1- Introduction

2- Material and methods

3- Results

4- Discussion

5- Conclusions

References

Abstract

Hydrodynamic fluctuations can trigger sediment suspension concomitantly with internal phosphorus release, while the interactive effect of turbulence mixing and sediment suspension on the regulation of phosphorus dynamics is in need of deep understanding. This study addressed the changes in total phosphorus (TP), phosphate (PO43–P) and suspended sediment (SS) in the overlying water, and measured the profile of dissolved oxygen (DO), Fe(II) and soluble reactive phosphorus (SRP) across the sediment-water interface in the simulated environmental turbulence scenario, For a turbulence intensity (ε) of 3.6 × 10−3 m2/s3, the SRP flux increased hence PO43–P showed a 36.36% increase relative to its initial level. Although ε of 1.3 × 10−2 m2/s3 benefited the delivery of oxygen from the bulk aqueous phase to the upper sediment which can trigger the formation of Fe oxides and hydroxides, the turbulence-induced phosphorus diffusion from the sediment exceeded its inactivation and resulted in a large SRP flux. However, a protion of the released PO43–P can be immobilized through SS adsorption and biotic (likely cyanobacteria) assimilation. Higher turbulence intensities (ε of 3.3 × 10−2 and 7.4 × 10−2 m2/s3) led to an approximately 40-fold increase in TP concentration and a significant increase in sediment suspension, which contributed to the immobilization of a majority of the phosphate through adsorption; thus, the PO43–P concentrations in the overlying water displayed 47.75% and 41.67% decline, respectively. This study also confirmed the sequential phosphorus buffer mechanisms associated with increasing turbulence intensities. With an ε of 3.6 × 10−3 m2/s3, bounding to Fe ion had a significant impact on phosphorus inactivation but with an ε of 7.4 × 10−2 m2/s3, the main immobilization mechanism is switched to phosphorus adsorption from the large quantity of suspended sediment.

Introduction

Phosphorus is essential for the support of phytoplankton growth and is a major driving force for water eutrophication (Søndergaard et al., 2013; Conley et al., 2009). Sediment can act as a sources of pollutants in aquatic environments, and the pollutants accumulated in the sediments can be released into the overlying water, contributing a considerable quantity of phosphorus to the water column (Zhang et al., 2018). For most eutrophic waters, the upper 0e10 cm sediment layer typically has much higher phosphorus levels than that is found in the lower sediment layers; and the most heavily contaminated sediment layer is the top 0e5 cm (Tang et al., 2019), where phosphorus is mobile and can be easily released into the overlying water due to changes in the physicochemical characteristics of the water (Soto-Jimenez et al., 2003; Wu et al., 2001). Thus, even if phosphorus input is effectively reduced, the eutrophication level may remain relatively high, as internal phosphorus loading from sediments may lead to algae blooming and cause a decline in the water quality (Huser and Pilgrim, 2014; Yin et al., 2018; Yin et al., 2016). Therefore, understanding the process of phosphorus release from the sediments is crucial for the management and recovery of eutrophic waters.

1- Introduction

2- Materials and methods

3- Results

4- Discussion

References

Abstract

Photosynthesis is an important driver of calcium carbonate deposition on tropical coral reefs largely due to the symbiosis of numerous invertebrates with photosynthetic dinoflagellates in the family Symbiodiniaceae. In bioeroding sponges, however, similar symbioses appear to support the decalcification of carbonate substrates. Compared to its role in calcification, the relative importance of photosynthesis to decalcification processes is less known. Here, the daytime and night-time chemical bioerosion rates of the common Indo-Pacific sponge Cliona orientalis were examined under varying levels of photosynthetic activity and/or varying abundances of intracellular dinoflagellates. Photosynthesis was manipulated either by preconditioning the sponges with the photosynthetic inhibitor diuron (DCMU), or by exposing them to short-term heat stress to achieve bleaching (loss of symbionts). DCMU reduced symbiont numbers and diminished their ability to evolve oxygen. Thermal stress caused a significantly greater loss of symbionts, but photosynthesis was less inhibited. In both cases, decreases in photosynthetic activity and symbiont densities led to proportionately lowered daytime chemical bioerosion rates. Moreover, increased rates of night-time bioerosion were linked to greater daytime rates of photosynthesis, rather than to the night-time respiration of the sponge holobiont. Our findings support the conclusion that photosynthetic products (photosynthates) and/or by-products (oxygen) stimulate sponge bioerosion. This work further reveals the importance of symbionts in the ecology of such sponges and in their ability to sustain high bioerosion activity in otherwise nutrient-poor ecosystems.

Introduction

Tropical coral reefs thrive in shallow clear waters where sunlight powers productive and diverse food chains. Symbioses between invertebrates and a variety of photosynthetic microorganisms enable high rates of productivity on reefs despite low environmental concentrations of inorganic nutrients (Muscatine and Porter, 1977; Seckbach, 2004; Yellowlees et al., 2008). The evolutionary success that results from hosting phototrophic microbes such as dinoflagellates, diatoms, chlorophytes, rhodophytes, or cyanobacteria is seen in a wide range of diverse invertebrates belonging to taxa such as Cnidaria, Mollusca, Foraminifera, Platyhelminthes, and Porifera (Trench, 1993; Stat et al., 2006; Lipps and Stanley, 2016). Presently almost 200 sponge species (Porifera) are known to host microbial photosynthetic symbionts (“photosymbiotic” sponges), which may contribute to the nutrition and health of the sponge (e.g. Wilkinson, 1987; Hill et al., 2011; Thacker and Freeman, 2012). Sponge species that receive substantial contributions in this way are often named “phototrophic” after the nutritional mode of their symbionts, or “mixotrophic” given that they also depend on filter-feeding by specialized food-trapping sponge cells. The nutrition of sponges is often further supplemented by inputs from intimate associations formed with heterotrophic bacterial populations of varying density and diversity (e.g. Hentschel et al., 2003; Taylor et al., 2007).