Search

Salt marsh ponds expand and deepen over time, potentially reducing ecosystem carbon storage and resilience. The water filled volumes of ponds represent missing carbon due to prevented soil accumulation and removal by erosion and decomposition. Removal mechanisms have different implications as eroded carbon can be redistributed while decomposition results in loss. We constrained ponding effects on carbon dynamics in a New England marsh and determined whether expansion and deepening impact nearby soils by conducting geochemical characterizations of cores from three ponds and surrounding high marshes and models of wind‐driven erosion. Radioisotope profiles demonstrate that ponds are not depositional environments and that contemporaneous marsh accretion represents prevented accumulation accounting for 32%–42% of the missing carbon. Erosion accounted for 0%–38% and was bracketed using radioisotope inventories and wind‐driven resuspension models. Decomposition, calculated by difference, removes 22%–68%, and when normalized over pond lifespans, produces rates that agree with previous metabolism measurements. Pond surface soils contain new contributions from submerged primary producers and evidence of microbial alteration of underlying peat, as higher levels of detrital biomarkers and thermal stability indices, compared to the marsh. Below pond surface horizons, soil properties and organic matter composition were similar to the marsh, indicating that ponding effects are shallow. Soil bulk density, elemental content, and accretion rates were similar between marsh sites but different from ponds, suggesting that lateral effects are spatially confined. Consequently, ponds negatively impact ecosystem carbon storage but at current densities are not causing pervasive degradation of marshes in this system. Plain Language Summary: Ponds are natural features of salt marshes but their expansion may be an indicator of ecosystem deterioration because they impede the marsh's ability to keep pace with sea‐level rise and remove decades of buried soil carbon. The water filled holes created by ponds represent volumes of marsh soil carbon that are missing due to prevented accumulation or lost through erosion and decomposition. These loss pathways have different implications for coastal carbon cycling as eroded soils can be redeposited elsewhere while microbial decomposition represents permanent loss. We used geochemical and modeling approaches to assess how much of the carbon missing from ponds can be attributed to prevented soil accumulation, erosion, and decomposition as well as whether ponds reduce the integrity of the surrounding marsh. We estimate that these processes represent 32%–42%, 0%–38%, and 22%–68%, respectively, of soil carbon missing from three ponds in a New England salt marsh. The range of potential erosion losses reflect differences in fetch and wind‐driven waves used in the models. Decomposition was calculated by subtracting the contributions of prevented accretion and erosion from the volume of missing carbon and, while the range is large, losses normalized over time are comparable to previously measured respiration rates. Comparisons of soil properties and composition between the ponds and surrounding marsh demonstrate that the effects of expansion are confined to within a 10 m perimeter. Consequently, in this system, ponds represent net losses from the carbon budget and at current densities are not causing pervasive degradation of the marsh. Key Points: Salt marsh ponds deepen and expand over time but their effects are localized and do not result in deterioration of the surrounding marshThree processes account for soil carbon missing from deepening ponds: prevented deposition, erosion, and decompositionEroded soils may be redistributed and retained within the marsh, while decomposition represents carbon loss [ABSTRACT FROM AUTHOR]

Disturbances are one of the most important processes affecting natural systems, but there is a gap between simple conceptual models of disturbance and complex empirical studies. We studied the perturbation caused by headward‐eroding creeks in southeastern USA salt marshes. We measured disturbance responses (magnitude and recovery trajectory) of 19 variables. Some variables (shoot density, root biomass, snail density, soil pH, soil strength, soil temperature, elevation) declined sharply, while other variables (crab burrow density, soil organic matter, soil redox) increased sharply, in response to the burrowed and grazed conditions at the creek head. These variables recovered over subsequent years or decades. Other variables (shoot height, aboveground biomass, rhizome biomass, light interception) declined sharply in the creek head, then overshot control values before recovering. Some variables (benthic algae, soil salinity) did not appear to be disturbed by the creek head. As hypothesized, plants recovered before soils and snails. Disturbance magnitude and time to recovery were often greater directly adjacent to the new creekbank than for the same variables in a parallel transect further away from the creekbank, and in some cases variables never converged with control values, indicating a persistent state change. Reducing the dimensionality of the data set into principal component axes obscured the diverse ways in which different aspects of the system responded to and recovered from the perturbation. Our study illustrates the challenges in moving from simple conceptual models of disturbance to empirical studies in which multiple variables are likely to be affected differently and follow different recovery trajectories. [ABSTRACT FROM AUTHOR]

