Biodeposition, respiration, and excretion rates of an introduced clam Mercenaria mercenaria in ponds with implications for potential competition with the native clam Meretrix meretrix in Shuangtaizi estuary, China

This study aimed to evaluate the potential impacts of an introduced clam Mercenaria mercenaria on estuarine ecosystem, and implications for the niche competition with a native clam Meretrix meretrix. The biodeposition, respiration, and excretion rates of 34. mercenaria were determined seasonally using a sediment trap and a closed respirator in field. The biodeposition rates ofM. mercenaria were 0.06-0.37 g/ （ind.·d）, and the respiration rates were 0.31-14.66 mg/（ind.·d）. The ammonia and phosphate excretion rates were 0.18-36.70 and 1.44-14.87 μg/（ind.·d）, respectively. The hard clam M. mercenaria may discharge dry deposits up to 2.1 × 10^5 t, contribute 18.3 t ammonia and 9.0 t phosphate to culture ponds, and consume 7.9×10^3 t O2 from ponds annually. It suggested that the hard clam M. mercenaria might play an important role in pelagic-benthic coupling in pond ecosystem through biodeposition and excretion. A comparison of the key physiological parameters of the introduced clam M. mercenaria and the native clam Meretrix meretrix suggested that M. mercenaria had a niche similar to that of Meretrix meretrix in Shuangtaizi estuary and might have a potential competition with Meretrix meretrix for habitat and food ifM. mercenaria species escaped from the culture pond or artificially released in estuarine ecosystem.

A 50-year sedimentary record of heavy metals and their chemical speciations in the Shuangtaizi River estuary(China):implications for pollution and biodegradation

Two parallel sediment cores collected from tidal flat located in the Shuangtaizi River estuary were analyzed for heavy metal concentrations and chemical speciations.Based on the ^(137)Cs activity profile,mean sedimentation rate at the sampling site during the past 50 years was estimated to be 1.3 cm·a^(-1).Correlation analyses show that almost all the metals are associated with each other,suggesting that these metals might be derived from same sources and/or affected by same geochemical processes.Influence of total organic carbon(TOC)content on the concentrations of Cr,Ni,Cu and Cd is evident.Silt and clay contents,instead of sand content,play an important role in the distribution of these metals.The dominant binding phases for most of the metals(except for Cd)are the residual.The relative decrease of the residual fraction of Cd and Pb in the upper 66 cm of the core is striking.The distribution of chemical fraction confirms that the residual fractions of these metals have a natural origin,while only the non-residual fractions of Cd and Pb increased upward the core due to pollution in the past five decades.Pollution assessment on these heavy metals based on Index of Geoaccumulation(I_(geo))also demonstrates that most of the metals are unpolluted.The weak pollution as observed in the sediments is perhaps related to a local plant,the Suaeda heteroptera Kitag,which may have played a significant role in the biodegradation of these metals and the metal distribution in the estuary.