Fate of macroalgae in benthic systems: carbon and nitrogen cycling within the microbial community

Abstract

High nutrient loading to coastal bays is often accompanied by the presence of bloomforming macroalgae, which take up and sequester large amounts of C and N while growing. This pool is temporary, however, as nuisance macroalgae exhibit a bloom and die-off cycle, influencing the biogeochemical functioning of these systems in unknown ways. The objective of this study was to trace the C and N from senescing macroalgae into relevant sediment pools. A macroalgal die-off event was simulated by the addition of freeze-dried macroalgae (Gracilaria spp.), pre-labeled with stable isotopes ( 13 C and 15 N), to sediment mesocosms. The isotopes were traced into bulk sediments and partitioned into benthic microalgal (BMA) and bacterial biomass using microbial biomarkers to quantify the uptake and retention of macroalgal C and N. Bulk sediments took up label immediately following the die-off, and macroalgal C and N were retained in the sediments for at least 2 wk. Approximately 6 to 50% and 2 to 9% of macroalgal N and C, respectively, were incorporated into the sediments. Label from the macroalgae appeared in both bacterial and BMA biomarkers, suggesting that efficient shuttling of macroalgal C and N between these communities may serve as a mechanism for retention of macroalgal nutrients within the sediments. KEY WORDS: Stable isotopes · Macroalgae · Benthic microalgae · Bacteria · Biomarker · Coastal eutrophication Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 414: [41][42][43][44][45][46][47][48][49][50][51][52][53][54][55] 2010 Studies of macroalgal bloom decay have demonstrated rapid breakdown of biomass, resulting in release of both inorganic and organic nutrients to the water column (Buchsbaum et al. 1991, Castaldelli et al. 2003, García-Robledo et al. 2008, supporting phytoplankton and bacterial metabolism , Nedergaard et al. 2002. Fewer studies have focused on macroalgal decay within the sediments (Nedergaard et al. 2002, Lomstein et al. 2006, Rossi 2007, García-Robledo et al. 2008, where heterotrophic bacterial densities are significantly higher than in the water column (Deming & Baross 1993, Schmidt et al. 1998, Ducklow 2000. In addition, most of the sediment studies have been conducted in low or no light environments, even though light is typically available to shallow sediments where macroalgal die-offs occur and sediment biogeochemistry is largely affected by benthic microalgal (BMA) activity (Underwood & Kromkamp 1999). While nutrients associated with senescent macroalgal blooms are recycled and can have a positive feedback on phytoplankton production in the water column, nutrients released during macroalgal decay in the sediments may support BMA and bacterial production, which could intercept the return of nutrients to the overlying water column. Thus if shallow-water sediments behave as a nutrient filter, the response by phytoplankton may be reduced, and benthic production could effectively buffer the system from further eutrophication. In order to be...

Ecosystem metabolism in shallow coastal lagoons: patterns and partitioning of planktonic, benthic, and integrated community rates

“…A potential source of allochthonous organic loads is marsh organic carbon. However, a previous study in the fringing tidal marshes of Hog Island Bay found that these marshes were not sources of organic carbon to the lagoon (Neikirk 1996).Internal production of dissolved organic matter (Rochelle-Newall et al 2007, Anderson et al 2010 could also regulate NEM in our study lagoons, as suggested by the relatively high measured DON. DON was the predominant N species of TDN in all 4 lagoons (Table 2), derived largely from internal recycling.…”

“…The close proximity of shallow marine systems to land, their small volumes and relatively long residence times, and the penetration of light to the benthos make these systems particularly susceptible to nutrient enrichment (Duarte 1995, Bricker et al 1999). Microphytobenthos (MPB), however, may help mediate the response of shallow systems to nutrient loading (Tobias et al 2003, Anderson et al 2010.Although nitrogen loads to coastal lagoons are of a magnitude similar to those of deeper estuaries, responses in shallow lagoons appear to be quite different, likely because of enhanced benthic− pelagic coupling (Nixon et al 2001). An illuminated benthos results in a significant contribution of MPB, seagrasses and macroalgae to total system production.…”

Paleoenvironmental conditions define current sustainability of microbial populations in Laguna Potrok Aike sediments, Argentina

“…Salinity in the water column increases with evaporation while a long residence time in the water column also leads to increased internal recycling of nutrients (Howarth and Marino 2006), possibly making Nfixing cyanobacteria the major suppliers of organic N to other algae (Marcarelli et al 2006;Håkanson et al 2007). Microalgal OM is easily degraded in the water column and may have provided only minor amounts of the N available to bacteria (Hardison et al 2010), thus engendering N-limiting conditions in the water column and sediments.…”

Effects of grazing and fertilization on epiphyte growth dynamics under moderately eutrophic conditions: implications for grazing rate estimates

“…Discrepancies between targeted and realized increases in nutrient concentration often occur in fertilization experiments due to factors that are not well known and/or cannot be controlled with accuracy , Stutes et al 2006). Here, it is possible that nutrient efflux from the settling sediment into the overlying water within the aquaria may have contributed to the higher level of enrichment found in some of the experiments (Sundbäck et al 1991, Rizzo et al 1992, Hardison et al 2010. In spite of this, the level of increase in nutrient concentration attained with our initial additions still remains moderate in relation to other fertilization experiments (Downing et al 1999.…”