# Reality Check: Experimental Studies on Microplastics Lack Realism
## 4 Concentration  
### Aquatic Effects  
MP concentrations in various marine environments ranges from approximately 1 mg/L-1 ng/L, generally <20 MPs/ m\({}^{3}\) in water [39] but most lab exposures use far higher concentrations. [PERSON] et al. (2016) state that experimental exposures are two to seven orders-of-magnitude higher than concentrations in the environment [39]. [PERSON] et al. (2017) reviewed particle counts in marine and freshwater surface water and found them to be extremely variable [40]. MP concentrations in marine surface waters are reported from 0.0005 particles L\({}^{-1}\) to 16 particles L\({}^{-1}\) with intermediate concentrations. Freshwater surface samples have concentrations around the lower end of marine surface concentrations, in general. According to [PERSON] et al. (2020) only 17% of concentrations used in laboratory experiments have been found in nature, and 80% of particle sizes used in experiments fall below the size range seen in the majority of environmental sampling [2]. In their review article, [PERSON] et al. (2021) found 14 papers that found exposure with no or minor effects [41]. Some of these used low concentrations, but some used high concentrations. In contrast they found 81 papers that reported adverse effects, most of which used unrealistically high concentrations (Figure 1). However, current MP sampling is likely to underestimate concentrations in the water [42]. Using different mesh sizes for collection results in greatly different counts. MPs collected with a 100 \(\upmu\)m net were 10-times greater than samples collected with a 500 \(\upmu\)m net. [PERSON] et al. (2019), comparing samples collected with a standard plankton net (300 \(\upmu\)m mesh) verses a bulk sample filtered through an 8 \(\upmu\)m filter, found 8.5 times as many in the filtered sample, including a much greater proportion of microfibers [43]. This effect is important when comparing surface water samples, since differences in sampling methods lead to a lack of comparability. Sediment concentrations tend to be higher than in water in UK river sediments, which was also observed in coastal sediments in Slovenia [44; 45]. High concentrations (thousands of particles kg\({}^{-1}\) of dry sediment) were found in river sediments in Germany, similar to the 2000-8000 particles kg\({}^{-1}\) reported in coastal sediments in Canada [46; 47]. Experiments do not always use unreasonably high concentrations, and there are many studies that find deleterious effects at ranges close to environmental concentrations. Lobster (_Homarus americanus_) larvae were exposed to MFs at 0, 1, 10 and 25 MF mL\({}^{-1}\)[11]. Only the highest MF concentration decreased early larval survival. MFs did not affect the timing or rate of molting. Mussels (_Mytilus edulis_) were fed a diet of plankton with MF concentrations up to 30 MF mL\({}^{-1}\) and filtration rates were greatly reduced [48]. [PERSON] et al. (2020) used environmentally relevant concentrations of MFs (1 mm long 0.1 mm diameter) to expose mole crabs, _Emerita analoga_[8]. The site from which crabs were collected had an average of 15 pieces/100 mL of sand, water had 3-7 MF/L water. They found that crabs ingested MFs and showed deleterious effects on survival, reproduction, and embryo development. [PERSON] et al. (2021) performed 60-day exposures of sea cucumbers, _Apostichopus japonicus_ to MFs [49]. Their food was mixed with MFs at 0.6 and 1.2 MFs g\({}^{-1}\), based on environmentally relevant concentrations. In the third treatment, MFs were mixed in the diet at 10 MF g\({}^{-1}\) to simulate the worst-case scenario. Exposures did not significantly affect growth or fecal production rate. However, acid phosphatase and alkaline phosphatase activity were altered, and total antioxidant capacity was reduced in juveniles and adults. Hermit crabs (_Pagurus bernhardus_) living in suboptimal shells were exposed to polyethylene particles at 25 particles L\({}^{-1}\)[50], which is lower than most exposure studies. After five days, crabs showed impaired shell selection.