ORIGINAL Decomposing macroalgae (Ulva spp.) impacts benthic macrofauna and surface sediment erosion Rachel J. Harris 1,2 & Clarisse Niemand 2 & Conrad A. Pilditch 2 Received: 13 February 2019 /Accepted: 26 August 2019 # Springer-Verlag GmbH Germany, part of Springer Nature 2019 Abstract Patches of decomposing macroalgae can be beneficial to intertidal regions, providing habitat and food, or can create anoxic conditions inhospitable to benthic organisms. These opposing outcomes warrant further investigations into biotic-abiotic processes associated with macroalgae. Here, differences in surface sediment erosion measures (erosion threshold (Ʈ c ;Nm -2 ), erosion rate (ER;gm -2 s -1 ), and sub-surface erosion rates (m e ;gN -1 s -1 )) were examined at sites dominated by the suspension-feeding clam or deposit-feeding bivalve (Austrovenus stutchburyi and Macomona liliana, respectively) after 30 days of exposure to decomposing macroalgae (Ulva spp.). The Austrovenus site was chosen to represent a species-rich, functionally diverse macrofaunal community. The nearby Macomona site had similar sediment characteristics, yet had a less abundant and diverse faunal community. Despite the equal amounts of Ulva recovered from both sites (< 3% of the initial 3 kg wet weight m -2 added), differences in surface erosion were measured. One day after Ulva removal, an initial increase in surface erosion (-Ʈ c and +ER) was measured at the Austrovenus site, but after 14 days there was no difference compared with control plots. At the Macomona site, Ulva addition stabilized sediments (+Ʈ c and -ER), an effect that persisted for 14 days. These differences were reflected in the macrofaunal community. At the Austrovenus site, the macrofaunal community recovered within 14 days whereas differences between Ulva and controls persisted at the Macomona site. This work emphasizes how subtle differences in benthic community composition and recovery can result in spatial and temporal variations in sediment erosion potential on intertidal sandflats. Introduction Macroalgal blooms linked to coastal eutrophication have be- come increasingly common across the globe (Sfriso et al. 1992; Valiela et al. 1997; Raffaelli et al. 1998; Morand and Merceron 2005; Liu et al. 2013). When washed onshore, macroalgae can create large-scale catastrophic disturbances that cover thousands of kilometers (e.g., Liu et al. 2013), but also disturbances on a smaller (meter) scale that result in lo- calized organic enrichment or hypoxia (e.g., Norkko and Bonsdorff 1998a, b). Macroalgal blooms are typically season- al, coinciding with a decrease in current velocity and increase in salinity, nutrients, and/or light penetration, thus providing favorable conditions for growth (e.g., Martins et al. 2001). Ulva (Enteromorpha) spp. are the most common green macroalgae associated with prolific blooms and can be found from temperate to tropical regions worldwide (Teichberg et al. 2010). Since Ulva growth rapidly increases in the presence of dissolved inorganic nitrogen (Lapointe and Tenore 1981), this species has become a concern in nutrient-rich estuarine sys- tems. From a human perspective, the buildup of decomposing macroalgae creates an unpleasant smell (i.e., hydrogen sul- fides), resulting in a decline in the recreational use of beaches and waterways (Teichberg et al. 2010). However, the ecolog- ical effects are complex and will depend upon the amount of algae, the stage of decomposition, and the response in the benthic macrofaunal population (Norkko et al. 2000; Orr et al. 2005). Decomposition of macroalgal blooms primarily occurs at the sea bed and the consequences to benthic ecosystems can be positive or negative, determined by ecological processes. Small fishes and mobile macrofauna (e.g., gastropods, crusta- ceans) use near-bed drifting macroalgae for shelter and protec- tion from predators (Raffaelli et al. 1998; Salovius et al. 2005). If the overall estuarine nitrogen supply is low, macrofaunal * Rachel J. Harris harris.rachelj@gmail.com 1 Loxahatchee River District, 2500 Jupiter Park Dr, Jupiter, FL 33458, USA 2 School of Science, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand Geo-Marine Letters https://doi.org/10.1007/s00367-019-00593-z