Effects of Detrital Subsidies on Soft-Sediment Ecosystem Function Are Transient and Source-Dependent

Abstract

Detrital subsidies from marine macrophytes are prevalent in temperate estuaries, and their role in structuring benthic macrofaunal communities is well documented, but the resulting impact on ecosystem function is not understood. We conducted a field experiment to test the effects of detrital decay on soft-sediment primary production, community metabolism and nutrient regeneration (measures of ecosystem function). Twenty four (2 m2) plots were established on an intertidal sandflat, to which we added 0 or 220 g DW m-2 of detritus from either mangroves (Avicennia marina), seagrass (Zostera muelleri), or kelp (Ecklonia radiata) (n = 6 plots per treatment). Then, after 4, 17 and 46 d we measured ecosystem function, macrofaunal community structure and sediment properties. We hypothesized that (1) detrital decay would stimulate benthic primary production either by supplying nutrients to the benthic macrophytes, or by altering the macrofaunal community; and (2) ecosystem responses would depend on the stage and rate of macrophyte decay (a function of source). Avicennia detritus decayed the slowest with a half-life (t50) of 46 d, while Zostera and Ecklonia had t50 values of 28 and 2.6 d, respectively. However, ecosystem responses were not related to these differences. Instead, we found transient effects (up to 17 d) of Avicennia and Ecklonia detritus on benthic primary production, where initially (4 d) these detrital sources suppressed primary production, but after 17 d, primary production was stimulated in Avicennia plots relative to controls. Other ecosystem function response variables and the macrofaunal community composition were not altered by the addition of detritus, but did vary with time. By sampling ecosystem function temporally, we were able to capture the in situ transient effects of detrital subsidies on important benthic ecosystem functions.

The Dynamics of Expanding Mangroves in New Zealand

“…Globally, mangroves are considered to be an important ecological habitat, through the provision of safe haven, habitat and food for juvenile stages of numerous fish, and a diversity of crustaceans and terrestrial species (Claudino et al 2015;Gladstone-Gallagher et al 2016;Nagelkerken et al 2008). However, studies of mangrove productivity and related ecosystem services are predominantly conducted in tropical settings, or where extensive muddy coastlines or deltas support vast mangrove forests (Lee et al 2014;Nagelkerken et al 2008).…”

Changes in benthic community structure and sediment characteristics after natural recolonisation of the seagrass Zostera muelleri

“…Changes in community composition could also be influenced by a cascade effect of seagrass detritus in the food web as macrofaunal structure is modified by detrital enrichment, in which seagrass is known to be important 40 . Detrital inputs also vary with sediment properties, i.e., mud compared to sand, with this affecting macrofaunal community response 41,42 .…”

“…One New Zealand study found that seagrass beds appear to provide feeding and hiding grounds for organisms within a wide range of functional groups, including deposit feeders, scavengers, grazers and predators, which may help to explain the increase in total species over time observed in our study 25 . Changes in community composition could also be influenced by a cascade effect of seagrass detritus in the food web as macrofaunal structure is modified by detrital enrichment, in which seagrass is known to be important 40 . Detrital inputs also vary with sediment properties, i.e., mud compared to sand, with this affecting macrofaunal community response 41 , 42 .…”

Implications of nutrient enrichment for the conservation and management of seagrass Zostera muelleri meadows

“…To simulate a procedural control, inert sand of similar grain size was applied in control plots with the same method. No additional control was used because with a similar level of disturbance in a prior experiment, no procedural effects were detected on intertidal macrofaunal community composition, primary production, respiration, or nutrient fluxes, and no trace of procedural disturbance was visually apparent 1–7 weeks following disturbance, even in plots containing seagrass (Douglas et al, ; Gladstone‐Gallagher, Lohrer, Lundquist, & Pilditch, ; Gladstone‐Gallagher, Lundquist, & Pilditch, ). The quantity of fertilizer used in different treatment levels was 400 g N/m 2 for the high‐enrichment treatment, 266 g N/m 2 for the medium‐enrichment treatment, and 133 g N/m 2 for the low‐enrichment treatment.…”