Long-term variation in phytoplankton assemblages during urbanization: A comparative case study of Deep Bay and Mirs Bay, Hong Kong, China

Highlights

• A 23-year phytoplankton dynamics and water quality data set was present.

• Deep Bay and Mirs Bay significantly differ in nutrients and physical factors.

• Phytoplankton community was less diverse with higher biomass in Deep Bay.

• Significant variables were identified to explain part of phytoplankton dynamics.

• Organic nutrients may be a potential factor to influence phytoplankton dynamics.

Abstract

A long-term dataset, including physicochemical, nutrient, and phytoplankton assemblages from 1994 to 2016, was analyzed to investigate the response of the algal community to variations in environmental factors in Deep Bay and Mirs Bay in southern China. These bays differ in their overall nutrient loadings, as well as in physical factors. The results showed that diatoms were numerically dominant in Mirs Bay, while other minor phytoplankton groups, including eutrophication-tolerant species, constituted the majority in Deep Bay. Phytoplankton community composition tended to be less complex in Deep Bay, suggesting a stressed, unstable and unbalanced ecosystem compared to that in Mirs Bay. Algal blooms occurred more frequently in Mirs Bay, whereas fewer but larger-scale blooms occurred in Deep Bay. Statistically, the combination of all explanatory variables accounted for approximately 55% of the variation in Chlorophyll-a (Chl-a) concentration and less than 20% of the total phytoplankton variation over the 23-year period in the two bays. The high level of nutrients caused by urbanization was not the driving force in the formation of blooms but presumably provided a nutrient base that resulted in blooms with longer durations and covering larger areas.

Introduction

Coastal seas are important areas of oceans and one of the most valuable and vulnerable habitats that are increasingly affected by near-shore human activities (Jickells, 1998; Spatharis et al., 2007; Shen et al., 2011; Wu et al., 2016). Urbanization, which is a major pressure on aquatic ecosystems, has resulted in extensive water quality problems in numerous cities (Xu et al., 2017). Hong Kong shares the waters of two important bays, Deep Bay and Mirs Bay, with Shenzhen, a quickly growing city in mainland China. Deep Bay is a shallow embayment with sediment-laden water in the eastern Pearl River estuary and northwestern part of the New Territories, located at 113°53′–114°0.6′E and 22°24′–22°31′N. It has very low discharge and sustains a unique aquatic ecosystem with the ecologically sensitive Mai Po Inner Deep Bay Ramsar Site and oyster culture areas (Xu et al., 2010; EPD (Environmental Protection Department of Hong Kong), 2018). This bay also has the largest and most important mangrove habitat in Hong Kong, as well as an important feeding ground for a huge number of resident and wintering birds (EPD (Environmental Protection Department of Hong Kong), 2018). Due to rapid economic development and urbanization in Shenzhen and Hong Kong, Deep Bay is becoming increasingly polluted by domestic sewage, industrial waste, and stormwater runoff (Huan et al., 2016). In contrast, Mirs Bay, the second major estuarine ecosystem in Shenzhen, is located at 114°13′–114°32′E and 22°38′–22°24′N, subject to less pollution, and maintains a good ecological environment. In general, Mirs Bay has healthy coastlines and good water quality, supporting a great diversity of marine life (EPD (Environmental Protection Department of Hong Kong), 2016).

Eutrophication caused by increased anthropogenic nutrient inputs has been considered a major threat to marine ecosystems for decades (e.g., Bachmann et al., 2006; Xu et al., 2010). Nutrient enrichment (mainly nitrogen, phosphorus and silica) stimulates excessive algal (phytoplankton) growth, forming blooms that result in the deterioration of water quality and the ecosystem. Phytoplankton are critical components of marine ecosystems and represent the first biological response to nutrient enrichment and other environmental problems through changing population dynamics and community structure (Blanco et al., 2008; Shen et al., 2010). Many studies have shown that nutrient enrichment can significantly affect phytoplankton at the species and genus levels or cause variation in chlorophyll concentration, a proxy for phytoplankton biomass (Jiang et al., 2014; Godhe et al., 2015; López-Abbate et al., 2017; Xiao et al., 2018). Phytoplankton monitoring is thus crucial for tracking the biological consequences of nutrient enrichment over time. It provides information on the status and trends of water quality and plays an essential role in the management of coastal eutrophication. In this context, it is informative to analyze long-term ecological monitoring data for specific locations/ecosystems of interest to better understand the effects of environmental factors on phytoplankton communities.

The Environmental Protection Department (EPD) of the Hong Kong government initiated a comprehensive sampling program to monitor water quality and the phytoplankton community in territorial waters in the late 1980s. These time-series data reveal the response of phytoplankton assemblages to long-term environmental changes, which allow us to better understand the long-term impact of urbanization on the phytoplankton community in Deep Bay and Mirs Bay. This study attempts to determine to what extent the variation in phytoplankton biomass, abundance and composition can be explained by changes in nutrients and other physicochemical variables in Deep Bay and Mirs Bay.