Measuring the Effects of Microplastics on Sponges

Microplastics (MP’s) are ubiquitous throughout the marine environment, and are derived from either direct production or from the fragmentation (to <5mm) of larger plastic pollution. Recently concern has intensified as the extent of MP pollution and its presence in the marine environment has been highlighted. Literature concerning concentrations of microplastics indicates an increasing occurrence in the marine environment, from coastal beaches to deep sea sediments. In addition, the effects microplastics have on marine organisms are well documented, with studies ranging from large pelagic animals to benthic filter feeders. However to date, there are few data on how MPs influence Porifera. Sponges are an important component of temperate benthic ecosystems, providing a range of important functional roles. Sponges are able to adapt to many environments by exploiting a variety of food sources, from dissolved organic matter to small crustaceans. Regardless of this, sponges feed primarily on picoplankton, and are able to retain up to 99% of these from seawater. The impact microplastics have on these suspension feeders is becoming of increasing concern, and previous research has centred primarily on sponge feeding or responses to sediments. As such, this thesis is the first to focus on the metabolic responses of sponges to MPs. To examine this, two response variables were measured: O₂ consumption (Respiration) and feeding (Retention efficiency). To examine the effects of MP on sponge respiration, two temperate sponge species (Tethya bergquistae and Crella incrustans) were exposed to two different sized plastic particles (1 μm and 6 μm) at two different concentrations (200,000 and 400,000 beads per mL). Results indicate that sponges are resilient to MP pollution. The only significant result was the effect of MP size on the respiration rates on Tethya bergquistae (P = 0.001), but there were no other significant main effects or interactions. Marine particulates come in many shapes and sizes, as such the retention abilities of temperate sponges were tested after exposure to different types and sizes of particulates. This was achieved by subjecting the same two sponge species (Crella incrustans and Tethya bergquistae) to two microplastic (1 μm & 6 μm), two sediment (1 μm & 6 μm) and two “Food” (raw sea water and Isochrysis galbana) treatments. This experiment showed some significant retention differences, but these differences were difficult to explain and largely inconclusive. This has highlighted the need for further investigation into the effects of: mixed treatments (i.e. sediments + plastics together) and varying plastic shapes (sphere + fibre + fragment). Finally, there is a crucial gap in knowledge regarding the fate of microplastics after ingestion by sponges. This research outlines the potential for temperate sponges to be resilient to microplastics particles when considering respiration rates. In addition, this study also outlines the variable nature of Crella incrustans and Tethya bergquistae concerning particulate retention. As the MP concentrations used in this thesis are very high and are unlikely to be found in New Zealand in the near future, this thesis therefore demonstrates the capability for sponges to be resilient to microplastic pollution. The outcomes of my thesis highlight the importance of understanding the impacts of microplastics on benthic organisms. The marine environment is dynamic and organisms are susceptible to a multitude of stressors. As such, there is a need to explore interactions between multiple factors at the same time.