Unusual pattern in characteristics of the eelgrass Zostera marina L. in a shallow lagoon (Swan Lake), north China: Implications on the importance of seagrass conservation

Since the 1980s, seagrass populations have declined greatly in most coastal waters of China. Nevertheless, we documented, in this study, an unusual occurrence of the eelgrass Zostera marina L. in north China, with biomass markedly higher than other temperate areas in the world. Seasonal changes in biometric variables and the biomass of Z. marina were examined in a shallow marine lagoon of Swan Lake, an important nature reserve for swans (Cygnus cygnus). Results showed that shoot height of eelgrass varied seasonally, with maximum values (>170 cm) being recorded in June 2009 and minimum values (<25 cm) in January-March 2010. The highest shoot density was recorded in August 2009 (645 shoots m−2) and the lowest in March 2010 (334 shoots m−2). Eelgrass biomass also varied seasonally, from >1000 g DW m−2 in August 2009 to <100 g DW m−2 in March 2010. Carbon, nitrogen and phosphorus content in different parts of eelgrass also varied seasonally. The N and P contents in rhizomes were significantly lower than that in leaves and leaf sheaths. Annual average C/N ratios in leaves, leaf sheaths, and rhizomes were 14.99 ± 2.97, 13.31 ± 4.59, and 18.50 ± 4.99, respectively; and annual average C/P ratios were 248.82 ± 81.7, 199.8 ± 87.5, and 289.2 ± 95.1, respectively; the abovementioned deviations suggest P enrichment of eelgrass in the lagoon, compared with the global range for the species. Water temperature is the most important environmental factor that impacts the seasonal variation in this species. The increased distribution area and high productivity of Z. marina were mostly attributed to local conservation efforts.