Landscape patterns shape wetland pond ecosystem function from glacial headwaters to ocean

Examining patterns and processes along the aquatic continuum from headwaters to ocean can benefit from a landscape ecology approach, where hydrologic and ecological processes depend on landscape position. We conducted a study of freshwater wetland ponds subject to similar climatic conditions but distributed along a 20-km trajectory from glacial headwaters to ocean in southcentral Alaska, U.S.A. Specifically, we investigated how proximity to glaciers and ocean influenced physical, chemical, and biological characteristics of ponds. Physicochemical patterns along a distance gradient from the ocean supported the hypothesized influence of elevation and potential atmospheric deposition of marine-derived nitrogen, whereas those related to glacial flow path length may reflect inputs from glacial weathering. We expected that the effects of landscape and hydrology on physicochemical patterns would provide a template for shaping ecosystem processes, but ecosystem processes also appeared to contribute to physicochemical patterns across this landscape. Ponds more heavily influenced by glaciers tended to be more heterotrophic exhibiting greater rates of organic-matter decomposition and ecosystem respiration, which were positively correlated with phosphorus and iron concentrations likely due to glacial weathering and remineralization processes. In contrast, ponds near the ocean tended to be more autotrophic exhibiting greater gross primary production and net ecosystem production, processes that may have contributed to greater total nitrogen, nitrogen to phosphorus ratios, and dissolved organic carbon concentrations. Consideration of the relative importance of hydrologic inputs across the landscape is needed because the acceleration of glacial melt and sea-level rise by climate change may alter future broad-scale patterns of ecosystem processes. Landscape ecology aims to explore spatial and temporal patterns across heterogeneous landscapes and to examine their influences on biotic and abiotic processes (Risser 1987; Turner 2005). The exploration of these patterns is a classical theme in aquatic ecology. For example, the “River Continuum Concept,” a foundation for many studies in stream ecology, posits that the varying conditions along a river from headwaters to the mouth shape its chemical and biological processes (Vannote et al. 1980). Similarly, limnologists have documented the influence of landscape position (e.g., elevation or lake network number) on the chemical and biological properties of lakes (Kratz et al. 1997; Soranno et al. 1999; Sadro et al. 2011). Landscape patterns can therefore affect physical, chemical, and biological aspects of aquatic ecosystems. Hydrology is particularly important in shaping landscape patterns in both rivers and lakes. For example, headwater streams tend to be dominated by inputs of coarse particulate organic matter (e.g., leaves and woody debris) from the surrounding forest, but downstream where the channel widens, the influence of the forest declines and primary producers within the stream (e.g., algae and aquatic plants) begin to play a larger role (Vannote et al. 1980). The “River Continuum Concept” thereby relates the physics and hydrology of flowing waters to their biological characteristics (e.g., invertebrate and fish diversity) and ecosystem processes (i.e., the relative importance of production vs. respiration). Similarly in lakes, landscape position or elevation can determine the relative importance of precipitation, runoff, and groundwater inputs, which in turn shape pH, dissolved organic carbon *Correspondence: [email protected] This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Special Issue: Headwaters to Oceans: Ecological and Biogeochemical Contrasts Across the Aquatic Continuum Edited by: Marguerite Xenopoulos, John A. Downing, M. Dileep Kumar, Susanne Menden-Deuer, and Maren Voss S207 LIMNOLOGY and OCEANOGRAPHY Limnol. Oceanogr. 62, 2017, S207–S221 VC 2017 The Authors Limnology and Oceanography published by Wiley Periodicals, Inc. on behalf of Association for the Sciences of Limnology and Oceanography doi: 10.1002/lno.10575 (DOC), nutrient availability, and base cation concentrations (Kratz et al. 1997; Soranno et al. 1999; Sadro et al. 2011). While studies of landscape position relate hydrology to chemical and biological characteristics of lakes such as fish species richness (Kratz et al. 1997), chlorophyll (Soranno et al. 1999), and bacterioplankton abundance (Sadro et al. 2011), very few studies have directly examined the landscape influences on ecosystem processes. One exception is Kling et al. (2000) who found that lake primary productivity tended to decrease lower in the landscape likely due to the corresponding decreases in particulate carbon and nitrogen. Therefore, hydrology and its influence on a landscape can shape both chemical and biological patterns as well as ecosystem processes in aquatic ecosystems. Proximity to hydrologic sources, such as the ocean, can also play an important role in shaping chemical and biological processes. For example, in estuarine systems of the Chesapeake Bay, Jordan et al. (2008) observed that as salinity and porewater sulfate availability increased, pore-water concentrations of dissolved iron and ammonium decreased, while dissolved inorganic phosphorus increased, thereby exhibiting lower N : P ratios. They hypothesized that the mechanism behind these patterns was geological and that these patterns could contribute to the shift from nitrogen limitation in coastal marine water to phosphorus limitation in freshwater (Jordan et al. 2008). Additionally, we might expect that freshwater systems closer to the ocean would tend to exhibit higher sulfate due to sea spray and marine atmospheric deposition (Junge and Werby 1958). Because of the ocean’s potential influence on both atmospheric and aquatic chemistry, it is reasonable to assume that these changes in nutrient stoichiometry could also affect ecosystem processes across the landscape (e.g., Vitousek et al. 1997; Sim o and Pedr os-Ali o 1999). Proximity to glaciers could also alter chemical and biological processes of freshwater ecosystems. For example, phosphorus availability in glacially influenced Alaskan rivers peaked when runoff was highest due to glaciated watersheds having higher yields of phosphorus, which tend to be associated with poorly weathered calcite/apatite-rich minerals (Hood and Scott 2008). Additionally, glacially influenced streams of the Copper River, Alaska, receive high loads of suspended sediment and colloidal iron associated with silicates because of the physical processes of mechanical weathering (Schroth et al. 2011). Because glacier-fed streams tend to have higher turbidity, lower temperatures, and therefore less suitable habitat for algae and invertebrates than groundwater-fed streams (Malard et al. 2006), one might expect that glacially influenced aquatic ecosystems may tend toward heterotrophy rather than autotrophy. Although past research demonstrates that oceanic and glacial inputs can affect their respective aquatic ecosystems, we know less about how these influences interact in a single broad-scale landscape (O’Neel et al. 2015). The Copper River Delta (CRD) in southcentral Alaska is a unique landscape that allows us to examine the interactive influences of both ocean and glaciers across an aquatic continuum. To examine the glacier-to-ocean continuum, we surveyed physicochemical properties and ecosystem function in 15 freshwater ponds. We hypothesized that landscape position would strongly influence physicochemical properties in these ponds, which in turn would affect ecosystem processes such as organic-matter decomposition and ecosystem metabolism. Because glacially influenced systems may be less suitable for primary and secondary producers (Malard et al. 2006) and headwaters generally tend to exhibit greater heterotrophy (Vannote et al. 1980), we also hypothesized that the relative importance of heterotrophic vs. autotrophic processes would increase closer to the glaciers and decrease closer to the ocean.