Influences of N-Fixing and Non-N-Fixing Vegetation and Invasive Fish on Water Chemistry of Hawaiian Anchialine Ponds

Abstract

In coastal waters, it remains unclear how terrestrial invasive species might alter nutrient availability and thus affect bottom-up control of primary production. Anchialine ponds are tidal- and groundwater-fed coastal water bodies without surface connections that provide convenient model systems in which to examine terrestrial to aquatic nutrient flow. To investigate how N-fixing and non-N-fixing terrestrial vegetation and invasive consumers affect water quality, we compared dissolved nutrient concentrations and physicochemical parameters among Hawaiian anchialine ponds surrounded by: (1) the nonnitrogen (N)-fixing tree Thespesia populnea (milo), (2) the exotic N-fixing tree Prosopis pallida (kiawe), (3) no tree cover, and (4) no tree cover but containing populations of invasive poeciliid fish. Average (±1 SE) concentration of dissolved inorganic nitrogen (DIN) across all ponds (51.15 µM ±3.1; n = 17) was high, but there was no discernible pattern among pond types. Model results suggest that leaf litter breakdown from the exotic N-fixing tree contributed <0.02% of total dissolved nitrogen (TDN) in the water column at high tide. However, additions of phosphorus (P), especially from litter of non-N-fixing T. populnea, resulted in ratios of DIN to soluble reactive phosphorus (SRP) that varied between 4.5:1 and 35:1 across pond types. Our study suggests that: (1) DIN concentrations were more dependent on watershed-scale N inputs than local tree cover or the presence of poeciliids, (2) low water residence times in these ponds reduce effects of local biota on water nutrient chemistry, and (3) P from anthropogenic inputs and/or biota cause alterations to DIN:SRP ratios in anchialine pond water that may potentially alter primary production rates in these coastal ecosystems.