blooms Altered Phytoplankton Biomass in Summer in the Yellow Sea: Satellite Observations

Since 2008, the worlds largest blooms of the green macroalgae, Ulva prolifera, have occurred every summer in the Yellow Sea, posing the question of whether these macroalgal blooms (MABs) have changed the phytoplankton biomass due to their perturbations of nutrient dynamics. We have attempted to address this question using long-term Moderate Resolution Imaging Spectroradiometer (MODIS) observations. A new MODIS monthly time-series of chlorophyll-a concentrations (Chl-a, an index of phytoplankton biomass) was generated after removing the macroalgae-contaminated pixels that were characterized by unexpectedly high values in the daily Chl-a products. Compared with Chl-a during July of 2002-2006 (the pre-MAB period), Chl-a during July of 2008-2012 (the MAB period) exhibited significant increases in the offshore Yellow Sea waters (rich in macroalgae), with mean Chl-a increased by 98% from 0.64 mu g/L to 1.26 mu g/L in the study region. In contrast, no significant Chl-a changes were observed during June between the two periods. After analyzing sea surface temperature, photosynthetically available radiation, and nutrient availability, we speculate that the observed Chl-a changes are due to nutrient competition between macroalgae and phytoplankton: during the MAB period, the fast-growing macroalgae would uptake the increased nutrients from the origin of Jiangsu Shoal; thus, the nutrients available to phytoplankton were reduced, leading to no apparent increases in biomass in the offshore Yellow Sea in June.

Using in situ and Satellite Hyperspectral Data to Estimate the Surface Suspended Sediments Concentrations in the Pearl River Estuary

In situ remote sensing reflectance (Rrs) collected during 2004-2006 and the planetary reflectance (Rp) derived from EO-1/Hyperion image, are tested for estimating the surface total suspended matter (TSM), total inorganic particles (TIP) and water turbidity in the Pearl River Estuary (PRE). The in situ data show that the content of TIP and turbidity is proportional to the concentration of TSM which ranges from 6 mg/L to 140 mg/L. The band-subtraction of Rrs at 610 nm and 600 nm, [Rrs(610)-Rrs(600)], and the subtraction of the 26th and 25th Hyperion bands (609.97 nm and 599.80 nm), [Rp(B26)-Rp(B25)], are used in an exponential regression model to estimate the TSM concentrations, the mean relative errors between the estimated and measured TSM are 27.2% and 23.3%, respectively for Rrs and Rp, and the root mean square errors of estimation are 12.6 mg/L and 5.9 mg/L, respectively. This band-subtraction of two neighboring bands shows better performance than several popular single-band and band-combination models. This good performance may be mainly attributed to the band-subtraction of the two neighboring bands which improves the sensitivity of reflectance to suspended sediments by reducing the background impacts from water surface reflection and path radiance at the specific wavelengths. These methods and findings with the high spatial and high spectral resolution data may be used for the remote sensing of turbid estuary waters although further validation work with a wider range of TSM concentration may be necessary.

Method of integrating landsat-5 and landsat-7 data to retrieve sea surface temperature in coastal waters on the basis of local empirical algorithm

A useful radiance-converting method was developed to convert the Landsat-7 ETM+thermal-infrared (TIR) band’s radiance (Lλ,L7/ETM+) to that of Landsat-5 TM TIR (Lλ,L5/TM) as: Lλ,L5/TM=0.9699xLλ,L7/ETM++0.1074 (R2=1). In addition, based on the radiance-converting equation and the linear relation between digital number (DN) and at-satellite radiance, a DN-converting equation can be established to convert DN value of the TIR band between Landsat-5 and Landsat-7. Via this method, it is easy to integrate Landsat-5 and Landsat-7 TIR data to retrieve the sea surface temperature (SST) in coastal waters on the basis of local empirical algorithms in which the radiance or DN of Lansat-5 and 7 TIR band is usually the only input independent variable. The method was employed in a local empirical algorithm in Daya Bay, China, to detect the thermal pollution of cooling water discharge from the Daya Bay nuclear power station (DNPS). This work demonstrates that radiance conversion is an effective approach to integration of Landsat-5 and Landsat-7 data in the process of a SST retrieval which is based on local empirical algorithms.

Estimation of Chlorophyll-a Concentrations in the Pearl River Estuary Using In Situ Hyperspectral Data: A Case Study

Taking Pearl River Estuary (PRE), China as an example, we explored the potential of in situ hyperspectral data in estimating chlorophyll-a concentrations of turbid waters. Two cruises were conducted on August 21, 2006 and May 18, 2004 to collect the data of water quality and remote sensing reflectance (Rrs). The field surveys showed that chlorophyll-a concentration ranged from 2.97 mu g/L to 49.97 mu g/L, and turbidity 13.6-128.9 NTU. The Rrs spectra were binned to 10 nm resolution, and then processed to be first-order and second-order derivatives. A linear algorithm is developed to estimate chlorophyll-a concentrations based on second order derivative at 670 nm; its mean relative errors of estimation is less than 58% and the root mean square error is 6.69 mu g/L, which is better than other popular algorithms for turbid waters, i.e., the ratio of Rrs at 700 nm and 670 nm. The Case-I algorithm of blue-green band ratio is also proved to be a failed application in PRE, and so does the algorithm of fluorescence line height (FLH), which is questionable for its application in waters with strong light scattering and absorption. All the above work was done without classification of cloud conditions. This suggests that the second-order derivative at 670 nm could be effective for estimation of chlorophyll-a concentrations in turbid waters especially in situ.

A review of environment problems in the coastal sea of South China

The coastal sea of South China provided an important habitat for protection and propagation of marine organisms. Rapid economic development and human activities, such as wastewater discharge, reclamation, overfishing and aquaculture in the South China Sea had already resulted in environmental degradation, and thus caused sharp contradictions between exploitation and protection of the coastal sea of South China. In this present article, the main environment problems and degradation trends were reviewed based on literatures and other sources of information, which mainly referenced nutrient pollution, persistent organic pollution and metal pollution, decrease of biodiversity, reduction of marine habitat and frequent natural and ecological disasters. The current efforts in China on protecting the environment in the coastal sea of South China were discussed, which included improving legislation by formulating a series of laws and regulations at national or local level, setting up natural reserves, and supporting research projects. There were many challenges regarding policy, management and science research to protect and sustain the coastal sea of South China, such as imperfect legal and administrative systems, lack of public participation, poor financial support and lack of monitoring and evaluation. Finally, some recommendations were put forward for the sake of the sustainable use of the environment in the coastal sea of South China, including reinforcing the planning of marine resource exploitation and use through integrated coastal zone management, strengthening the marine environment and protection awareness of the public, and scientifically establishing the fishery spawning spots and aquatic reserves.