Massive loss and microbial decomposition in reproductive biomass of Zostera marina

The biomass of Zostera marina in the temperate coastal zone has seasonal variations in their dry weight, with a peak during the spring season when a large section of the shoots develops reproductive organs. However, the period of high biomass is only short-term, after which large numbers of reproductive shoots drastically dis-appeared. Dislodgement is an important fate of the reproductive biomass. The fraction of biomass that reaches the deep sea contributes to carbon sequestration, however the processes of dislodgement and transportation of reproductive shoots of Z. marina has been rarely studied. Here, based on a monitoring survey of two Z. marina meadows, we estimated the net loss of biomass, including their reproductive one. In addition, microbial decomposition was assessed by dark incubation to evaluate the fate of the dislodged biomass. The apparent loss of biomass from the Z. marina meadows in the Declining Period, defined with the most rapid decrease in biomass, accounted for 82-100% of the biomass at the initial time-point of this period. By subtracting the biomass production from the apparent loss, we calculated the net loss of biomass in the Declining Period, ac-counting for 14-81% of the annual production. Approximately half of the biomass loss in the Declining Period was attributed to reproductive biomass, suggesting that dislodgement of reproductive shoots is an important fate of photosynthetic products of Z. marina. The carbon to nitrogen (CN) ratio of the reproductive shoots was found to be significantly higher than that of vegetative shoots. Since nitrogenous materials are relatively labile, we expected a lower bioavailability of reproductive shoots. However, microbial decomposition did not reflect the CN ratio, probably due to the heterogeneity of nitrogenous materials and/or due to the contribution of carbohy-drates consisting mostly of non-nitrogenous compounds.

Automated summary

The biomass of Zostera marina in temperate coastal zones shows seasonal variation with a peak during spring, largely due to the development of reproductive organs in shoots. However, this high biomass period is short-lived, with many reproductive shoots disappearing after. Loss of biomass, especially reproductive, is significant during the Declining Period, accounting for 14-81% of annual production. Dislodgement of reproductive shoots plays a key role in the fate of photosynthetic products, with microbial decomposition not reflecting nitrogen content in reproductive shoots.