Salt marsh survival with sea‐level rise (SLR) increasingly relies on soil organic carbon (SOC) accumulation and preservation. Using a novel combination of geochemical approaches, we characterized fine SOC (≤1 mm) supporting marsh elevation maintenance. Overlaying thermal reactivity, source (δ13C), and age (F14C) information demonstrates several processes contributing to soil development: marsh grass production, redeposition of eroded material, and microbial reworking. Redeposition of old carbon, likely from creekbanks, represented ∼9%–17% of shallow SOC (≤26 cm). Soils stored marsh grass‐derived compounds with a range of reactivities that were reworked over centuries‐to‐millennia. Decomposition decreases SOC thermal reactivity throughout the soil column while the decades‐long disturbance of ponding accelerated this shift in surface horizons. Empirically derived estimates of SOC turnover based on geochemical composition spanned a wide range (640–9,951 years) and have the potential to inform predictions of marsh ecosystem evolution. Plain Language Summary: Salt marsh survival with rising sea levels increasingly depends on the accumulation and preservation of buried organic carbon. Marsh soil organic carbon development reflects at least three processes: burial of differently reactive compounds that derive from local grasses, redeposition of old carbon (9%–17%), and microbial reworking. Decomposition results in a progressive decrease in the thermal reactivity of soil organic carbon, and disturbances such as ponding can accelerate this shift. Modeled rates of geochemically defined soil organic carbon pools turnover on the order of centuries‐to‐millennia and can refine predictions of salt marsh sustainability. Key Points: Salt marsh soils preserved compounds with a range of reactivities that were derived from local grasses9%–17% of surface soil carbon was old, likely reflecting redeposition of eroded creekbank materialDecomposition decreases soil organic carbon thermal reactivity, and disturbances accelerate this shift, particularly in surface horizons [ABSTRACT FROM AUTHOR]

Shallow ponds are expanding in many salt marshes with potential impacts on ecosystem functioning. Determining how pond characteristics change over time and scale with physical dimensions and other spatial predictors could facilitate incorporation of ponds into projections of ecosystem change. We evaluated scaling relationships across six differently sized ponds in three regions of the high marshes within the Plum Island Ecosystems‐Long Term Ecological Research site (MA, USA). We further characterized diel fluctuations in surface water chemistry in two ponds to understand short‐term processes that affect emergent properties (e.g., habitat suitability). Primary producers drove oxygen levels to supersaturation during the day, while nighttime respiration resulted in hypoxic to anoxic conditions. Diel swings in oxygen were mirrored by pH and resulted in successive shifts in redox‐sensitive metabolisms, as indicated by nitrate consumption at dusk followed by peaks in ammonium and then sulfide overnight. Abundances of macroalgae and Ruppia maritima correlated with whole‐pond oxygen metabolism rates, but not with surface area (SA), volume (V), or SA:V. Moreover, there were no clear patterns in primary producer abundances, surface water chemistry, or pond metabolism rates across marsh regions supplied by different tidal creeks or that differed in distance to upland borders or creekbanks. Comparisons with data from 2 years prior demonstrate that plant communities and biogeochemical processes are not in steady state. Factors contributing to variability between ponds and years are unclear but likely include infrequent tidal exchange. Temporal and spatial variability and the absence of scaling relationships complicate the integration of high marsh ponds into ecosystem biogeochemical models. Plain Language Summary: The spatial extent of shallow ponds is expanding in many salt marshes, due to hydrologic management practices and sea‐level rise, among other factors. Accounting for ponds in ecosystem biogeochemical models is important for predicting how marshes may change in the future. It is impractical to characterize every marsh pond because they can account for a large fraction of the landscape. Developing predictive relationships between pond properties and easily measured attributes, such as dimensions or distance from marsh landscape features, could facilitate integration of ponds into ecosystem models. We found that pond biogeochemistry changes dramatically day to night, reflecting a combination of primary production and heterotrophic (i.e., microbial) respiration. However, abundances of primary producers, and their effects on whole‐pond oxygen metabolism, did not change predictably with pond surface area or volume. Pond properties also did not vary according to location within the marsh. Instead, each pond was unique. The processes affecting pond development are therefore highly localized and might reflect long periods of tidal isolation in the high marsh. Key Points: Spatial and temporal variability in the biogeochemistry of ponds makes it difficult to estimate their contributions to salt marsh functioningPond water oxygen levels swing from supersaturation to anoxia over daily cycles, causing successive shifts in redox‐sensitive metabolismsBiogeochemical processes reflect localized conditions and do not scale with dimensions or vary predictably with marsh spatial properties [ABSTRACT FROM AUTHOR]

