Tracy N. Wiegner

Contribution of dissolved organic C to stream metabolism: a mesocosm study using 13C-enriched tree-tissue leachate

Abstract Dissolved organic C (DOC) is metabolically important in streams, but its contribution to ecosystem metabolism is not well known because it is a complex mixture of mostly unidentified molecules. The uptake of bioavailable DOC in White Clay Creek (WCC), a 3rd-order stream in Pennsylvania, was estimated from the results of an experiment using 13C-labeled tree-tissue leachate and streambed sediments in recirculating mesocosms. The contribution of DOC in transport to stream metabolism was estimated from measurements of 13C-DOC uptake, 12C-DOC concentrations, and diel changes in dissolved O2 in the mesocosms. Eighty percent (±5) of the DOC in the 13C-tree-tissue leachate was bioavailable and belonged to 1 of 2 distinct lability classes, readily and intermediately labile. These components made up 88% (±0.6) and 12% (±0.6), respectively, of the biodegradable DOC in the leachate. Uptake mass transfer coefficients for the readily and intermediately labile components were 55 (±24) μm/s and 2.6 (±0.13) μm/s, respectively. Based on our mesocosm measurements, DOC in transport could support 33 to 54% of the bacterial C demand and up to 51% of the community respiration in WCC. Extrapolation of our results to WCC indicates that readily and intermediately labile DOC similar in quality to the 13C-DOC would travel 175 and 3692 m downstream in WCC before being taken up by the sediments. These distances represent ∼7% and >150% of the length of the 3rd-order reach. Our results suggest that readily labile DOC is an important energy source at the reach scale, whereas intermediately labile DOC serves as an energy subsidy from upstream to downstream reaches.

The Effects of Multiple Stressors on the Balance between Autotrophic and Heterotrophic Processes in an Estuarine System

Responses of autotrophic and heterotrophic processes to nutrients and trace elements were examined in a series of experimental estuarine food webs of increasing trophic complexity using twenty 1-m3 mesocosms. Nutrients (nitrogen and phosphorus) and trace elements (a mix of arsenic, copper, cadmium) were added alone and in combination during four experimental runs spanning from spring 1997 to spring 1998. Diel changes in dissolved oxygen were used to examine whole system gross primary production (WS-GPP), respiration (WS-RESP), and net ecosystem metabolism (NEM). Nutrient and trace element additions had the greatest effect on WS-GPP, WS-RESP, and NEM; trophic complexity did not significantly affect any of these parameters (p>0.3). Effects of trophic complexity were detected in nutrient tanks where bivalves significantly (p=0.03) reduced WS-GPP. Nutrient additions significantly enhanced WS-GPP and to a lesser extent WS-RESP during most mesocosm runs. The system shifted from net heterotrophy (−17.2±1.8 mmol C m−3 d−1) in the controls to net autotrophy (29.1±7.6 mmol C m−3 d−1) in the nutrient tanks. The addition of trace elements alone did not affect WS-GPP and WS-RESP to the same extent as nutrients, and their effects were more variable. Additions of trace elements alone consistently made the system more net heterotrophic (−24.9±1.4 mmol C m−3 d−1) than the controls. When trace elements were added in combination with nutrients, the nutrient-enriched system became less autotrophic (1.6±3.1 mmol C m−3 d−1). The effects of trace elements on NEM occurred primarily through reductions in WS-GPP rather than increases in WS-RESP. Our results suggest that autotrophic and heterotrophic processes respond differently to these stressors.

Leaf-litter inputs from an invasive nitrogen-fixing tree influence organic-matter dynamics and nitrogen inputs in a Hawaiian river.

Abstract.  We examined how invasion of tropical riparian forests by an exotic N-fixing tree (Falcataria moluccana) affects organic-matter dynamics in a Hawaiian river by comparing early stages of leaf-litter breakdown between the exotic F. moluccana and native Metrosideros polymorpha trees. We examined early decomposition stages because of low leaf-litter retention rates (<20 d) that result from the flashy nature of tropical Pacific Island streams. Leaf breakdown rates, fungal biomass, and invertebrate abundances were 40, 120, and 30% greater, respectively, for F. moluccana than M. polymorpha leaves. Leaf-litter breakdown was largely a result of stream flow and to a lesser extent fungal colonization. Invertebrates were not an important factor in leaf-litter breakdown. Initial tannin content, leaf C∶N, and toughness were important intrinsic factors inhibiting leaf breakdown and fungal colonization. Regression analyses between remaining N content (%) and ash-free dry mass of leaf litter revealed that the early stages of F. moluccana leaf-litter breakdown are a source of N to streams invaded by F. moluccana and contribute a conservatively estimated 2.1 to 5.7% to the available total dissolved N pool. Direct input of F. moluccana leaf litter influences early stages of leaf-litter breakdown in tropical streams with low leaf-litter retention rates. Direct input of leaf litter also contributes somewhat to N inputs, but subsurface flows through N-rich soils of F. moluccana-invaded riparian forests probably are a greater source.

