Candida Savage

Experimenting with ecosystem interaction networks in search of threshold potentials in real-world marine ecosystems

Thresholds profoundly affect our understanding and management of ecosystem dynamics, but we have yet to develop practical techniques to assess the risk that thresholds will be crossed. Combining ecological knowledge of critical system interdependencies with a large-scale experiment, we tested for breaks in the ecosystem interaction network to identify threshold potential in real-world ecosystem dynamics. Our experiment with the bivalves Macomona liliana and Austrovenus stutchburyi on marine sandflats in New Zealand demonstrated that reductions in incident sunlight changed the interaction network between sediment biogeochemical fluxes, productivity, and macrofauna. By demonstrating loss of positive feedbacks and changes in the architecture of the network, we provide mechanistic evidence that stressors lead to break points in dynamics, which theory predicts predispose a system to a critical transition.

Partitioning of organic carbon among density fractions in surface sediments of Fiordland, New Zealand

Transport of particles plays a major role in redistributing organic carbon (OC) along coastal regions. In particular, the global importance of fjords as sites of carbon burial has recently been shown to be even more important than previously thought. In this study, we used six surface sediments from Fiordland, New Zealand, to investigate the transport of particles and OC based on density fractionation. Bulk, biomarker, and principle component analysis were applied to density fractions with ranges of 2.5 g cm−3. Our results found various patterns of OC partitioning at different locations along fjords, likely due to selective transport of higher density but smaller size particles along fjord head-to-mouth transects. We also found preferential leaching of certain biomarkers (e.g., lignin) over others (e.g., fatty acids) during the density fractionation procedure, which altered lignin-based degradation indices. Finally, our results indicated various patterns of OC partitioning on density fractions among different coastal systems. We further propose that a combination of particle size-density fractionation is needed to better understand transport and distribution of particles and OC.

Erosion of Modern Terrestrial Organic Matter as a Major Component of Sediments in Fjords

Fjords have recently been recognized as “hotspots” of carbon burial. In this study, we investigated organic carbon (OC) and biomarker radiocarbon values in fjord sediments from New Zealand. Our results showed that OC was mostly modern with the most aged OC in middle reaches of fjords, likely related to hydrodynamic sorting and inputs along adjacent slopes. Radiocarbon ages of sedimentary OC increased from north-to-south, consistent with the Fiordland regional gradients of lower fjord slopes and less rainfall. Our biomarker results suggested that lignin and long-chain fatty acids were preferentially linked with fresh terrestrial debris and degraded soil, respectively, likely due to their chemical and physical properties. Finally, we propose that fjords are a significant sink of modern OC, in contrast to large lowland coastal systems as a major sink of pre-aged OC. Overall, this study indicated that radiocarbon techniques are critical in investigating carbon dynamics in coastal systems.

Historical Reconstruction of Phytoplankton Composition in Estuaries of Fiordland, New Zealand: the Application of Plant Pigment Biomarkers

Phytoplankton pigments in sediment cores from four New Zealand fjords were quantified to investigate community composition and primary production in this pristine and remote region. Downcore sediment records from Doubtful Sound, Fiordland, were also compared with phytoplankton pigments in sediment traps to investigate pathways of phytodetritus flux from the upper water column to the sediment. Historic primary production was estimated using downcore chlorophyll a (Chl a) and β-carotene as proxies for total algal biomass. Sedimentary Chl a was similar across Fiordland (p = 0.09), but β-carotene was significantly different in Broughton Arm (Kruskal–Wallis p < 0.01). Dominant transformation products of Chl a were steryl chlorin esters and carotenol chlorin esters, indicating the importance of grazing as a sink for phytoplankton in Fiordland. Carotenoids indicative of diatoms and dinoflagellates were recovered in the sedimentary records of Doubtful Sound. Pigment biomarkers were observed in higher quantities in the sediment traps than in the surface sediment. In particular, grazing biomarkers were abundant in the sediment, highlighting the importance of fecal pellet export in phytodetritus preservation. Phytoplankton pigment preservation in Fiordland sediments is good as indicated by little variation in Chl a/pheopigment ratios throughout the cores. This research supports other new work, which has shown that carbon preservation in southern hemisphere fjord ecosystems is more efficient than previously thought.

Modern deposition rates and patterns of organic carbon burial in Fiordland, New Zealand

Fjords are disproportionately important for global organic carbon (OC) burial relative to their spatial extent and may be important in sequestering atmospheric CO2, providing a negative climate feedback. Within fjords, multiple locally variable delivery mechanisms control mineral sediment deposition, which in turn modulates OC burial. Sediment and OC sources in Fiordland, New Zealand, include terrigenous input at fjord heads, sediment reworking over fjord-mouth sills, and landslide events from steep fjord walls. Box cores were analyzed for sedimentary texture, sediment accumulation rate, and OC content to evaluate the relative importance of each delivery mechanism. Sediment accumulation was up to 3.4 mm/yr in proximal and distal fjord areas, with lower rates in medial reaches. X-radiograph and 210Pb stratigraphy indicate mass wasting and surface-sediment bioturbation throughout the fjords. Sediment accumulation rates are inversely correlated with %OC. Spatial heterogeneity in sediment depositional processes and rates is important when evaluating OC burial within fjords.

Physical and biological drivers of sediment oxygenation and denitrification in a New Zealand intermittently closed and open lake lagoon

ABSTRACT Intermittently closed and open lakes and lagoons (ICOLLs) are shallow estuarine ecosystems, many of which show eutrophication symptoms. The physical and biological drivers of sediment oxygenation and denitrification were examined in a New Zealand ICOLL using in situ enclosures for one year. Denitrification was seasonally and spatially variable, and higher denitrification rates were driven by organic matter availability, temperature and nitrate flux. The bulldozing invertebrates generally deepened oxygen penetration in 18% of oxygen profiles, consistent with their effect in multiple regressions. Chironomid larvae (Chironomidae) dominated the benthic community, and their tubular burrows modified oxygen penetration into the sediment in 18% of profiles, potentially affecting denitrification in the ICOLL through bioirrigation. Our study shows that macroinvertebrates support increased sediment oxygenation; therefore the use of artificial opening regimes in many ICOLLs may have negative effects on macroinvertebrate community composition and may consequently inhibit denitrification.