## 6 Challenges and Limitations
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**No** & **Solution** & **Research Area** & **Source** \\ \hline \multirow{3}{*}{1} & Building a reef from piles of car tires placed on a wooden & A river in Brazil where the impact of introducing artificial & \multirow{3}{*}{[4]} \\  & support—dimensions 2.45 m \(\times\) 1.05 m \(\times\) 1.25 m & & reservoir was assessed & \\ \hline \multirow{3}{*}{2} & Reefs made of piles of car tires placed on a wooden & The impact of habitat complexity induced by artificial reef & \multirow{3}{*}{[8]} \\  & support—dimensions 2.45 m \(\times\) 1.05 m \(\times\) 1.25 m & & construction on fish communities in a reservoir in Brazil & \\ \hline \multirow{3}{*}{3} & Structure consisting of cement discs (weight: 25 g, diameter & \multirow{3}{*}{Assessment of the suitability of different cements as \multirow{3}{*}{[94]}} \\  & 39 mm, thickness: 13 mm). cements enriched with silica & & substrates for the construction of artificial reefs based on \\  & (porzzolanic), blast furnace slag, and fly ash were used for the construction & & the occurrence of microalgae populations on the structures \\ \hline \multirow{3}{*}{4} & An artificial reef built of granite rubble & Impact on the number of different fish species in the USA & [7] \\ \hline \multirow{3}{*}{5} & A round frame made of 19 mm diameter polyvinyl & \multirow{3}{*}{Effects on the number of different fish species in the \multirow{3}{*}{[5]}} \\  & chloride (PVC) pipe with standard dimensions (diameter & \multirow{3}{*}{4.8 m and surface area 2.5 m\({}^{2}\)). Polyethylene ropes were & oligotrophic reservoir in Brazil & \\  & attached radially to the rope to the bunches of plants & & \\ \hline \multirow{3}{*}{6} & The artificial reefs were constructed from ceramics, & \multirow{3}{*}{Influence of reef material (ceramics, concrete, and PVC) on \multirow{3}{*}{[6]}} \\  & concrete, and polyvinyl chloride (PVC) by arranging pipes & & \\ \cline{1-1}  & of the individual materials with a length of 1.0 m and a diameter of 0.3 m in a pyramidal frame (10 pipes in a & \\ \cline{1-1}  & 4:3:2:1 arrangement from the base to the top) & \\ \hline \multirow{3}{*}{7} & Two artificial structures: pipes and trees and two & \multirow{3}{*}{Experimental verification of the use of artificial habitats by \multirow{3}{*}{[95]}} \\  & semi-natural rocks. The pipe structure was built from & fish in two neotropical reservoirs (2 water reservoirs in \\ \cline{1-1}  & ceramic pipes 0.8 m long and 0.25 m in diameter. The structure is in the shape of a pyramid made of 10 pipes. & Brazil). Determination of the relationship between the type \\ \cline{1-1}  & The wooden structure was composed of four pine trees & \multirow{3}{*}{Research on the structure of phytoplankton communities around an artificial reef and its adjacent waters in China} & \\ \cline{1-1} \cline{2-2} \cline{4-4} \cline{1-1}  & Artificial reef consisting of biomass modules, each & & \\ \cline{1-1}  & measuring 900 mm \(\times\) 900 mm \(\times\) 120 mm. Raw materials & \multirow{3}{*}{[93]} \\  & were mainly derived from by-products of biomass energy & & \\ \cline{1-1}  & production from crops and forest residues & & \\ \hline \multirow{3}{*}{9} & Four types of artificial reefs (emented rope, brick, wood, and ceramic) were tested over a period of six months & & \\ \cline{1-1}  & & Benthic species were studied. Colonization was faster and & \\ \cline{1-1}  & & denser on bricks and ceramics. The study was conducted & \\ \cline{1-1}  & in a freshwater pond filled with water from a river in Iran & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Work to date on artificial reefs dedicated to freshwater. material parameters [103; 104]. Others are very specific, including the application of 3D printing technology in underwater environments [13; 47]. An important group of limitations is also connected with current knowledge about different ecosystems and limitations in predicting the changes in the whole complex net of connections between the various organisms.