A Comparison of Water Quality Between Low- and High-Flow River Conditions in a Tropical Estuary, Hilo Bay, Hawaii

Effects of storms on the water quality of Hilo Bay, Hawaii, were examined by sampling surface waters at 6 stations 10 times during low-flow and 18 times during high-flow (storms) river conditions. The direction of a storm’s impact on water quality parameters was consistent among storms and most stations; however, direction of the impact varied with the parameter. High river flow conditions increased concentrations of nitrate and decreased those of dissolved organic nitrogen (N); effects on ammonium and particulate N were station specific. Storms also increased dissolved organic and particulate carbon (C) concentrations. Dissolved phosphorus (P) concentrations were not affected by high river flow events. Dissolved organic forms dominated the N, C, and P pools under both low- and high-flow river conditions. Soil-derived particles and fecal indicator bacteria increased during storms, while chlorophyll a concentrations and bacterial cell abundances decreased. Our results suggest that an increase in storms with global warming could impact water quality of tropical estuaries.

Synthesis of a 13C-Labeled Tracer for Stream DOC: Labeling Tulip Poplar Carbon with 13CO2

Ecosystem tracer-level additions would benefit from a stable isotope-labeled source of complex organic molecules. We tested a method to label tree C with 13C and create a stable isotope tracer for stream dissolved organic carbon (DOC) using tulip poplar (Liriodendron tulipifera L.) seedlings. In 2000, seedlings were grown with 0.82 moles of 13CO2 to assess the distribution and level of 13C enrichment in the tree tissues. In 2001, seedlings were grown with 25 times more 13CO2 to generate tissues with a 13C signal strong enough for a 13C-DOC stream tracer addition. 13C enrichment in the trees varied in each year and by tissue age and type. Tissues formed during labeling (new) were more enriched in 13C than tissues established prior to the 13CO2 injection (old). Stems were most enriched in 13C in both new and old tissues. A higher percentage of 13CO2 was incorporated into seedlings in 2000 (59% ±1) than 2001 (43% ±0). Percent 13C incorporation among tree tissue types paralleled biomass distributions. Although tree C and 13C were equally soluble in both years, a greater percentage of tree C went into solution in 2001 (30%) than 2000 (20%). The water-soluble tree C accounted for approximately 12% of the injected 13CO2 and had both humic and polysaccharide components. Results from a whole-stream 13C-DOC tracer addition demonstrated that tree C could be sufficiently labeled with 13CO2 to create a stream DOC isotope tracer with some polymeric constituents.

Surface Water Metabolism Potential in Groundwater-Fed Coastal Waters of Hawaii Island, USA

Submarine groundwater discharge (SGD) has become increasingly recognized as an important source of freshwater and nutrients to coastal waters worldwide. Although groundwater nutrients have been found to cause algal blooms in many temperate coastal waters, little is known about the biological response to these nutrients in the tropics. On the leeward coast of Hawaii Island, SGD is the dominant freshwater and nutrient source to coastal waters. Kiholo Bay, HI and Kaloko-Honokohau National Historical Park, HI are two nearshore regions with well-documented SGD with high nutrient concentrations; however, little is known about how biological processes within the surface waters respond to these inputs. This study examined how potential gross primary production (pGPP), respiration (RESP), and potential metabolism (pMET) within surface waters differed inside and outside of groundwater plumes at these two sites and between wet and dry seasons. pGPP and RESP were both significantly higher within groundwater plumes, suggesting that SGD stimulated these biological processes; however, RESP responded to a much greater extent than pGPP, resulting in heterotrophic surface waters. RESP also varied seasonally, with greater rates during the dry season compared to the wet one; pGPP did not vary seasonally. Autotrophic conditions were found within groundwater plumes at Kiholo Bay, while heterotrophic conditions were found within them at Kaloko-Honokohau and were greater during the dry season. Overall, our results show that coastal biological processes respond to SGD and that their responses vary over short spatial and temporal scales.

A linked land-sea modeling framework to inform ridge-to-reef management in high oceanic islands

Declining natural resources have led to a cultural renaissance across the Pacific that seeks to revive customary ridge-to-reef management approaches to protect freshwater and restore abundant coral reef fisheries. Effective ridge-to-reef management requires improved understanding of land-sea linkages and decision-support tools to simultaneously evaluate the effects of terrestrial and marine drivers on coral reefs, mediated by anthropogenic activities. Although a few applications have linked the effects of land cover to coral reefs, these are too coarse in resolution to inform watershed-scale management for Pacific Islands. To address this gap, we developed a novel linked land-sea modeling framework based on local data, which coupled groundwater and coral reef models at fine spatial resolution, to determine the effects of terrestrial drivers (groundwater and nutrients), mediated by human activities (land cover/use), and marine drivers (waves, geography, and habitat) on coral reefs. We applied this framework in two ‘ridge-to-reef’ systems (Hā‘ena and Ka‘ūpūlehu) subject to different natural disturbance regimes, located in the Hawaiian Archipelago. Our results indicated that coral reefs in Ka‘ūpūlehu are coral-dominated with many grazers and scrapers due to low rainfall and wave power. While coral reefs in Hā‘ena are dominated by crustose coralline algae with many grazers and less scrapers due to high rainfall and wave power. In general, Ka‘ūpūlehu is more vulnerable to land-based nutrients and coral bleaching than Hā‘ena due to high coral cover and limited dilution and mixing from low rainfall and wave power. However, the shallow and wave sheltered back-reef areas of Hā‘ena, which support high coral cover and act as nursery habitat for fishes, are also vulnerable to land-based nutrients and coral bleaching. Anthropogenic sources of nutrients located upstream from these vulnerable areas are relevant locations for nutrient mitigation, such as cesspool upgrades. In this study, we located coral reefs vulnerable to land-based nutrients and linked them to priority areas to manage sources of human-derived nutrients, thereby demonstrating how this framework can inform place-based ridge-to-reef management.

