Assessing Herbivorous Impacts of Apohyale sp. on the Ulva prolifera Green Tide in China

Xiaoxiang Miao,Jie Xiao,Yu Zang,Zongling Wang,Xuelei Zhang,Shiliang Fan

doi:10.3389/fpls.2021.795560

Abstract

An epiphytic gammarid species, Apohyale sp., was abundant in the floating Ulva prolifera (U. prolifera), which forms large-scale green tides in the Yellow Sea (YSGT). Field observation and laboratory experiments were subsequently conducted to study the species identity, abundance, and grazing effects on the floating algal biomass. The abundance of Apohyale sp. showed great spatial variation and varied from 0.03 to 1.47 inds g−1 in the YSGT. In average, each gram of Apohyale sp. body mass can consume 0.43 and 0.60 g algal mass of U. prolifera per day, and the grazing rates varied among the algae cultured with different nutritional seawaters. It was estimated that grazing of Apohyale sp. could efficiently reduce ~0.4 and 16.6% of the algal growth rates in Rudong and Qingdao, respectively. The U. prolifera fragments resulting from gnawing of Apohyale sp. had a higher growth rate and similar photosynthetic activities compared to the floating algae, indicating probably positive feedback on the floating algal biomass. This research corroborated the significant impact of Apohyale sp. on the floating algal mass of YSGT through the top-down control. However, further research is needed to understand the population dynamics of these primary predators and hence their correlation with the expansion or decline of YSGT, especially under the complex food webs in the southern Yellow Sea.

Highlights

Green tides are ecological disasters caused by the aggregation of green macroalgae (Fletcher, 1996; Blomster et al, 2002), which are frequently occurring in coastal countries and regions around the world (Smetacek and Zingone, 2013)
We focused on the epiphytic gammarids in the floating U. prolifera in the southern Yellow Sea, studied the abundance and species composition of the populations in the upstream source (Subei Shoal) and downstream aggregation regions (Qingdao) of the Yellow Sea green tides (YSGT)
The main difference between Apohyale sp. and the closely related A. pugettensis is the shape of gnathopod

Summary

Introduction

Green tides are ecological disasters caused by the aggregation of green macroalgae (Fletcher, 1996; Blomster et al, 2002), which are frequently occurring in coastal countries and regions around the world (Smetacek and Zingone, 2013). Since 2008, the large-scale Ulva prolifera (U. prolifera) green tides recur annually in the southern Yellow Sea of China and are recognized to be the largest green tide of the world (Liu et al, 2009; Yu and Liu, 2016). Various studies have confirmed a general raft-origin and northward drifting process of the Yellow Sea green tides (YSGT) and revealed a number of physiological and molecular mechanisms of U. prolifera for blooming as well (Liu et al, 2009; Gao et al, 2010; Qiao et al, 2011; Wang et al, 2015; Liu D. et al, 2020). Distinct from the numerous local green tides around the world, the YSGT undergoes long-distance drifting and causes significant trans-regional impacts with huge floating biomass inundating the open and Apohyale sp. Little is known about the roles of biotic factors, such as predators, on the occurrence of the green tides in the complex ecosystem of the southern Yellow Sea

Methods

Results

Conclusion