Driven by speculation, not by impact - the effects of plastic on fish species

Microplastics, both non-biodegradable and biodegradable, do not affect the whole organism functioning of a marine mussel

Microplastics are ubiquitous in the marine environment, and their uptake by many organisms has been well documented. Concern about increasing plastic waste in ecosystems and organisms has led to the production of biodegradable alternatives. However, long breakdown times of biodegradable plastics in natural environments mean they still have the potential to induce ecological impacts. The impacts of microplastics on organisms remain unclear, especially as many experimental microplastic exposures employ particle concentrations orders of magnitude greater than those found in natural ecosystems. Here, we exposed the ecosystem engineer, the Asian green mussel Perna viridis, to non-biodegradable and biodegradable microplastics at two environmentally relevant concentrations (~17-20 particles L^-1 and ~ 135-140 particles L^-1). After four weeks of exposure, there were no significant effects of microplastic type or concentration on the mortality, oxygen consumption rate, clearance rate, or condition index of P. viridis. With the increasing body of microplastic literature, future exposure studies considering biotic effects should make efforts to employ environmentally relevant concentrations. Further, we suggest that, while a high-profile threat to ecosystems, investigating the effects of microplastics on ecosystems should be conducted alongside, and not draw focus away from, other major threats such as climate change.

Plastic debris increases circadian temperature extremes in beach sediments

Plastic pollution is the focus of substantial scientific and public interest, leading many to believe the issue is well documented and managed, with effective mitigation in place. However, many aspects are poorly understood, including fundamental questions relating to the scope and severity of impacts (e.g., demographic consequences at the population level). Plastics accumulate in significant quantities on beaches globally, yet the consequences for these terrestrial environments are largely unknown. Using real world, in situ measurements of circadian thermal fluctuations of beach sediment on Henderson Island and Cocos (Keeling) Islands, we demonstrate that plastics increase circadian temperature extremes. Particular plastic levels were associated with increases in daily maximum temperatures of 2.45°C and decreases of daily minimum by - 1.50°C at 5 cm depth below the accumulated plastic. Mass of surface plastic was high on both islands (Henderson: 571 ± 197 g/m²; Cocos: 3164 ± 1989 g/m²), but did not affect thermal conductivity, specific heat capacity, thermal diffusivity, or moisture content of beach sediments. Therefore, we suggest plastic effects sediment temperatures by altering thermal inputs and outputs (e.g., infrared radiation absorption). The resulting circadian temperature fluctuations have potentially significant implications for terrestrial ectotherms, many of which have narrow thermal tolerance limits and are functionally important in beach habitats.

Microplastics do not affect the feeding rates of a marine predator

Microplastics may affect the physiology, behaviour and populations of aquatic and terrestrial fauna through many mechanisms, such as direct consumption and sensory disruption. However, the majority of experimental studies have employed questionably high dosages of microplastics that have little environmental relevance. Predation, in particular, is a key trophic interaction that structures populations and communities and influences ecosystem functioning, but rarely features in microplastic research. Here, we quantify the effects of low (~65-114 MP/L) and high (~650-1140 MP/L) microplastic concentrations on the feeding behaviour of a ubiquitous and globally representative key marine predator, the shore crab, Carcinus maenas. We used a functional response approach (predator consumption across prey densities) to determine crab consumption rates towards a key marine community prey species, the blue mussel Mytilus edulis, under low and high microplastic concentrations with acute (8h) and chronic (120h) microplastic exposure times. For both the acute and chronic microplastic exposure experiments, proportional prey consumption by crabs did not differ with respect to microplastic concentration, but significantly decreased over increasing prey densities. The crabs thus displayed classical, hyperbolic Type II functional responses in all experimental groups, characterised by high consumption rates at low prey densities. Crab attack rates, handling times and maximum feeding rates (i.e. functional response curves) were not significantly altered under lower or higher microplastics concentrations, or by acute or chronic microplastic exposures. Here, we show that functional response analyses could be widely employed to ascertain microplastic impacts on consumer-resource interactions. Furthermore, we suggest that future studies should adopt both acute and chronic microplastic exposure regimes, using environmentally-relevant microplastic dosages and types as well as elevated future scenarios of microplastic concentrations.