Mangrove forests are among the world's most productive and carbon‐rich ecosystems. Despite growing understanding of factors controlling mangrove forest soil carbon stocks, there is a need to advance understanding of the speed of peat development beneath maturing mangrove forests, especially in created and restored mangrove forests that are intended to compensate for ecosystem functions lost during mangrove forest conversion to other land uses. To better quantify the rate of soil organic matter development beneath created, maturing mangrove forests, we measured ecosystem changes across a 25‐yr chronosequence. We compared ecosystem properties in created, maturing mangrove forests to adjacent natural mangrove forests. We also quantified site‐specific changes that occurred between 2010 and 2016. Soil organic matter accumulated rapidly beneath maturing mangrove forests as sandy soils transitioned to organic‐rich soils (peat). Within 25 yr, a 20‐cm deep peat layer developed. The time required for created mangrove forests to reach equivalency with natural mangrove forests was estimated as (1) <15 yr for herbaceous and juvenile vegetation, (2) ~55 yr for adult trees, (3) ~25 yr for the upper soil layer (0–10 cm), and (4) ~45–80 yr for the lower soil layer (10–30 cm). For soil elevation change, the created mangrove forests were equivalent to or surpassed natural mangrove forests within the first 5 yr. A comparison to chronosequence studies from other ecosystems indicates that the rate of soil organic matter accumulation beneath maturing mangrove forests may be among the fastest globally. In most peatland ecosystems, soil organic matter formation occurs slowly (over centuries, millennia); however, these results show that mangrove peat formation can occur within decades. Peat development, primarily due to subsurface root accumulation, enables mangrove forests to sequester carbon, adjust their elevation relative to sea level, and adapt to changing conditions at the dynamic land–ocean interface. In the face of climate change and rising sea levels, coastal managers are increasingly concerned with the longevity and functionality of coastal restoration efforts. Our results advance understanding of the pace of ecosystem development in created, maturing mangrove forests, which can improve predictions of mangrove forest responses to global change and ecosystem restoration. [ABSTRACT FROM AUTHOR]

High‐throughput sequencing has enabled robust shotgun metagenomic sequencing that informs our understanding of the genetic basis of important biogeochemical processes. Slower to develop, however, are the application of these tools in a controlled experimental framework that pushes the field beyond exploratory analysis toward hypothesis‐driven research. We performed flow‐through reactor experiments to examine how salt marsh sediments from varying depths respond to nitrate addition and linked biogeochemical processes to this underlying genetic foundation. Understanding the mechanistic basis of carbon and nitrogen cycling in salt marsh sediments is critical for predicting how important ecosystem services provided by marshes, including carbon storage and nutrient removal, will respond to global change. Prior to the addition of nitrate, we used metagenomics to examine the functional potential of the sediment microbial community that occurred along a depth gradient, where organic matter reactivity changes due to decomposition. Metagenomic data indicated that genes encoding enzymes involved in respiration, including denitrification, were higher in shallow sediments, and genes indicative of resource limitation were greatest at depth. After 92 d of nitrate enrichment, we measured cumulative increases in dissolved inorganic carbon production, denitrification, and dissimilatory nitrate reduction to ammonium; these rates correlated strongly with genes that encode essential enzymes in these important pathways. Our results highlight the importance of controlled experiments in linking biogeochemical rates to underlying genetic pathways. Furthermore, they indicate the importance of nitrate as an electron acceptor in fueling microbial respiration, which has consequences for carbon and nitrogen cycling and fate in coastal marine systems. [ABSTRACT FROM AUTHOR]

Abstract: Runaway expansion of shallow ponds can catalyze the conversion of vegetated marshes into open water environments. Predicting how this transition affects ecosystem functioning is difficult because little is known about pond biogeochemistry. We characterized sediment organic matter sources and transformations in three ponds with different plant communities, over alternating periods of tidal isolation and flushing, during summer and fall, using a combination of stable isotopes, lipid biomarkers, and benthic fluxes. Sediment respiration rates (1.66 ± 0.09 mmol C m−2 d−1 to 28.53 ± 7.76 mmol C m−2 d−1) were comparable to shallow estuaries and driven by sulfate reduction. Rates varied across ponds, reflecting differences in summertime Ruppia maritima and macroalgae abundances, but were similar between seasons. Interactions between aboveground plant and sediment bacterial communities translated into distinct biogeochemical processes across the three ponds. Tidal isolation and summer weather intensified plant and bacterial community effects on pond carbon dynamics, resulting in algal biomass and lipid δ13C values that were 3–12‰ enriched, compared to nearby habitats. Surface sediment organic matter mainly derived from pond microalgae and was compositionally distinct from tidal creeks and marshes. Surprisingly, sediment bacteria were not tightly coupled to benthic microalgae but decomposed multiple carbon sources in surface sediments and became increasingly reliant on buried peat at deeper horizons. Pond development over time could largely be explained by sediment respiration and the simultaneous accretion of the surrounding marsh platform. The role of decomposition in pond expansion is consistent with previous assessments based on whole‐pond metabolism rates. Consequently, future pond expansion could alter ecosystem biogeochemistry and reduce carbon storage. [ABSTRACT FROM AUTHOR]