Stream Nutrient Concentrations on the Windward Coast of Hawai'i Island and Their Relationship to Watershed Characteristics

Abstract: Dissolved inorganic and organic nutrients and physiochemical parameters were measured in 24 Hawai‘i Island streams. Particulate nutrients and instantaneous nutrient and sediment fluxes were measured in half of these streams. Stream waters were dilute and slightly alkaline and had low concentrations of ammonium, orthophosphate, dissolved organic phosphorus, and total suspended solids. Particulate matter comprised 45%, 73%, and 28% of nitrogen, phosphorus, and carbon pools, respectively. Dissolved nitrogen was comprised primarily of organic nitrogen (54%) and nitrate (34%). In some streams, nitrate and total nitrogen concentrations were slightly elevated relative to Hawai‘i Department of Health (HDOH) water quality standards. Instantaneous nitrate yields for the streams plus 26 HDOH stations were calculated, and the average from the combined data set was 7.1 (SD 11.1) moles N day-1 km-2. Nitrate concentrations and yields were 2.1 and 3.5 times higher, respectively, in Kohala watersheds than in Mauna Kea watersheds. Regression analysis was used to evaluate whether water quality parameters are predicted by watershed area, mean annual rainfall, population density, or percentage of agricultural land. Many water quality parameters were not predicted by these variables. In Mauna Kea streams, concentrations of dissolved organic nitrogen and dissolved organic carbon increased with increasing watershed area, nitrate concentrations increased with increasing population density, and both specific conductivity and nitrate yield increased with increasing percentage of agricultural lands. In Kohala streams, nitrate concentrations and yields were not predicted by watershed characteristics. Overall, watershed characteristics, as quantified in this study, were not strong predictors of water quality.

Leaf Litter Breakdown of Native and Exotic Tree Species in Two Hawaiian Streams that Differ in Flow

Abstract: Riparian leaf litter is a major source of allochthonous organic material to temperate and tropical streams, promoting primary and secondary productivity in lotic and nearshore habitats. In tropical island streams, where native leaf-shredding macroinvertebrates are absent, physical fragmentation from stream flow is an important factor affecting leaf litter breakdown and, thus, organic matter dynamics. Additionally, the invasion of exotic plants into riparian areas is expected to affect litter composition and, consequently, its degradation. We compared the interactions of stream flow and inputs of leaf litter from native and exotic plants on leaf litter breakdown in two streams of varying flows on Hawai‘i Island. Decay rates were greater in the high flow stream than in the low flow one for exotic Spathodea campanulata (0.037 vs. 0.023 day-1), but not significantly different for exotic Psidium cattleianum (0.003 vs. 0.003 day-1), and native Metrosideros polymorpha (0.005 vs. 0.002 days-1). In contrast, the exotic Falcataria moluccana (a nitrogen fixer) decomposed more rapidly in the low flow stream (0.017 day-1) than in the high flow stream (0.010 day-1). Breakdown rates also varied among species, with S. campanulata > F. moluccana > M. polymorpha > P. cattleianum. Breakdown rates were generally positively correlated to leaf nitrogen content and negatively correlated with leaf structure characteristics (toughness, organic carbon content, percentage lignin). Our findings indicate that stream flow regimes altered by climate change are likely to influence leaf litter decomposition, and S. campanulata and F. moluccana will likely impact organic matter dynamics in Hawaiian and other Pacific Island streams. However, predicted changes depend on the species composition of riparian leaf litter.

Spatial distribution of sewage pollution on a Hawaiian coral reef

While sewage pollution is contributing to the global decline of coral reefs, its offshore extent and direct reef impacts from water column mixing and benthic seeps are poorly documented. We addressed this knowledge gap on a Hawaiian coral reef using sewage indicator and benthic cover measurements, macroalgal bioassays, and a pollution scoring tool. Fecal indicator bacteria (FIB) and nutrient concentrations were spatially variable in surface and benthic waters, with shoreline values being highest. Shoreline macroalgae δ15N and %N indicated high nitrogen loads containing sewage, while offshore surface and benthic values suggested lower nitrogen loads from environmental sources. Coral cover was negatively correlated with FIB, macroalgal δ15N, and nutrient concentrations. Benthic salinity and temperature measurements detected daily tidal groundwater pulses which may explain these associations. While pollution scores revealed that sewage was largely concentrated along the shoreline, results showed some reached the reef and may be contributing to its declining condition.