To test how benthic biogeochemistry responds to changing nutrient availability and estuary trophic state, I conducted mesocosm and field experiments. In the mesocosm experiment, sediment cores from eutrophic and oligotrophic estuaries were incubated under high or low nutrient levels. Net production rates were greater in eutrophic vs. oligotrophic cores while respiration rates were greater in high nutrient treatments, indicating that sediment metabolism was affected by trophic history and nutrient availability. Sediment organic matter (SOM) composition was insensitive to the nutrient treatments but varied with trophic history as carbon : nitrogen ratios were lower, δ15N was higher, and algal and bacterial fatty acids (FAs) were more abundant in eutrophic sediments. In the field experiment, intact cores were reciprocally transplanted between eutrophic and oligotrophic estuaries and changes in SOM composition were followed over four months. SOM responses were more dramatic in cores transplanted to the oligotrophic vs. eutrophic estuary. In sediments moved from the eutrophic to oligotrophic estuary, carbon and nitrogen content as well as abundances of algal and bacterial FAs fell within one month. Conversely, sediment δ15N increased sharply while algal and bacterial FAs rose slightly in cores transplanted from the oligotrophic to eutrophic estuary. Different SOM responses between the experiments were likely due to direct vs. indirect effects of nutrient enrichment. Nutrient levels were manipulated in the mesocosm experiment while the estuaries differed in nutrients, macroalgal biomass, benthic animal communities, and oxygen levels. Consequently, sediment biogeochemical responses to watershed nutrient mitigation will likely follow broader changes in estuary habitat quality. [ABSTRACT FROM AUTHOR]

1. Aquatic ecologists use mesocosm experiments to understand mechanisms driving ecological processes. Comparisons across experiments, and extrapolations to larger scales, are complicated by the use of mesocosms with varying dimensions. We conducted a mesocosm experiment over a volumetric scale spanning five orders of magnitude (from 4 L to whole ponds) to determine the generality of algal responses to nutrient enrichment. Recognising that mesocosm dimensions may affect algal growth, we also manipulated the ratio of mesocosm surface area to volume (SA : V) over two levels (high versus low). We used mesocosm tanks of similar size and construction to those commonly used in aquatic experiments to increase the generality of our results. 2. Volume was generally a stronger determinant of algal responses than mesocosm shape (i.e. SA : V). However, the effects of both volume and shape on algae were weak and explained a small portion of the variance in response variables. In addition, there was no consistent, directional relationship (positive or neutral) between mesocosm volume and algal abundance (estimated by chlorophyll concentration). Combined, our findings suggest that results from small-scale experiments, examining the direct response of algae to nutrient enrichment, can probably be 'moved on up' and applied to larger, more natural aquatic systems. 3. Algal response to nutrient enrichment (e.g. nutrient use efficiency and effect size) varied strongly with time. This underscores the importance of choosing an experimental timescale appropriate to the biological and/or ecological process of interest. 4. We compared our results to those from a recent meta-analysis of nutrient-limitation studies that included 359 freshwater pelagic experiments, spanning a wide range of volumetric and temporal scales. Similar findings between this experiment and the meta-analysis indicate that algal response to nutrient enrichment varies little across spatial scales. Therefore, it is probable that results from small-scale pelagic algal nutrient-limitation experiments are relevant to large-scale processes, such as eutrophication. [ABSTRACT FROM AUTHOR]

We tested the singular and interactive effects of resource availability (light) and community composition (food chain length and herbivore species richness) on eelgrass (Zostera marina) ecosystem properties and functioning with an experimental mesocosm system. Food chain length was manipulated through the presence or absence of blue crab (Callinectes sapidus) predators, whereas grazer species richness varied across three levels (zero, two, or four crustacean species). We found important and interacting effects of bottom-up and top-down forcings on sediment organic matter (SOM) composition. Light increased eelgrass and algal biomass and sediment organic carbon and nitrogen content. Increasing grazer diversity generally decreased algal biomass and ecosystem production but interacted with food chain length (i.e., presence of predatory crabs) and light. Predators generally increased algal biomass and ecosystem production through a trophic cascade, which was stronger at high grazer diversity and under ambient light. SOM composition, determined with fatty acid (FA) biomarkers, was sensitive to all manipulated variables. Increasing grazer species richness often decreased the contributions of FAs derived from plant and algal sources, whereas increasing light had the opposite effect. Food chain length was generally a less important determinant of SOM composition than light, although predators did increase FAs representative of heterotrophic bacteria. Overall, resource availability and epibenthic community composition strongly influenced organic matter cycling, SOM composition, and the bacterial community in seagrass-bed sediments. [ABSTRACT FROM AUTHOR]